Wednesday, June 29, 2011
Garage Door Opener Transmitter
This electronic circuit is the newer and revised version of the garage door opener transmitter. The transmitter is a PIC based on 40 MHz Microchip’s 16F630 microcontroller. A 10 switch DIP-switch is used for setting the code. To save space, we used a different HF output coil. Schematic , PCB files and hex code are available.
VHF/UHF TV Modulator
The electronic circuit is a TV modulator that is really no more than a transmitter. It is a very small transmitter, admittedly, but none the less that is what it is. What does a modulator actually do? In general -and this design is no exception to the rule - it is a simple oscillator that generates a frequency somewhere in the VHF or UHF region. The oscillator is modulated with the video signal and the modulated carrier wave thus generated is fed into the TV set's aerial input via a cable. Then all that remains to do is tune the TV to the correct frequency.
Circuit Schematic
The crystal oscillator is based on a very fast HF transistor, Tl (BFR91), which performs the amplitude modulation. Apart from this there is little to be said about the oscillator except, perhaps, that it is essential to use the correct values for the components surrounding Tl. This is, of course, simply common sense in this sort of HF circuit.
The harmonics generator is formed by two Schottky diodes, Dl and D2. These diodes must switch very quickly in time with the 27 MHz signal so they provide strong harmonics up into the gigahertz range. The modulation depth can be set with Pl, while the oscillator's d.c. value can be varied by means of P2. The combination of these two presets enables either positive or negative amplitude modulation to be selected.
This is essential as the harmonics produced vary in this respect. We will discuss the calibration of Pl and P2 later in this article. The power for the circuit can be provided by either an unstabilized 8...30 V or a stabilized 5 V. The latter could be taken from a computer's power supply and in this case ICI is not needed.
The printed circuit board for the modulator is only single-sided. The largecopper surface acts as a ground plain.
Parts list
Resistors:
R1, R2 = 4k7
R3, R4 = 56ohm
P1 = 100 ohm preset
P2 = 500 ohm preset
Capacitors:
C1 = 4mf7/16 V
C2= 10p
C3 = 220p
C4 = 47p
C5 = 47n, ceramic
C6 = 100n*
C7 = 330n*
Inductors:
L1, L2 = 3.5 turns of 0.2 mm (SWG 35 or 36) CuL on a ferrite bead of about 3.5 x 3.5 mm
L3 = 1 microH
L4 = 1 turn of 0.8. . .1 mm (SWG 19...21) CuL, air wound with a diameter of 8 mm
Semiconductors:
D1, D2 = 1N6263 (Ambit/Cirkit)
D3 = lN4148
T1 = BFR91 (Ambit/Cirkit)
IC1 = 7805*
Miscellaneous:
X1 = crystal, 27 MHzd(3rd overtone) or other 3rd overtone crystal between 25 and 30 MHz
*= not needed if the circuit is powered from a stabilised 5 V supply
Source
Circuit Schematic
The crystal oscillator is based on a very fast HF transistor, Tl (BFR91), which performs the amplitude modulation. Apart from this there is little to be said about the oscillator except, perhaps, that it is essential to use the correct values for the components surrounding Tl. This is, of course, simply common sense in this sort of HF circuit.
The harmonics generator is formed by two Schottky diodes, Dl and D2. These diodes must switch very quickly in time with the 27 MHz signal so they provide strong harmonics up into the gigahertz range. The modulation depth can be set with Pl, while the oscillator's d.c. value can be varied by means of P2. The combination of these two presets enables either positive or negative amplitude modulation to be selected.
This is essential as the harmonics produced vary in this respect. We will discuss the calibration of Pl and P2 later in this article. The power for the circuit can be provided by either an unstabilized 8...30 V or a stabilized 5 V. The latter could be taken from a computer's power supply and in this case ICI is not needed.
The printed circuit board for the modulator is only single-sided. The largecopper surface acts as a ground plain.
Parts list
Resistors:
R1, R2 = 4k7
R3, R4 = 56ohm
P1 = 100 ohm preset
P2 = 500 ohm preset
Capacitors:
C1 = 4mf7/16 V
C2= 10p
C3 = 220p
C4 = 47p
C5 = 47n, ceramic
C6 = 100n*
C7 = 330n*
Inductors:
L1, L2 = 3.5 turns of 0.2 mm (SWG 35 or 36) CuL on a ferrite bead of about 3.5 x 3.5 mm
L3 = 1 microH
L4 = 1 turn of 0.8. . .1 mm (SWG 19...21) CuL, air wound with a diameter of 8 mm
Semiconductors:
D1, D2 = 1N6263 (Ambit/Cirkit)
D3 = lN4148
T1 = BFR91 (Ambit/Cirkit)
IC1 = 7805*
Miscellaneous:
X1 = crystal, 27 MHzd(3rd overtone) or other 3rd overtone crystal between 25 and 30 MHz
*= not needed if the circuit is powered from a stabilised 5 V supply
Source
Stereo FM Transmitter Based BA1404 Chips
This electronic circuit design is a stereo FM transmitter that improves sound quality, has very good frequency stability, maximizes transmitter's range, and is fairly simple for everyone to build. We are happy to announce that this goal and expectations have been met and even exceeded.
The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range.
Sound Quality and Frequency Stability
One of the qualities of BA1404 FM transmitter is excellent frequency stability. This is mainly due to a use of high quality 3.5 turn variable coil. Tunable RF coils are ideal for precise frequency tuning because their magnet wire is halfway embedded within the plastic, which minimizes frequency drifts. Regular air coils are not preferred for professional broadcasting because the coil expands and contracts with temperature changes. That's the very reason why variable coil was chosen as a substitution for an air coil and a variable capacitor.
Another quality of the presented BA1404 transmitter is a crystal clear stereo sound and improved sound separation. There are several factors that account for improved sound quality and a separation. First reason is the use of 38 KHz crystal which provides rock solid frequency for stereo encoder. Another reason is the use of two 1nF decoupling capacitors one for BA1404 chip and another for 3.5 variable coil. These capacitors have to be as close as possible to a BA1404 chip and a variable coil because this will GREATLY improve the sound quality, sound separation and even frequency stability as well. What they do is filter out the noise in the incoming DC voltage. If the noise enters BA1404 chip stereo generator will include it in a transmitted sound affecting both the sound and multiplex signal that is responsible for generation of the clear stereo signal. If that noise enters it will also be included in a generation of subcarrier frequency affecting the frequency stability. Most people are not aware of how important this is and might place them in a wrong location, away from the target components which provides no use, or worse decide not to use these capacitors at all.
Another factor that is extremely important and which improves overall quality of the whole BA1404 transmitter including frequency stability, sound quality and sound separation is the use of the ground plane on the transmitter’s PCB. It is recommended that ground plane should always be used in circuits that deal with higher frequencies.
Printed Circuit Board
This a suggested high-resolution PCB layout for BA1404 Transmitter. It is ready for printing and no further adjustments are necessary. Dimensions of the PCB should be 57 mm x 35 mm (W x H).
Source
The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range.
Sound Quality and Frequency Stability
One of the qualities of BA1404 FM transmitter is excellent frequency stability. This is mainly due to a use of high quality 3.5 turn variable coil. Tunable RF coils are ideal for precise frequency tuning because their magnet wire is halfway embedded within the plastic, which minimizes frequency drifts. Regular air coils are not preferred for professional broadcasting because the coil expands and contracts with temperature changes. That's the very reason why variable coil was chosen as a substitution for an air coil and a variable capacitor.
Another quality of the presented BA1404 transmitter is a crystal clear stereo sound and improved sound separation. There are several factors that account for improved sound quality and a separation. First reason is the use of 38 KHz crystal which provides rock solid frequency for stereo encoder. Another reason is the use of two 1nF decoupling capacitors one for BA1404 chip and another for 3.5 variable coil. These capacitors have to be as close as possible to a BA1404 chip and a variable coil because this will GREATLY improve the sound quality, sound separation and even frequency stability as well. What they do is filter out the noise in the incoming DC voltage. If the noise enters BA1404 chip stereo generator will include it in a transmitted sound affecting both the sound and multiplex signal that is responsible for generation of the clear stereo signal. If that noise enters it will also be included in a generation of subcarrier frequency affecting the frequency stability. Most people are not aware of how important this is and might place them in a wrong location, away from the target components which provides no use, or worse decide not to use these capacitors at all.
Another factor that is extremely important and which improves overall quality of the whole BA1404 transmitter including frequency stability, sound quality and sound separation is the use of the ground plane on the transmitter’s PCB. It is recommended that ground plane should always be used in circuits that deal with higher frequencies.
Printed Circuit Board
This a suggested high-resolution PCB layout for BA1404 Transmitter. It is ready for printing and no further adjustments are necessary. Dimensions of the PCB should be 57 mm x 35 mm (W x H).
Source
Stereo FM Transmitter Based BH1417 Chips
This electronic circuit is a latest BH1417 FM Transmitter design from RHOM that includes a lot of features in one small package. It comes with pre-emphasis, limiter so that the music can be transmitted at the same audio level, stereo encoder for stereo transmission, low pass filter that blocks any audio signals above 15KHz to prevent any RF interference, PLL circuit that provides rock solid frequency transmission (no more frequency drift), FM oscillator and RF output buffer.
There are 14 possible transmission frequencies with 200KHz increments that users can select with a 4-DIP switch. Lower band frequencies start from 88.7 up to 89.9 MHz, and upper band frequencies start from 107.7 up to 108.9 MHz.
BH1417 can be supplied with 4 - 6 voltage and consumes only around 30mA, providing 20mW output RF power. BH1417 provides 40dB channel separation which is pretty good, although older BA1404 FM Transmitter chip provides slightly better 45dB channel separation.
BH1417 is only available in SOP22 IC case so this may be an inconvenience for some folks. On the other hand, because the chip is smaller than regular DIP-based ICs it is possible to fit the entire transmitter on a small PCB.
The bad news is that BH1417 requires 7.6MHz crystal oscillator, which is very hard to find. The good news is that you can use 7.68 MHz crystal instead, which is easier to find. In fact our BH1417 transmitter prototype (schematic shown above) uses 7.68 MHz crystal. This has absolutely no effect on stereo encoding process, we have tested it and stereo sound is crystal clear. The transmitted frequency on the other hand will be shifted up by exactly 1MHz (example: 88.1 MHz to 89.1 MHz) which is perfectly fine. The frequencies that are used in this project have been adjusted by 1MHz already so no additional conversion is necessary.
BH1417 chip may also be used a stand alone stereo encoder. The advantage of that is that you have full freedom of using a transmitter & amplifier of your choice. You will still have a pre-emphasis, limiter, stereo encoder and low pass filter in one small package because very few external components are required for these blocks. PIN 5 is MPX output that can be directly connected to an external FM transmitter through a 10uF cap.
Parts List:
1x BH1417 - Stereo PLL Transmitter IC (Case SOP22) (datasheet)
1x 7.68 MHz Crystal
1x MPSA13 - NPN Darlington Transistor
1x 2.5 Turns Variable Coil
1x MV2109 - Varicap Diode
1x 4-DIP Switch
ANT - 30 cm of copper wire
1x 22K Resistor
7x 10K Resistor
1x 5.1K Resistor
2x 3.3K Resistor
1x 100 Ohm Resistor 1x 100uF Capacitor
3x 10uF Capacitor
2x 1uF Capacitor
1x 47nF Capacitor
3x 2.2nF Capacitor
1x 1nF Capacitor
1x 330pF Capacitor
2x 150pF Capacitor
1x 33pF Capacitor
2x 27pF Capacitor
1x 22pF Capacitor
2x 10pF Capacitor
Specifications:
Supply Voltage: 4 - 6V
Transmission Frequency: 87.7 - 88.9MHz, 106.7 - 107.9MHz (200kHz steps)
Output RF Power: 20mW
Audio Frequency: 20 - 15KHz
Separation: 40dB
Power Consumption: 30mA
Frequency Selection / Calibration
Frequency selection is very straight forward. Simply select transmission frequency at which you would like to transmit, set the combination for 4-DIP switch and BH1417 will immediately tune to that frequency. If you can't hear the transmitted audio signal on your FM receiver then re-adjust 2.5 turn variable coil until you can hear the signal. If you have a laboratory power supply you may try to vary the voltage supply from 4 to 6V. While doing that BH1417 will automatically vary the voltage for MV2109 varicap diode making sure that there's no frequency drift.
Source
There are 14 possible transmission frequencies with 200KHz increments that users can select with a 4-DIP switch. Lower band frequencies start from 88.7 up to 89.9 MHz, and upper band frequencies start from 107.7 up to 108.9 MHz.
BH1417 can be supplied with 4 - 6 voltage and consumes only around 30mA, providing 20mW output RF power. BH1417 provides 40dB channel separation which is pretty good, although older BA1404 FM Transmitter chip provides slightly better 45dB channel separation.
BH1417 is only available in SOP22 IC case so this may be an inconvenience for some folks. On the other hand, because the chip is smaller than regular DIP-based ICs it is possible to fit the entire transmitter on a small PCB.
The bad news is that BH1417 requires 7.6MHz crystal oscillator, which is very hard to find. The good news is that you can use 7.68 MHz crystal instead, which is easier to find. In fact our BH1417 transmitter prototype (schematic shown above) uses 7.68 MHz crystal. This has absolutely no effect on stereo encoding process, we have tested it and stereo sound is crystal clear. The transmitted frequency on the other hand will be shifted up by exactly 1MHz (example: 88.1 MHz to 89.1 MHz) which is perfectly fine. The frequencies that are used in this project have been adjusted by 1MHz already so no additional conversion is necessary.
BH1417 chip may also be used a stand alone stereo encoder. The advantage of that is that you have full freedom of using a transmitter & amplifier of your choice. You will still have a pre-emphasis, limiter, stereo encoder and low pass filter in one small package because very few external components are required for these blocks. PIN 5 is MPX output that can be directly connected to an external FM transmitter through a 10uF cap.
Parts List:
1x BH1417 - Stereo PLL Transmitter IC (Case SOP22) (datasheet)
1x 7.68 MHz Crystal
1x MPSA13 - NPN Darlington Transistor
1x 2.5 Turns Variable Coil
1x MV2109 - Varicap Diode
1x 4-DIP Switch
ANT - 30 cm of copper wire
1x 22K Resistor
7x 10K Resistor
1x 5.1K Resistor
2x 3.3K Resistor
1x 100 Ohm Resistor 1x 100uF Capacitor
3x 10uF Capacitor
2x 1uF Capacitor
1x 47nF Capacitor
3x 2.2nF Capacitor
1x 1nF Capacitor
1x 330pF Capacitor
2x 150pF Capacitor
1x 33pF Capacitor
2x 27pF Capacitor
1x 22pF Capacitor
2x 10pF Capacitor
Specifications:
Supply Voltage: 4 - 6V
Transmission Frequency: 87.7 - 88.9MHz, 106.7 - 107.9MHz (200kHz steps)
Output RF Power: 20mW
Audio Frequency: 20 - 15KHz
Separation: 40dB
Power Consumption: 30mA
Frequency Selection / Calibration
Frequency selection is very straight forward. Simply select transmission frequency at which you would like to transmit, set the combination for 4-DIP switch and BH1417 will immediately tune to that frequency. If you can't hear the transmitted audio signal on your FM receiver then re-adjust 2.5 turn variable coil until you can hear the signal. If you have a laboratory power supply you may try to vary the voltage supply from 4 to 6V. While doing that BH1417 will automatically vary the voltage for MV2109 varicap diode making sure that there's no frequency drift.
Source
Transistor Schmitt Trigger Oscillator
The Schmitt Trigger oscillator below employs 3 transistors, 6 resistors and a capacitor to generate a square waveform. Pulse waveforms can be generated with an additional diode and resistor (R6). Q1 and Q2 are connected with a common emitter resistor (R1) so that the conduction of one transistor causes the other to turn off. Q3 is controlled by Q2 and provides the squarewave output from the collector.
In operation, the timing capacitor charges and discharges through the feedback resistor (Rf) toward the output voltage. When the capacitor voltage rises above the base voltage at Q2, Q1 begins to conduct, causing Q2 and Q3 to turn off, and the output voltage to fall to 0. This in turn produces a lower voltage at the base of Q2 and causes the capacitor to begin discharging toward 0. When the capacitor voltages falls below the base voltage at Q2, Q1 will turn off causing Q2 and Q3 to turn on and the output to rise to near the supply voltage and the capacitor to begin charging and repeating the cycle. The switching levels are established by R2,R4 and R5. When the output is high, the voltage at the base of Q2 is determined by R4 in parallel with R5 and the combination in series with R2. When the output is low, the base voltage is set by R4 in parallel with R2 and the combination in series with R5. This assumes R3 is a small value compared to R2. The switching levels will be about 1/3 and 2/3 of the supply voltage if the three resistors are equal (R2,R4,R5).
There are many different combinations of resistor values that can be used. R3 should low enough to pull the output signal down as far as needed when the circuit is connected to a load. So if the load draws 1mA and the low voltage needed is 0.5 volts, R3 would be 0.5/.001 = 500 ohms (510 standard). When the output is high, Q3 will supply current to the load and also current through R3. If 10 mA is needed for the load and the supply voltage is 12, the transistor current will be 24 mA for R3 plus 10 mA to the load = 34 mA total. Assuming a minimum transistor gain of 20, the collector current for Q2 and base current for Q3 will be 34/20 = 1.7 mA. If the switching levels are 1/3 and 2/3 of the supply (12 volts) then the high level emitter voltage for Q1 and Q2 will be about 7 volts, so the emitter resistor (R1) will be 7/0.0017 = 3.9K standard. A lower value (1 or 2K) would also work and provide a little more base drive to Q3 than needed. The remaining resistors R2, R4, R5 can be about 10 times the value of R1, or something around 39K.
The combination of the capacitor and the feedback resistor (Rf) determines the frequency. If the switching levels are 1/3 and 2/3 of the supply, the half cycle time interval will be about 0.693*Rf*C which is similar to the 555 timer formula. The unit I assembled uses a 56K and 0.1 uF cap for a positive time interval of about 3.5 mS. An additional 22K resistor and diode were used in parallel with the 56K to reduce the negative time interval to about 1 mS.
In the diagram, T1 represents the time at which the capacitor voltage has fallen to the lower trigger potential (4 volts at the base of Q2) and caused Q1 to switch off and Q2 and Q3 to switch on. T2 represents the next event when the capacitor voltage has risen to 8 volts causing Q2 an Q3 to turn off and Q1 to conduct. T3 represents the same condition as T1 where the cycle begins to repeat. Now, if you look close on a scope, you will notice the duty cycle is not exactly 50% This is due to the small base current of Q1 which is supplied by the capacitor. As the capacitor charges, the E/B of Q1 is reverse biased and the base does not draw any current from the capacitor so the charge time is slightly longer than the discharge. This problem can be compensated for with an additional diode and resistor as shown (R6) with the diode turned around the other way.
Source
In operation, the timing capacitor charges and discharges through the feedback resistor (Rf) toward the output voltage. When the capacitor voltage rises above the base voltage at Q2, Q1 begins to conduct, causing Q2 and Q3 to turn off, and the output voltage to fall to 0. This in turn produces a lower voltage at the base of Q2 and causes the capacitor to begin discharging toward 0. When the capacitor voltages falls below the base voltage at Q2, Q1 will turn off causing Q2 and Q3 to turn on and the output to rise to near the supply voltage and the capacitor to begin charging and repeating the cycle. The switching levels are established by R2,R4 and R5. When the output is high, the voltage at the base of Q2 is determined by R4 in parallel with R5 and the combination in series with R2. When the output is low, the base voltage is set by R4 in parallel with R2 and the combination in series with R5. This assumes R3 is a small value compared to R2. The switching levels will be about 1/3 and 2/3 of the supply voltage if the three resistors are equal (R2,R4,R5).
There are many different combinations of resistor values that can be used. R3 should low enough to pull the output signal down as far as needed when the circuit is connected to a load. So if the load draws 1mA and the low voltage needed is 0.5 volts, R3 would be 0.5/.001 = 500 ohms (510 standard). When the output is high, Q3 will supply current to the load and also current through R3. If 10 mA is needed for the load and the supply voltage is 12, the transistor current will be 24 mA for R3 plus 10 mA to the load = 34 mA total. Assuming a minimum transistor gain of 20, the collector current for Q2 and base current for Q3 will be 34/20 = 1.7 mA. If the switching levels are 1/3 and 2/3 of the supply (12 volts) then the high level emitter voltage for Q1 and Q2 will be about 7 volts, so the emitter resistor (R1) will be 7/0.0017 = 3.9K standard. A lower value (1 or 2K) would also work and provide a little more base drive to Q3 than needed. The remaining resistors R2, R4, R5 can be about 10 times the value of R1, or something around 39K.
The combination of the capacitor and the feedback resistor (Rf) determines the frequency. If the switching levels are 1/3 and 2/3 of the supply, the half cycle time interval will be about 0.693*Rf*C which is similar to the 555 timer formula. The unit I assembled uses a 56K and 0.1 uF cap for a positive time interval of about 3.5 mS. An additional 22K resistor and diode were used in parallel with the 56K to reduce the negative time interval to about 1 mS.
In the diagram, T1 represents the time at which the capacitor voltage has fallen to the lower trigger potential (4 volts at the base of Q2) and caused Q1 to switch off and Q2 and Q3 to switch on. T2 represents the next event when the capacitor voltage has risen to 8 volts causing Q2 an Q3 to turn off and Q1 to conduct. T3 represents the same condition as T1 where the cycle begins to repeat. Now, if you look close on a scope, you will notice the duty cycle is not exactly 50% This is due to the small base current of Q1 which is supplied by the capacitor. As the capacitor charges, the E/B of Q1 is reverse biased and the base does not draw any current from the capacitor so the charge time is slightly longer than the discharge. This problem can be compensated for with an additional diode and resistor as shown (R6) with the diode turned around the other way.
Source
FM Transmitter with 2 Transistor
This FM ransmitter circuits may be tuned to operated over the range 88-108 MHz. Although only low power this circuit can be transmitted with a range of 20 or 30 metres. It's suitable for use as wireless microphone.
This circuit used a pair of BC548 transistors. Although not strictly RF transistors, they still give good results. Use an ECM Mic with a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug.
The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.
One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations.
Source
This circuit used a pair of BC548 transistors. Although not strictly RF transistors, they still give good results. Use an ECM Mic with a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug.
The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.
One final point, don't hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations.
Source
Tuesday, June 28, 2011
Arduino + Laser Pointer + Servos = Homemade Laser Cannon
After discovering how ridiculously easy it was to control a servo motor from the Arduino, I wondered what 2 servos and a laser pointer could do...
Long story short, I now have a computer controlled laser cannon muhahha!
The whole thing is controlled via serial commands, which let me:
I started by sending the commands by hand from the Arduino IDE, but then I wrote a Java app that let me control all the options using a GUI, which made things a lot easier. It can also record the position of the laser at given times, and play these back, allowing the laser to follow a path:
Finally, I added some physical aiming controls (two variable resistors to control X and Y). This makes it a lot easier to point it at objects, and I can use it along side the Java interface.
If you want to build your own, the Arduino PDE file is here. There's not much actual "building" required - if you have two servos, Blu Tac them together and sticky tape a laser pointer on the side. I'm using a heavy shot glass as a frim base for mine ;)
The laser pointer and servos drew too much power for my Arduino to handle on its own, so I have them hooked up to an external 4.5V supply. The servos have a separate +V and signal wire so they were easy enough to wire up, but I had to use a transistor to switch the laser pointer.
I'm not quite finished the Java app yet, but if you're interested leave a comment and I'll post what I have so far.
Long story short, I now have a computer controlled laser cannon muhahha!
The whole thing is controlled via serial commands, which let me:
- Control pitch (Y) and yaw (X)
- Turn the laser on and off
- Make the laser flash or not
- Follow a pre-programmed "sweep" pattern
- Control whether or not the laser turned off while moving and on again
I started by sending the commands by hand from the Arduino IDE, but then I wrote a Java app that let me control all the options using a GUI, which made things a lot easier. It can also record the position of the laser at given times, and play these back, allowing the laser to follow a path:Finally, I added some physical aiming controls (two variable resistors to control X and Y). This makes it a lot easier to point it at objects, and I can use it along side the Java interface.
If you want to build your own, the Arduino PDE file is here. There's not much actual "building" required - if you have two servos, Blu Tac them together and sticky tape a laser pointer on the side. I'm using a heavy shot glass as a frim base for mine ;)
The laser pointer and servos drew too much power for my Arduino to handle on its own, so I have them hooked up to an external 4.5V supply. The servos have a separate +V and signal wire so they were easy enough to wire up, but I had to use a transistor to switch the laser pointer.
I'm not quite finished the Java app yet, but if you're interested leave a comment and I'll post what I have so far.
Monday, June 27, 2011
88-108 MHz Voltage Controlled Oscilator for PLL Controller
This Circuit will explain the PLL unit and the VCO (Voltage Controlled Oscillator) which will create the FM modulated RF signal up to 400mW. the schematic to follow my function description. The main oscillator is based around the transistor Q1. This oscillator is called Colpitts oscillator and it is voltage controlled to achieve FM frequency modulation) and PLL control.
Q1 should be a HF transistor to work well, but in this case I have used a cheap and common BC817 transistor which works great. The oscillator needs a LC tank to oscillate properly. In this case the LC tank consist of L1 with the varicap D1 and the two capacitor (C4, C5) at the base-emitter of the transistor. The value of C1 will set the VCO range.
The large value of C1 the wider will the VCO range be. Since the capacitance of the varicap (D1) is dependent of the voltage over it, the capacitance will change with changed voltage. When the voltage change, so will the oscillating frequency. In this way you achieve a VCO function. You can use many different varicap diod to get it working. In my case I use a varicap (SMV1251) which has a wide range 3-55pF to secure the VCO range (88 to 108MHz).
Inside the dashed blue box you will find the audio modulation unit. This unit also include a second varicap (D2). This varicap is biased with a DC voltage about 3-4 volt DC. This varcap is also included in the LC tank by a capacitor (C2) of 3.3pF. The input audio will passes the capacitor (C15) and be added to the DC voltage. Since the input audio voltage change in amplitude, the total voltage over the varicap (D2) will also change. As an effect of this the capacitance will change and so will the LC tank frequency.
You have a Frequency Modulation of the carrier signal. The modulation depth is set by the input amplitude. The signal should be around 1Vpp. Just connect the audio to negative side of C15. Now you wonder why I don't use the first varicap (D1) to modulate the signal? I could do that if the frequency would be fixed, but in this project the frequency range is 88 to 108MHz.
If you look at the varicap curve to the left of the schematic. You can easily see that the relative capacitance change more at lower voltage than it does at higher voltage. Imagine I use an audio signal with constant amplitude. If I would modulated the (D1) varicap with this amplitude the modulation depth would differ depending on the voltage over the varicap (D1). Remember that the voltage over varicap (D1) is about 0V at 88MHz and +5V at 108MHz. By use two varicap (D1) and (D2) I get the same modulation depth from 88 to 108MHz.
Now, look at the right of the LMX2322 circuit and you find the reference frequency oscillator VCTCXO. This oscillator is based on a very accurate VCTCXO (Voltage Controlled Temperature controlled Crystal Oscillator) at 16.8MHz. Pin 1 is the calibration input. The voltage here should be 2.5 Volt. The performance of the VCTCXO crystal in this construction is so good that you do not need to make any reference tuning.
A small portion of the VCO energy is feed back to the PLL circuit through resistor (R4) and (C16). The PLL will then use the VCO frequency to regulate the tuning voltage. At pin 5 of LMX2322 you will find a PLL filter to form the (Vtune) which is the regulating voltage of the VCO. The PLL try to regulate the (Vtune) so the VCO oscillator frequency is locked to desired frequency. You will also find the TP (test Point) here.
The last part we haven't discussed is the RF power amplifier (Q2). Some energy from the VCO is taped by (C6) to the base of the (Q2). Q2 should be a RF transistor to obtain best RF amplification. To use a BC817 here will work, but not good.
The emitter resistor (R12 and R16) set the current through this transistor and with R12, R16 = 100 ohm and +9V power supply you will easy have 150mW of output power into 50 ohm load. You can lower the resistors (R12, R16) to get high power, but please don't overload this poor transistor, it will be hot and burn up… Current consumption of VCO unit = 60 mA @ 9V.
Printed Circuit Board (PCB.pdf)
This is how the real board should look when you are going to solder the components.
It is a board made for surface mounted components, so the cuppar is on the top layer.
Parts List
100 = R7, R12, R16
330 = R4
1k = R1, R2, R3, R10
3.3k = R11
10k = R5, R6, R14, R17
20k = R13
43k = R9
100k = R8, R15
3.3pF = C2, C16
15pF = C4, C6
22pF = C5
1nF = C1, C3, C8, C17, C22, C23
100nF = C7, C9, C11, C12, C13, C14, C19, C20
2.2uF = C15, C18
220uF = C10, C21
L1 = 3 turns diam 6.5mm (Everything from 6 to 7 mm will work good!)
L2, L3, L4 = 10uH
D1, D2 = SMV1251
Q1 = BC817-25
Q2 = BFG193
X1 = 16.800 MHz VCTCXO Reference oscillator
V1 = 78L05
IC1 = LMX2322
Source
Q1 should be a HF transistor to work well, but in this case I have used a cheap and common BC817 transistor which works great. The oscillator needs a LC tank to oscillate properly. In this case the LC tank consist of L1 with the varicap D1 and the two capacitor (C4, C5) at the base-emitter of the transistor. The value of C1 will set the VCO range.
The large value of C1 the wider will the VCO range be. Since the capacitance of the varicap (D1) is dependent of the voltage over it, the capacitance will change with changed voltage. When the voltage change, so will the oscillating frequency. In this way you achieve a VCO function. You can use many different varicap diod to get it working. In my case I use a varicap (SMV1251) which has a wide range 3-55pF to secure the VCO range (88 to 108MHz).Inside the dashed blue box you will find the audio modulation unit. This unit also include a second varicap (D2). This varicap is biased with a DC voltage about 3-4 volt DC. This varcap is also included in the LC tank by a capacitor (C2) of 3.3pF. The input audio will passes the capacitor (C15) and be added to the DC voltage. Since the input audio voltage change in amplitude, the total voltage over the varicap (D2) will also change. As an effect of this the capacitance will change and so will the LC tank frequency.
You have a Frequency Modulation of the carrier signal. The modulation depth is set by the input amplitude. The signal should be around 1Vpp. Just connect the audio to negative side of C15. Now you wonder why I don't use the first varicap (D1) to modulate the signal? I could do that if the frequency would be fixed, but in this project the frequency range is 88 to 108MHz.
If you look at the varicap curve to the left of the schematic. You can easily see that the relative capacitance change more at lower voltage than it does at higher voltage. Imagine I use an audio signal with constant amplitude. If I would modulated the (D1) varicap with this amplitude the modulation depth would differ depending on the voltage over the varicap (D1). Remember that the voltage over varicap (D1) is about 0V at 88MHz and +5V at 108MHz. By use two varicap (D1) and (D2) I get the same modulation depth from 88 to 108MHz.
Now, look at the right of the LMX2322 circuit and you find the reference frequency oscillator VCTCXO. This oscillator is based on a very accurate VCTCXO (Voltage Controlled Temperature controlled Crystal Oscillator) at 16.8MHz. Pin 1 is the calibration input. The voltage here should be 2.5 Volt. The performance of the VCTCXO crystal in this construction is so good that you do not need to make any reference tuning.
A small portion of the VCO energy is feed back to the PLL circuit through resistor (R4) and (C16). The PLL will then use the VCO frequency to regulate the tuning voltage. At pin 5 of LMX2322 you will find a PLL filter to form the (Vtune) which is the regulating voltage of the VCO. The PLL try to regulate the (Vtune) so the VCO oscillator frequency is locked to desired frequency. You will also find the TP (test Point) here.
The last part we haven't discussed is the RF power amplifier (Q2). Some energy from the VCO is taped by (C6) to the base of the (Q2). Q2 should be a RF transistor to obtain best RF amplification. To use a BC817 here will work, but not good.
The emitter resistor (R12 and R16) set the current through this transistor and with R12, R16 = 100 ohm and +9V power supply you will easy have 150mW of output power into 50 ohm load. You can lower the resistors (R12, R16) to get high power, but please don't overload this poor transistor, it will be hot and burn up… Current consumption of VCO unit = 60 mA @ 9V.
Printed Circuit Board (PCB.pdf)This is how the real board should look when you are going to solder the components.
It is a board made for surface mounted components, so the cuppar is on the top layer.
Parts List
100 = R7, R12, R16
330 = R4
1k = R1, R2, R3, R10
3.3k = R11
10k = R5, R6, R14, R17
20k = R13
43k = R9
100k = R8, R15
3.3pF = C2, C16
15pF = C4, C6
22pF = C5
1nF = C1, C3, C8, C17, C22, C23
100nF = C7, C9, C11, C12, C13, C14, C19, C20
2.2uF = C15, C18
220uF = C10, C21
L1 = 3 turns diam 6.5mm (Everything from 6 to 7 mm will work good!)
L2, L3, L4 = 10uH
D1, D2 = SMV1251
Q1 = BC817-25
Q2 = BFG193
X1 = 16.800 MHz VCTCXO Reference oscillator
V1 = 78L05
IC1 = LMX2322
Source
88-108 MHz PLL Controller for FM Transmitter
This circuit will explain the PLL controller unit for the FM transmitter. It is very important since the transmitter frequency is digitally controlled and thereby very stable. The heart of this unit is a PIC processor called PIC16F870, a 2 x 16 char display and four buttons.
Circuit Schematic
You will also find a 2 line 16 char display based on " HD44780-based LCD Modules" which is very common. Most LCD displays are based on this circuit.
Printed Circuit Board (PCB file for LCD controller pdf)
The connection of this PCB is matched to the 16x2 LCD display I have on my component page.
The LCD is places on the backside of this PCB (See photos). A jumper J1 is added to choose if you want strong backlight or not.
If jumper J1 is disconnected the LCD will have soft backlight because a low current will pass through R6. If jumper J1 is connected you will have strong backlight.
The component are not critial at all, the Pot (P1) can be from 1k-22k and (R1-R5) resistor can be changed to 1k-10k. The orange squars are the input from the buttons to select frequencies. The green squars are connection to the PLL at the VCO.
Custom Made Display Text
The display has 2 lines with 16 Chars. The first line of the display show "FM Radio station", next line will show the frequency. If you want, you can write your own text on the first line.
To modify the test, you press Inc 50 kHz button or Dec 50kHz button during power up. You can change the chars up/down with the two buttons. When you find the char you like you press Inc 1Mhz button to go to next char. When all 16 Char is entered, the unit restart with the new text.
All frequency register will also be reset back to factory settings 90.00 MHz. Pretty simple.
PIC16F870 Programs (INHX8M format)
The zip file contains hex file made for this project.
I have made two programs, each one are made for different crystal frequency of the PIC.
100 = R6
3.3k = R1, R2, R3, R4, R5
20k = P1
22pF = C1, C2
100nF = C5, C6
2.2uF = C3
220uF = C4, C7
X1 = 13.000 MHz
PZ = Piezo
V1 = 78L05
IC1 = PIC16F870
LCD 16x2 Char Blue type (VCO circuit)
Source
Circuit Schematic
You will also find a 2 line 16 char display based on " HD44780-based LCD Modules" which is very common. Most LCD displays are based on this circuit.
Printed Circuit Board (PCB file for LCD controller pdf)
The connection of this PCB is matched to the 16x2 LCD display I have on my component page.
The LCD is places on the backside of this PCB (See photos). A jumper J1 is added to choose if you want strong backlight or not.
If jumper J1 is disconnected the LCD will have soft backlight because a low current will pass through R6. If jumper J1 is connected you will have strong backlight.
The component are not critial at all, the Pot (P1) can be from 1k-22k and (R1-R5) resistor can be changed to 1k-10k. The orange squars are the input from the buttons to select frequencies. The green squars are connection to the PLL at the VCO.
Custom Made Display Text
The display has 2 lines with 16 Chars. The first line of the display show "FM Radio station", next line will show the frequency. If you want, you can write your own text on the first line.
To modify the test, you press Inc 50 kHz button or Dec 50kHz button during power up. You can change the chars up/down with the two buttons. When you find the char you like you press Inc 1Mhz button to go to next char. When all 16 Char is entered, the unit restart with the new text.
All frequency register will also be reset back to factory settings 90.00 MHz. Pretty simple.
PIC16F870 Programs (INHX8M format)
The zip file contains hex file made for this project.
I have made two programs, each one are made for different crystal frequency of the PIC.
- When you drive the PIC with a VCTCXO of 16.8 MHz. (PIC HEX)
- when you drive the PIC with a crystal of 2-5MHz. (PIC HEX)
- When you drive the PIC with a crystal of 13MHz.(PIC HEX)
100 = R6
3.3k = R1, R2, R3, R4, R5
20k = P1
22pF = C1, C2
100nF = C5, C6
2.2uF = C3
220uF = C4, C7
X1 = 13.000 MHz
PZ = Piezo
V1 = 78L05
IC1 = PIC16F870
LCD 16x2 Char Blue type (VCO circuit)
Source
Saturday, June 25, 2011
Arduino Based Data Logger
Well I thought it was about time I actually finished a project I started (enough to boast about online anyway). I recently bought myself an Arduino and just about every attachment under the sun, and have really been stumped for interesting projects to do with it.
I decided to build myself a makeshift data logger to sample the temperature and brightness of my bedroom during the night. Temperature was the main variable I was looking for, but I thought brightness would let me compare this to when the sun came up, etc.
Hardware used:
Arduino (www.arduino.cc)
"An Arduino is an open source electronics prototyping platform," according to their website. It consists of a circuit board with a processor and memory, some digital and analog inputs and outputs and usually a USB port. You can also get attachments that give them screens, ethernet ports, bluetooth, etc. People have done some pretty awesome things with them, from cat flaps that Twitter whenever their cats come home, to flight controllers for UAVs.
Breadboard
Pretty standard piece of electronic prototyping kit. A breadboard is a plastic board with holes in it that let you plug wires and other discrete electronic components together. I got mine from Maplin Electronics (in the UK), though you can get them at pretty any electrical retailler
Light Dependent Resistor (LDR) and Thermistor (RTD)
In case you're wondering, RTD stands for "Resistance Temperature Detector," and basically means a resistor that changes it's value based on how h ot it is. An LDR works on the same principal (except with light instead of heat). I got these off eBay, though again you can get them from anywhere
EEPROM Chips
EEPROM (Electronically Erasable Programmable Read Only Memory) chips are used to store the data once I've acquired and processed it. The chips I'm using are made by Microchip (the guys who make PIC processors), and communicate over I2C, which only uses two wires from the Arduino (saves on IO). I got mine for free 'cos im a student from Microchip Direct
Building the Circuit:
There's not a lot of point in me including a circuit diagram. If you're familiar at all with the Arduino you'll know it has a bunch of analog voltage inputs. I wired the RTD and LDR up in voltage divider style setups and plugged Vout from each into the an analog input (AN0 and AN1 in my case).
The EEPROM chips were wired up according to their datasheet. Basically this involves plugging each of the 8 pins into either 5V or 0V, except the two used for communications. No other discrete components were used.
Programming:
Now, the really fun part! The ATmega chip on the Arduino is programmed using the AVR C compiler. You can download this, and the programming environment from www.arduino.cc. Programming took me about 6-8 hours to write the program for the processor, and about 1 hour for the program running on my PC to graph the values.
Processor
Besides actually capturing the data every 15s and storing it to the EEPROMs, I had a couple of other requirements for the processor.
Graphing Software
I needed a way to show these results. I used Java, and an open source graphing package called JFreeChart. The Java program sends a serial command to the Arduino telling it to send its data, and the Arduino sends a comma separated list of values to the PC, which the Java program then interprets and displays on the graph.
The source code can be found here, though you'll need a whole bunch of JAR files, and some of the Arduino config files. The entire Eclipse workspace folder can be found here.
Results:
The graph above shows the results from leaving my data logger running overnight in my bedroom. You can see the temperature barely drops to about 23'C during the night, even with my windows open. Considering this is Scotland, it's hard to believe. No wonder I'm having trouble sleeping!
I decided to build myself a makeshift data logger to sample the temperature and brightness of my bedroom during the night. Temperature was the main variable I was looking for, but I thought brightness would let me compare this to when the sun came up, etc.
Hardware used:
- 1x Arduino Duemilanove
- 1x Prototyping breadboard
- 1x Light Dependent Resistor (LDR)
- 1x Thermistor (RTD)
- 2x Resistors
- 2x 1024Kbit EEPROM chips (Microchip 24aa1025)
- Some jumper wires
- An LED
Arduino (www.arduino.cc)
"An Arduino is an open source electronics prototyping platform," according to their website. It consists of a circuit board with a processor and memory, some digital and analog inputs and outputs and usually a USB port. You can also get attachments that give them screens, ethernet ports, bluetooth, etc. People have done some pretty awesome things with them, from cat flaps that Twitter whenever their cats come home, to flight controllers for UAVs.
Breadboard
Pretty standard piece of electronic prototyping kit. A breadboard is a plastic board with holes in it that let you plug wires and other discrete electronic components together. I got mine from Maplin Electronics (in the UK), though you can get them at pretty any electrical retailler
Light Dependent Resistor (LDR) and Thermistor (RTD)
In case you're wondering, RTD stands for "Resistance Temperature Detector," and basically means a resistor that changes it's value based on how h ot it is. An LDR works on the same principal (except with light instead of heat). I got these off eBay, though again you can get them from anywhere
EEPROM Chips
EEPROM (Electronically Erasable Programmable Read Only Memory) chips are used to store the data once I've acquired and processed it. The chips I'm using are made by Microchip (the guys who make PIC processors), and communicate over I2C, which only uses two wires from the Arduino (saves on IO). I got mine for free 'cos im a student from Microchip Direct
Building the Circuit:
There's not a lot of point in me including a circuit diagram. If you're familiar at all with the Arduino you'll know it has a bunch of analog voltage inputs. I wired the RTD and LDR up in voltage divider style setups and plugged Vout from each into the an analog input (AN0 and AN1 in my case).
The EEPROM chips were wired up according to their datasheet. Basically this involves plugging each of the 8 pins into either 5V or 0V, except the two used for communications. No other discrete components were used.
Programming:
Now, the really fun part! The ATmega chip on the Arduino is programmed using the AVR C compiler. You can download this, and the programming environment from www.arduino.cc. Programming took me about 6-8 hours to write the program for the processor, and about 1 hour for the program running on my PC to graph the values.
Processor
Besides actually capturing the data every 15s and storing it to the EEPROMs, I had a couple of other requirements for the processor.
- I had to be able to calibrate my sensors - i.e. I had to be able to stick the RTD in the fridge with a thermometer, and tell the processor what temperature it was actually at. Similarly while holding it in my hands to keep it warm. The processor then had to translate the raw analog values into actual temperature readings.
- I needed to get the data out of the EEPROMs somehow, which meant getting the processor to read the data off the chips and dump it over serial to the PC.
Graphing Software
I needed a way to show these results. I used Java, and an open source graphing package called JFreeChart. The Java program sends a serial command to the Arduino telling it to send its data, and the Arduino sends a comma separated list of values to the PC, which the Java program then interprets and displays on the graph.
The source code can be found here, though you'll need a whole bunch of JAR files, and some of the Arduino config files. The entire Eclipse workspace folder can be found here.
Results:
The graph above shows the results from leaving my data logger running overnight in my bedroom. You can see the temperature barely drops to about 23'C during the night, even with my windows open. Considering this is Scotland, it's hard to believe. No wonder I'm having trouble sleeping!Wednesday, June 22, 2011
Ring Circular Polarized Antenna for 88-108 MHz
This simple antenna called Ring Circular Polarized Antenna. You should construct this antenna with .5 inch copper. With a little experiment I did, antenna can be tuned in 88-108 MHz FM frequency range with only changing the vertical elements. The secret is a ratio of vertical and horizontal dimensions.
In FM transmitter you need signal transmitted to any direction so you need antenna type that transmitted with polarized circularly. Antenna Gain -3.2 dB, bandwith 500 KHz with maximum power handling 500 Watts. In order to use antenna on 88-108 MHz, broadband antenna type must be designed with no tune involved.
How to double Your Transmitter Power
Amplification of signal to transmit based on antenna gain in Decible. Power from transmitter be able to doubled, tripled or even become more power. The value is called ERP (Emmittion Radiating Power). For Doubled your transmitter power to be ERP Power need stacked 4 antennas (3.12 dB gain antenna), 6 antennas (5.12 dB), 8 antennas (6.4 dB).
How to stack your antenna?
See more : FM Transmitter - LCD FM Transmitter - FM Transmitter Antenna
Low Power VHF TV Transmitter
One of the most useful gadgets a video enthusiast can have is a low-power TV Transmitter. Such a device can transmit a signal from a VCR to any TV in a home or backyard. Imagine the convenience of being able to sit by the pool watching your favorite movie on a portable with a tape or laserdisc playing indoors. You could even retransmit cable TV for your own private viewing. Videotapes can be dubbed from one VCR to another without a cable connecting the two machines together.When connected to a video camera, a TV transmitter can be used in surveillance for monitoring a particular location. The main problem a video enthusiast has in obtaining a TV transmitter is that a commercial units are expensive. However, we have some good news! You can build the TV Transmitter described here for less.
Download Description and Construction Details
See more : FM Encoder - FM Transmitter - FM Stereo Transmitter
80mW FM Transmitter with Dipole Antenna
This electronic circuit is a fm transmitter circuit that suitable for beginners for stereo encoder testing. Provided the input stage is designed to accommodate line input levels i.e. approx. 2Vrms into 50kOhm. The transmitter output power is approx. 80mW, which is sufficient to cover an area of about one hundred meters.The RF portion consists of a totally screened oscillator which is operated by a stabilised voltage and a loosely coupled buffer stage. With this two-stage lay-out a good frequency stability and low harmonic interference radiation is achieved. The frequency of the oscillator can be adjusted in the commercial range (88-108MHz) by a trimmer capacitor Cl.
The FM-modulation of the transmitter is achieved by a varicap BA138. The frequency ratio can be adjusted by the capacitor C6.
To enlarge the transmission range, the output circuit which is tuned with capacitor C14, can be terminated into a full wavelength antenna.
RF filters (RFC1-3) minimize power supply hum in the transmitted signal.
Specifications:
Operating voltage: 9-12V
Operating current: 8mA
Frequency (adjustable): 88-108MHz
Inductors:
L1: 3 turns, 5mm coil diameter (air-core), 1mm copper wire with a tap-off at a wire length of 12 mm, measured from ground terminal.
L2: 2 turns, 5mm coil diameter (air core), 1.5mm copper wire.
RFC1-3: 5 turns, 6-hole ferroxcube, thin Copper wire.
Antenna construction:
The antenna consists of a full wavelength dipole configuration. The construction details are indicated in figure below.
See more : FM Stereo Encoder - FM Transmitters - Stereo Transmitter
FM Stereo Encoder for Beginner
This circuit is a simple fm stereo encoder. A method commonly used in (double side-band suppressed carrier) DSB-SC modulation to provide synchronisation between modulator and demodulator is to transmit a sinusoidal tone (pilot tone) whose frequency and phase are related to the carrier frequency. This tone is positioned at 19 kHz, outside the pass-band of the modulated signal. The carrier frequency is 38 kHz, double that of the pilot tone. The receiver circuitry detects the pilot tone and translates it to 38 kHz, which is then used to demodulated the encoded signal.
In stereo broadcasting it is necessary to transmit and receive both left (L) and right (R) audio channels while also providing the sum (L+R) to monophonic receivers. To serve both stereophonic and monophonic receivers, the (L+R) signal occupies the normal audio spectrum in the frequency range 20 Hz to 15 kHz and the (L-R) signal, also in the same frequency range, is shifted in frequency using DSB-SC modulation. The carrier frequency used in this process is 38 kHz. A typical block diagram of a FM stereo encoder is shown in figure 1 (a) and figure 1 (b) indicates the resultant composite spectrum.
In the receiver, the pilot tone is filtered out and is doubled in frequency which is then used to synchronise the demodulator to the modulator. Finally an addition and subtraction (matrixing) of the two signals yields the desired L and R audio signals.
Subtractor:
The subtracter consists of an op-amp configured as a one-to-one subtracter. The subtraction process yields the (L-R) signal which is the modulated with the carrier at a frequency of 38 kHz.
Adder:
The adder consists of an op-amp configured as a one-to-one adder. The addition process yields the (L+R) signal which is used in monophonic receivers.
Multiplier (modulator):
The multiplier consists of an analogue switch which chops the (L-R) signal at a frequency of 38 kHz.
Band-pass filter:
The band-pass filter is centred at 38 kHz and yields the desired DSB-SC signal.
Pilot tone generation:
An astable consisting of a 555 timer is set to generate a frequency of 76 kHz. This frequency is divided using two F/F's to produce 38 kHz and 19 kHz.
Carrier generation:
The carrier is generated by dividing the 76 kHz signal by two.
Low-pass filter:
The 19 kHz is passed through a low-pass filter to produce a sinusoidal pilot tone.
Mixer:
The final stage is the mixer. The mixer, by using an addition process, combines the monophonic (L+R) signal, DSB-SC (L-R) signal and pilot tone.
Specifications:
Input impedance: 47kOhm
Input level: less than 2Vrms
Output level: maximum 2Vrms into 50kOhm
Tuning and calibration:
- Adjust your FM receiver to the frequency of your transmitter.
- Select Mono reception mode.
- Adjust POT1, POT2, POT3, POT4 and POT5 to minimum.
- Connect a TAPE or CD player Left (L) channel to the encoder.
- Play a sound track.
- Adjust POT3 to 3/4 of the value.
- Adjust POT1 until the signal distorts on your monophonic receiver.
- Turn POT1 back until there is no distortion.
- Disconnect the (L) channel and connect the (R) channel.
- Adjust POT2 until the signal distorts on the monophonic receiver.
- Turn POT2 back until there is no distortion.
- Adjust C7 until the frequency output (PIN3) of the 555 is 76 kHz (here you'll need a frequency counter).
- Select Stereophonic reception mode on your receiver.
- Adjust POT5 until the pilot tone indicator switches on.
- Adjust POT4 until there is no signal on the left (L) channel and there is a signal on the right (R) channel of the receiver.
- All adjustments required are now done.
- Re-connect the left (L) channel to the encoder.
- Should the callibration be correct you will hear the sound track in stereo on you receiver.
See more : FM Encoder - FM Transmitter - FM Stereo Transmitter
