RF Circuits

The Tuning Coil
The coil version uses a 1/4" (4mm) diameter coil wound on a drill bit, although the PCB version has the coil fabricated on the PCB itself. A simple piece of insulated wire about 60 cm (2 feet) was fine for the antenna, and is connected to a 1-turn tapping of the coil. Use tinned copper wire for the tuning coil and not the enamelled wire for kids to build. It is much easier for them to solder the antenna, without "mashing-up" the coil, whilst trying to remove the enamel. The PCB version is ideal for the kits as there is no coil to wind, see the photographs on this page.


Since the antenna is coupled directly to the tuned circuit coil, the final frequency of the oscillator will vary if the antenna or the battery is touched. This could make this little circuit seem unstable. This is a normal feature of this type of circuit. To avoid this you will need to add an antenna buffer/amplifer but this only adds to the complexity - not exactly ideal for beginners. See this FM wireless microphone if you need a more stable circuit.

Finished Mic
For those who would like to see what a finished microphone lookes like, here is one ready built. Note that there is no pad for the microphone, the wires are to be fitted accross the 1n0 capacitor.Download PCB


Microphones
The 4K7 resistor feeds the Electret condenser microphone with DC and the internal FET amplifer will develop an AF signal accross this resistor. I originally used a 47K resistor here. It worked fine but many electret microphones require up to 1mA to operate. Reducing the value to 4K7 increases the microphone sensitivity quite a lot. If you want to use a dynamic microphone then you may omit this resistor completely although it will do no harm to leave it in circuit. Please note that the audio sensitivity of the microphone is not fantastic - i.e. you cannot use it to bug a room. It is intended for you to speak directly into the microphone as you would with any other normal microphone. Do NOT expect to hear a whisper at 5 meters (15 feet) because you won't.

The microphone sensitivity is quite low. This is normal for this type of circuit not incorporating a microphone amplifer. A high-output type microphone is therefore required. If you need a microphone amplifer then see this FM wireless microphone for a suitable circuit.

Source: Simple FM Mic