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FM wireless Mike is a small electronic project based on a radio transmitter. It is a wonderful idea to make a wacky talky through which you can talk to your friend, next door. Just speak or play into the microphone and you’ll broadcast to an FM receiver at distances up to 50 feet (maybe 100 feet if the wind is right). Use a standard FM radio to receive the sound.

Circuit Operation:-

The circuit is a combination of a Microphone preamplifier and a FM transmitter.


Microphone Preamplifier is a simple audio amplifier. Transistor Q1 is operated in Voltage Divider Mode. We use a dynamic microphone in the amplifier. The small alternating current produced in the microphone is amplified and fed in to the FM transmitter.
FM transmitter is a tuned collector radio frequency oscillator. The coil and trimmer is in parallel tank circuit. We specify the value of trimmer and size of the coil such that the resonant frequency will be near 88-108 MHz (in the FM radio Band). The oscillator oscillates with a constant resonant frequency and we can change the operating frequency by changing the trimmer capacitance. This frequency is the center frequency of the FM. If we want the FM as a modulation we have to apply the modulating signal (audio signal from amplifier) to the base of the transistor of the oscillator. We all know the base-emitter junction acts like a diode. If we apply the signal the voltage across the junction will change and this particular transistor have some property of changing base capacitance according to the applied input voltage. Thus we change the capacitance which is in series with the resonant tank circuit causing a change in operating frequency. This is actually the FM modulation. Now the modulated signal will be transmitted via a small radio antenna.

 

Circuit Diagram:-

FM Wireless Mike Circuit Diagram

Component Required:-

Parts List for FM Wireless Mike

  B1 - 9.V battery, Type 2U6
  C1 - 0.05.uF, 3.V DC capacitor
  C2 - 20.uF, 3 V DC electrolytic capacitor
  C3 - 5.uF, 12 V DC electrolytic capacitor
  C4 - 47.pF, 25 V DC capacitor
  C5 - 5.30 pF trimmer capacitor
  C6 - 6.3.pF ceramic capacitor
  C7 - 0.01.uF, 10 V DC capacitor
  L1 - see pictorial detail
  MIC - Crystal or ceramic microphone element
  Q1,Q2 - npn transistor HEP - 720
  R1 - 47,000.ohm, 1/2. watt resistor
  R2 - 33,000.ohm, 1/2. watt resistor
  R3 - 1500.ohm, 1/2. watt resistor
  R4 - 3300.ohm, 1/2. watt resistor
  R5 - 100,000.ohm, 1/2. watt resistor
  R6 - 470.ohm, 1/2. watt resistor
  S1 - SPST switch
 

Testing:- Use standard RF wiring precautions and make coil L1 exactly as shown. Best speech clearly is obtained by using crystal or ceramic mike. For music reproduction, substitute a dynamic mike element.

The unit can be assembled on a cardboard using push in terminals for tie points. The case must be metal to prevent hand capacitance from continuously changing the output frequency. Pass the 6-in. solid ware antenna through the metal case using 1/4th- in. whole and a matching rubber grommet for an insulator.
 

Application:- Use the circuit to broadcast your voice to your neighborhood. Talk to your friend next door. One thing you should keep in your mind -don't tune in a frequency where a station is broad casting otherwise you may disturb other FM listeners in your neighborhood.