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Wireless microphone schematic

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Simple wireless microphone

This wireless FM microphone is simple to build and it has a useful transmitbng rangę (over 300 meters in the open air). Despite its smali component count and a 3V operating yoltage it will easily penetrate over three floors of an apartment building. It may be tuned anywhere m the FM band (87-108MHz) and tts transmissions can be picked up on any standard FM receiver.

The coil (LI) should be about 3mm in diameter wlth 5 turns 0.61 mm copper wire. You can vary the Tx frequency by simply adjustmg the spacing of the coils. The antenna should be a half or quarter wavelength long (for 100 MHz 150 cm or 75 cm).

Circuit description:The audio amplificabon stage (Tl) is a standard common emitter amplifier. The 47nF capacitor isolates the microphone from the base yoltage of the transistor and only allows AC signals to pass.

The LC tank Circuit Is constructed with T2, the feedback capacitor C5 and the parallel LC Circuit LI, C4. The coupling capacitor(C6) directs the signal to the RF amplifier (T3).

Circuit calibration: Place the transmltter about 10 feet from a FM radio. Set the radio to somewhere about 89-90 MHz. Spread the winding of the LI coil apart to tune to the desired frequency.

Parts list:

•    T1,T2,T3: 2N2222 transistor

•    Ri: lOk 5%

•    R2: 33k lin.

•    R3: 12k 5%

•    R4: 5.6k 5%

•    R5: 2.2k 5%

•    R6,R8: 47k 5%

•    R7: 470 ohms 5%

•    R9: 180 ohms 5%

•    Cl,C2: 47nF

•    C3:InF

•    C4: 33pF

•    C5: 5.6pF

•    C6:8.2pF

•    C7: lOnf

•    LI: 3mm in diameter with 5 turns 0.61 mm copper wire

•    Kl: SPDT toggle switch

•    Other parts: 2 AA battery holder, Eiectret microphone, antenna wire

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