FM Transmitter Circuit

Nothing critical here. To get a bit of tuning out of the coil you could put a 4-40pF trimmer capacitor (optional) parallel over the 1 μH coil, L1. C1/C4 and C5/C6 are ceramic capacitors, preferably NPO (low noise) types. C2/C3 are electrolytic or can be tantalum types. The antenna is nothing more than a piece of 12" wire or a piece of piano wire from 6" to 12".

To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit wont work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. Im sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

Parts List

R1,R3 = 100K

R2 = 10K

R4 = 470 ohm

C1,C4 = 470pF

C2,C3 = 4.7μF, 16V, electrolytic

C5,C6 = 4.7pF

C7 = 4-40pF trimmer cap (optional, see text)

L1 = 1μH

Q1,Q2 = 2N2222, NPN transistor

Mic = Electret Microphone

B1 = 9 Volt, Alkaline battery

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Fm Wireless Microphone Circuit

Easy FM wireless microphone

This FM wireless microphone is easy to build and has a large benefit of transmission (about 300 meters, while outdoor). Regardless of its small component count and 3V operating voltage that will easily penetrate the excess of some floors of an apartment development. It can be adjusted everywhere, while in the FM band (87-108MHz) and its transmissions can be picked up at any ordinary FM receiver. The coil (L1) should be about 3 mm in diameter, with five rounds of 0.61 mm copper wire. You are able Tx frequency range by simply adjusting the distance between the coils. The antenna should be half or quarter of an extended wave (100 MHz 150 cm or seventy-five centimeters).

FM wireless microphone circuit description: The audio amplifier stage (T1) is a conventional common emitter amplifier. The 47nF capacitor isolates the microphone from the base voltage to the transistor and only allows AC signals to pass. The LC tank circuit T2 occurs, the feedback capacitor C5 and the parallel LC circuit L1, C4. The coupling capacitor (C6) directs the signal to your amplifier RF (T3).

FM Wireless Microphone calibration circuit: Location of the transmitter 10 feet of a FM radio. Place the radio in a 89 to 90 MHz Spread the coil in the coil L1 frequency tuning also sought.

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Low Power FM Transmitter Schematic

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receivers mute circuit to hear this).

Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF. A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz - this is a benefit for a simple circuit like this, and even commercial FM is usually limited to a 15kHz bandwidth.

The Principle of works this application;
Q1 is the oscillator, and is a conventional design. L1 and C3 (in parallel with C2) tune the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistors collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver - usually ±75kHz.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarise. If polarised, the positive terminal must connect to the 10k resistor.

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