Micro FM Transmitter

posted in: Electrical Engineering | 0

Wouldn’t it be cool to transmit your voice or other sounds over the FM airwaves to a nearby radio? Building the circuit from this micro FM transmitter schematic will allow you to hide it almost anywhere and will broadcast it’s signal over 40 feet. You can tune the transmitter in the range of 91 and 97 MHz by adjusting the coil L using a tiny, non-metal screw driver. Clockwise will raise the frequency and counterclockwise will of course lower it.

Transistors, as you may know, can be used as either a switch to turn a signal on or off, based on a third input, or as an amplifier. In this circuit, we’ll actually need to have transistors that perform both of these tasks. Transistors are incredibly powerful tools in our electronic toolbox and are worth serious study to learn about their intricacies and properties.

The first transistor is a very high frequency oscillator, which will oscillate between 91-97 MHz. The microphone will apply varying pulses to the base of the transistor’s base, affecting the oscillating frequency. We call this frequency modulation or FM for short. The stronger the signal from the microphone, the more the variation of the frequency. This is how sounds are transmitted from the micro FM transmitter to the radio, through these pulses. The pulses then drive the speaker on the other end in the receiving radio.

The second transistor, TR2, is used as an amplifier. It amplifies the signal from the oscillator/modulator, putting it out to the antenna. This signal travels over the air in the form of radio waves, a type of electromagnetic radiation. Electromagnetic radiation is all around us, even coming from space. Visible light is a form of electromagnetic radiation as well.

If you build this micro FM transmitter, take a picture and let me know in the comments below! Happy transmitting.

Micro FM Transmitter

Micro FM Transmitter
Micro FM transmitter schematic

Parts list:

Capacitors:

2 – 10µ
2 – 4p
1 – 10p
1 – 0.01µ
1 – 100p

Resistors:

1 – 180
2 – 33k
1 – 100
1 – 10k
1 – 1.2k

Transistors:

2 – 2N2222

Misc:

1 – L2474/10 coil

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