SHOWALL FUN PRANKS HIGH VOLTAGE LAZARUS-64 PHOTOGRAPHY SPY GADGETS VIDEO GAME
Figure 2 - The electret microphone will drive the audio preamplifier stage
Figure 2 - The electret microphone will drive the audio preamplifier stage

This transmitter can be thought of as two independent stages that work together: an audio preamplifier stage which feeds a radio frequency oscillator stage. The audio preamplifier stage consists of most of the semiconductors in the circuit, including the small electret microphone, which is basically a tiny microphone in a can with its own built in amplification circuit. Because of this built in amplifier, the electret microphone is able to drive the single transistor preamplifier circuit to a very decent level, enough to hear just about every whisper in a room.

The output from the audio preamplifier is then sent to the radio frequency stage in order to create the needed frequency modulation. To make it easy to build and test this project, the audio stage will be built first and the tested before any of the radio frequency components are added to the circuit.

Electret microphones can be salvaged from most small consumer electronic devices that record audio. Answering machines, old tape decks, dictation machines, and even kids' toys will have one inside. The electret microphone is very easy to identify - it will be the pencil eraser sized metal can with a felt pad on one side and two wires or terminals on the other. Sometimes the microphone will be wrapped in a rubber casing, which can easily be removed.



Figure 3 - The radio frequency coil is extremely easy to make
Figure 3 - The radio frequency coil is extremely easy to make

Coils are usually the most difficult part of any radio frequency circuit as most hardware hackers do not own equipment to measure and test coils. The good news is that there is only a single coil in this circuit, and it is so easy to make that it is almost impossible to do it wrong! All you need is some small enameled copper wire and a 1/4 inch diameter bolt or dowel to wind it on. This copper wire can be pulled from an old transformer, toy motor, relay, solenoid, or purchased new at most electronics supply outlets. As for the wire gage (thickness), don't worry too much about it - 1mm or somewhere near 1mm is close enough. I have built many versions of this transmitter using all kinds of varying scrap parts and it usually works. Most times, errors are due to wiring, not the parts used in the circuit.

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