Figure 0 - Build the 2 transistor Spy Transmitter
Build the 2 transistor Spy Transmitter

Radio frequency projects can seem more difficult than most electronics projects because most of the time you cannot build them on a solderless breadboard and there may be parts used that are not easy to source such as coils and adjustable capacitors. This project is focused towards those who have not yet attempted to build any kind of RF project, and it is laid out in such a way as to make it easy to explore the basic principles of RF circuitry and ensure a successful final product.

This simple 2 transistor audio transmitter will send the sounds picked up in a room to any FM radio tuned to the same frequency as the transmitter, somewhere between 80 and 100 Megahertz. The expected range will be at least 100 feet and could be substantially longer depending on the parts used and the quality of your final product. This circuit is based on one that has been around since the 1960s and published thousands of times, so it is tried, tested and guaranteed to work if you follow the instructions. Performance is "OK", but since this is the one of the most basic transmitter circuits possible, don't expect high quality or rock solid performance.

Figure 1 - You can salvage most of the parts needed from an old radio
Figure 1 - You can salvage most of the parts needed from an old radio

Since many of the parts only need to be "close enough", you will probably be able to salvage all that you need from any old radio, TV, or RF based circuit board. Even the two transistors used are generic, and practically any small signal NPN transistor will work here. Looking up datasheets on the various transistors pulled form old circuit boards is a great way to learn the important parameters such as VCEO (Collector Emitter Voltage), VCBO (Collector base Voltage), VEBO (Emitter Base Voltage), and IC (Continuous Collector Current). Anything close to the specs shown in the parts I am using for this project will be good enough for this transmitter. Out of the 10 or more random NPN transistors, I tried, only two of them would not transmit onto the FM radio band.

Resistors, capacitors, and even the wire needed to wind the small coil can also be salvaged from an old circuit board if you hunt around for the correct values. The only component you may have to order from a supplier will be the small 10-50 pF adjustable capacitor, but I will offer an alternative solution to the circuit that will not require the trimmer capacitor at all. Read over the entire project before heating up your soldering iron so you know what parts you may need and how the circuit can be modified to suit your own junk box inventory.

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