SHOWALL FUN PRANKS HIGH VOLTAGE LAZARUS-64 PHOTOGRAPHY SPY GADGETS VIDEO GAME
Figure 8 - The basic 2 transistor Spy Transmitter schematic
Figure 8 - The basic 2 transistor Spy Transmitter schematic

The schematic for the 2 transistor Spy Transmitter is shown with the audio preamplifier stage in green and the radio frequency stage in red. As you can see, the audio preamplifier stage takes up most of the schematic real estate, and parts count. Transistor Q1 forms a simple audio amplifier that is fed directly from the output of the electret microphone. Since the electret microphone also contains its own built in amplifier, the system becomes very sensitive to any nearby sounds, often able to hear a whisper in a large room. Most of the capacitors in the audio preamplifier stage are there to stabilize the circuit. Because the transmit frequency is so dependent on voltage and loading, changes in current consumption form the amplifier would make the RF stage unstable without the capacitors to act as a buffer. You will see this as you tune up the circuit for the first time.

The radio frequency stage consists of the transistor Q2 and a "tank circuit" made from the hand wound coil (L1) and the variable capacitor (C5). The coil and capacitor form a tuned circuit, which will oscillate at a frequency somewhere in the FM radio band, dependant on the setting of the variable capacitor. Since the tuned circuit is switched by transistor Q2, which is in turn switched by transistor Q1, changes in the audio preamplifier result in modulation of the RF stage. This is about as simple as transmitters get! Commercial transmitters must include a lot more circuitry and quality components in order to achieve any kind of real stability, and as you will see when experimenting with the finished product, this unit is very prone to drifting if the voltage changes or if anything comes close to the antenna or RF stage. Our little room bug certainly works well if left undisturbed, but simplicity is exchanged for quality here.



Figure 9 - Laying out the parts inventory before the build
Figure 9 - Laying out the parts inventory before the build

Laying out all of the parts inventory right onto the printed schematic is a system I have used for many years, and makes it easy when scavenging for parts or possible replacement values. A resistor color chart will be handy unless you already have all of the color codes memorized since you can't just drop your ohm meter across any resistor that is still soldered into a circuit board and expect a valid reading. A small magnifier will help as well, since resistors and capacitors are so small on most modern boards.

As far as capacitor codes go, the numbers state the value in microfarads (uF) and include the number of places the decimal is moved to the left. I know...why not just put the actual value??!? Hey, that would be too logical and use less ink, so let's not go there! The capacitor codes will be as follows; C1 is .047uF (#473), C2 is electrolytic and will be marked as 10uF, C3 is .22uF (#224), C4 is electrolytic and will be marked as .47uF, C6 is 5pF and will be simply marked as 5. C5 is an adjustable trimmer capacitor (approximately 10pF to 50pF), and will probably have no markings at all. You may have to order one from a supplier or just take one from some old RF circuit board and hope for the best. The good news is that most of them will work anyway, as they all have very small values.

As an option to avoid having to source an adjustable capacitor for C5, you could just use a six turn coil and insert a ferrite slug into it to allow the tuning to be done by the coil instead. This will also require that you replace C5 with a 10pF fixed capacitor instead of the adjustable capacitor. Try to find the adjustable capacitor first though, as slug coil tuning is even more finicky than capacitor tuning.

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