The Electronic Bug Detector is basically a broad-band receiver. It picks up frequencies on the FM band between 80Mhz and 150MHz. If a bug with a sensitive microphone is transmitting nearby, the result will be a feedback whistle on the detector. The frequency range of this device is limited and well under what any real surveillance equipment uses. But it is great for detecting small DIY FM transmitters such as my FM Listening Bug.
In this Instructable we will cover:
- The Schematic and some basics of how it works
- Parts needed
- Getting the PCB layout onto a prototyping board (pegboard)
- Laying out the components on the board
- Tools needed to assemble the kit
- Soldering everything together.
Step 1: Schematic
Note: The circuit will not drive a speaker.
Each stage provides high gain and the signal is picked up by an untuned front end. You can see the front end is untuned as the inductor does not have a capacitor across it. You would think the 20 turn inductor would create a short-circuit to ground. But signals at 100MHz will oscillate in the inductor and can be picked off via the 470p capacitor. Signals of a low frequency will be taken to ground. The 100MHz signal is amplified by the first two stages and the audio component is detected by the diodes on the base of the third transistor. The fourth transistor is an audio amplifier to drive the piezo diaphragm. The inductor across the piezo provides a load for the transistor and creates a high voltage during part of each cycle to increase the volume. The .022uF capacitor removes the high frequency component of the signal. The 47uF across the power supply improves performance of the circuit by preventing signals from the output stage passing to the front end. All of this will run of a small CR2032 3V button battery.
Here is the Schematic from Upverter: