Build a Tiny (Unstable) Bugging Device

We don’t know who the [amgworkshop] wanted to listen in on, but they apparently went searching for a small FM wireless transmitter. There’s plenty of circuits around, but they wanted something smaller. The original circuit had a variable capacitor to tune the output frequency. The new design uses a fixed capacitor and a spring for an antenna. You can see the build steps in the video below, but don’t expect a lot of frequency stability or fidelity out of a single transistor transmitter.

The parts list is minimal. In addition to a coin cell holder (which serves as the construction base), you need a transistor, two resistors, three capacitors, a homemade inductor (very easy to make with some wire and a drill bit), and an electret microphone. Of course, you need a battery, too. The whole thing is potted with hot glue.

If you want a better circuit (and longer battery life) you might look at [Angelo’s] similar build. Not quite as tiny, though. There’s no shortage of other examples out there, many using different construction methods.

28 thoughts on “Build a Tiny (Unstable) Bugging Device

    1. Well your largest part is always going to be the 6mm by 8mm coil. Can probably halve the size. Not sure you can go much lower than a 3v batt though, because the electrehet mic needs 1.5 v and the transistor will drop 0.7 If I’m thinking straight this morn.

      1. I remember making a very unstable FM transmitter with no microphone, uses a very fine magnet wire, and works down to 1.5v. The coil itself acts as the microphone. Best when placed on surfaces like tables, or window panes, as it amplifies the vibration from the surface.

      1. In the US it’s mostly state laws that will get you into trouble I think, plus I think a Federal statute that prohibits transporting bugging devices across state lines. IANAL tho’.

          1. I believe it’s court determined, for instance you can get a simple audio amp and headphones and sell it “for listening to wildlife” and be in the clear, but package it as “spy ears” and you’re in the crap.

          2. No, they only care about they bugs if they can’t use them too.

            More seriously, many localities allow recording of conversations as long as at least one party is aware of the recording. Most conveniently, the knowledgeable party is the allowed to be the person doing the recording.

  1. Stability of a LC oscillator is good enough for a transmitter on the fm broadcast band – the receiver will have such a wide bandwidth to cope with any drift it wouldn’t be noticeable.

        1. Analog receivers with automatic frequency control (AFC) will attempt to follow the signal within certain limits. It can be done with PLL controlled receivers too, but I’m not sure how many implement this feature since today they take for granted that all transmitters are extremely accurate.

  2. I guess one could build a stable bug by using a crystal oscillator, “pulled” around 24.576MHz via the microphone audio driving a varicap. It would then be easy to quadruple into 98.3MHz in the LC-loaded PA stage. I wonder would the modulation be strong enough? Only one way to check!

  3. I got building these decades ago when we had parrots. They would never talk around us, so we thought it might be fun to listen in when we were out of the house. They got progressively smaller and better. The last bunch were smaller than a 9V battery clip, with the exception of the coil, and the mic. They were also 3 transistor units, with an audio amp, the oscillator, and an RF amp. I used tiny pieces of perf board, but used the same holes for multiple components. As for our original use, the parrots did in fact make a lot more noise when we were not around.

  4. The almost legendary Dick Smith Electronics used to sell bug kits in Oz, if my memory serves, there were two or three different varieties. Illegal in the UK of course, so importing them back here would have been illegal. Importing the PCBs, transistors etc without assembling them however was not. I think I may still have a couple of them in my junk boxes somewhere.

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