An interesting product (one of many) of my experiments with survival electronics

OK folks, this is probably the simplest bug or wireless microphone you have ever seen. I invented it a few years ago. It's a modified common base Colpitts oscillator that serves also as a common base AF amplifier. This simple circuit uses no resistors (the speaker/microphone serves as resistor) and works from less than 1 volt (the optimal supply voltage is around 1.5 V). Despite its simplicity, it is quite stable (if properly made).

The basic circuit:

The transistor T can be any cheap BC55x, BC54x, BC23x or BC63x (or similar NPN AF transistor, but the transmitter also works with PNP transistors, if the polarity of the power supply is reversed). Then the optimal value of the feedback capacitor C seems to be around 15 - 22 pF for the FM broadcast band (65,8 - 108 MHz). Inductance value of the coil L for the desired frequency depends mostly on the collector-base and emmiter-base capacitances of the chosen transistor (they, together with the coil's inductance, make a LC resonant circuit). The coil can have a ferrite core. Impedance of the speaker that serves as microphone must be at least 32 Ω (speaker from headphones), but higher impedance is much better (you can connect more 32 Ω​ speakers in series). With 45 - 260 Ω speakers this circuit works very well. The 1.5 volt cell (or solar panel or fruit battery) must be as near as possible to the circuit or the supply must be connected by a twisted wire pair.
I found that this circuit after a small change can also act as the simplest, one transistor superregenerative receiver (still in development) without any additional AF amplifier (the receiver uses a speaker instead of a choke coil)! Of course, it is not very loud. Please feel free to experiment with the circuit I described above and to share the results.

Some examples:

The circuit in an old and obsolete TESLA 50 Ω telephone speaker:

Transmitter with 260 Ω telephone speaker. Note the ferrite core secured by wax in the inductor:

Videos:


Test of the transmitter with the same 260 Ω​ speaker shown above:

This 45 Ω​ speaker also works very well:

Marián Fajner, july 2011.