Surveillance microphone the size of a matchstick
This tiny microphone the size of a matchstick is only 3mm in diameter, and is an excellent accessory for audio surveillance equipment. The microphone uses a standard-sized 3.5mm socket with a clip to allow you to connect it to your audio equipment. The audio socket does need to provide a power supply, but that’s very common for sound recorders these days.
A microphone of this size is suited to concealment in a variety of devices. Since its so small, it does attract a price tag of nearly $150, so you might want something a little cheaper (and larger) at around $40 such as this standard amplified microphone. These sorts of microphones can be connected to minidisc recorders and digital dictaphones to allow covert recording.
If you fancy a little bit of DIY, you can make your own covert microphone that’s only 6mm in diameter using these electret microphone inserts (i.e. the active component in the mics available above). The circuit diagram shows how you can connect the microphone to a battery to make it highly sensitive. You could make your own mics for about $10 using inserts such as I show above.