wireless FM bug

Posted on Feb 7, 2013

This project is a miniature, VHF FM (wideband) Wireless Microphone transmitter of the type that are commonly refered to as BUG's. Note that `BUGS` are illegal but `Wide-Band Frequency Modulation Wireless Microphones` (WBFMWMs) are not, as so many people have told me (including the RSGB!). Besides, the AF sensitivity of this transmitter prevents it from being an effective bug for eaves-dropping! I personally use one of these WBFMWMs plugged into my HF rig headphone socket so that I can `earwig` QSO's and nets when sitting on the toilet, washing the dishes, bringing in the coal, etc.

wireless FM bug
Click here to download the full size of the above Circuit.

I know from experience that this project can be used to stimulate interest in Radio in older children, and this was also one of the projects given to a group of scouts and girl- guides to construct. Surprisingly, I found the girls made a better job of soldering than the boys! The circuit is very simple and needs no explanation for construction although the kids needed some guidance when soldering. The coil version uses a 1/4" (4mm) diameter coil wound on a drill bit, although the PCB version has the coil fabricated on the PCB itself. A simple piece of insulated wire about 60 cm (2 feet) was fine for the antenna, and is connected to a 1-turn tapping of the coil. Use tinned copper wire for the tuning coil and not the enamelled wire for kids to build. It is much easier for them to solder the antenna, without "mashing-up" the coil, whilst trying to remove the enamel. The PCB version is ideal for the kits as there is no coil to wind, see the photographs on this page. If you wish to use the BUG (sorry!) FM Wireless Microphone from the headphone socket of an HF rig, then delete the 4K7 resistor (and reverse the 1uf capacitor). The circuit shown is for an "Electret" condenser microphone. The transmitter may be received by any VHF FM radio, but the pocket radio I use about the house was free with WEETABIX box-tops a few years ago. The unit should have a range of at least 100 meters (250 feet) but increase the "220 ohm" emitter...

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