FM wireless transmitter

Written by Joe Farr November 28, 2009 Hits: 2182 Favorites

5.0 (2)

I was rummaging around though some old files and found this. When I was a lad, I made hundreds of these wee beasties.

These FM transmitters can be made from very common components, will run for several days on a 9v battery and have a range of anywhere from a couple of hundred yards to over a mile if you can have a long enough aerial. We had our own school pirate radio station based on one of these.

FM Bug
Figure 1. Circuit diagram of the FM transmister

Using the component values specified it should transmit at around 100MHz to 110MHz on the FM band.

L1 is simply 5 turns of copper wire wound around the shaft of a jeweler’s screwdriver and then the 22pf capacitor soldered onto the coil around the midpoint.

The transistors can be any low power NPNs; the originals were either BC108s or ZTX300s

Lowering the value of R4 will increase the power output and hence the range, but will cause a higher drain on the battery and if the value is too low, will blow transistor Q2.

If you want it to be super sensitive i.e. a room type bug, you need to replace the microphone with a built-in FET amplifier like theses: http://www.maplin.co.uk/Module.aspx?ModuleNo=4566

You can also replace the microphone with a “beeper” circuit to make a great tracker device.

The longer the aerial, the better!!

You can construct this on either a piece of Vero (matrix board) or just point to point solder the components, but keep the component leads as short as possible. Once it all works, an old matchbox makes a great projet box. If just soldering the components together with no circuit board, drip some candle wax to hold everything in place once everything is working.

To set one up, connect it to a battery (they don’t always work very well when connected to a mains powered PSU), and place an FM radio close by. Slowly tune the radio around the top end of the FM band until you either get lots of audio feedback or complete silence; the silence means that something’s wrong with your microphone circuit and you are just picking up the carrier signal.

You can also add an additional meatier stage after Q2 if you want to experiment with additional power outputs and range.

Be warned, these devices are illegal in some countries.

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5.0

FM Wireless Transmitter

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5.0

Reviewed by Claude Haridge
December 03, 2009

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Yes, I remember building these as well when I was in my teens. The first one I made in a metal orange juice can with the antenna sticking out. I had it connected to my record player, then went walking down the road in wintertime with a FM radio pressed against my ear. I managed to get about 1/4 mile range with it. Then I miniaturized it and it didn't perform as well. Unfortunately I didn't have the background at the time to understand the poor performance other than the close proximity and layout of the components was somehow to blame. Great circuit to introduce someone to radio.

FM Xmitters

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5.0

Reviewed by Henry Davis
December 01, 2009

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Boy that takes me back a good number of years.

Good warning about using the circuit in some locations may be illegal. Kep the radiated power down below limits in teh US and you're OK - so long as you don't trash a commercial broadcaster.

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Joe Farr