This article shows you how to build a very simple FM transmitter from thirteen components, a Printed Circuit Board (PCB) and a 9v battery. This project was designed to be mounted on a PCB, however you don’t have to. You could construct the project on Vero board (strip board) or any other 0.1” pitch style of project board. If you just want to experiment with this circuit, you don’t even need a board; you can just solder the component s together and let the completed project just rest on the work top.
No matter which style you choose, try to keep all component leads nice and short. You could also make the PCB much smaller than the one shown here which is approx. 3 cm square. This is a good size to keep the unit small but nicer to work on for beginners. If you wanted to make one really small, you could use all SMT parts.
Selecting the operating frequency range
The value of capacitor C5 controls the transmission frequency range.
In the UK, domestic FM radio receivers cover from around 88 – 108MHz.
The following table shows an approximate frequency range that can be expected for different values of C5.
These are only approximate as frequency is determined by the L1 and the specification of the transistors, but these ranges were observed in the prototype unit. Also note that the closer the coil windings are, the lower the frequency will be. Just slightly compressing the coil dropped the transmission frequency by over 1 MHz.
C5 Value Lower Freq. Upper Freq.
5pf 130MHz 180MHz
10pf 115MHz 152MHz
22pf 106MHz 124MHz
47pf 89MHz 97MHz
100pf 73MHz 75MHz
Again this is just a rough guide. Different makes of capacitors will give different frequencies.
I personally picked a frequency that was outside of domestic FM receives so that I wouldn’t bother anybody; and nobody else can “tune-in” by accident. However, if you don’t have a communications receiver then you will have to pick a frequency range that you can receive with your FM radio equipment.
Winding the coil
The first think to do is wind and mount the coil. The coil is simply a length of 0.6mm / 22swg copper wire wound into a coil. Take a 10cm length of bare copper wire and wind it around a suitable former; the blade of a jewelers screw driver or knitting needle is ideal.
Read More: Building Simple FM Transmitter