I created a Tech Tip some time ago on building a simple FM transmitter that could be used with any domestic FM receiver. This is a more complete article on how to go about the building this device, and it contains a PCB layout.

Build a simple FM Transmitter

This article shows you how to build a very simple FM transmitter from thirteen components (one of these you make yourself), a Printed Circuit Board (PCB) and a 9v battery.

It should be mentioned that this unit will be illegal in some countries as it transmits on the FM band anywhere from around 70 to 180MHz, however, it puts out very little RF power and shouldn’t really interfere with anything or anybody.

 
Figure 1. Circuit diagram of the complete FM Transmitter.

Below is a parts list. I’ve included suitable part numbers from both Rapid Electronics in the UK, and Digikey to make part selection a little easier.

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.


 
Figure 2. Underside of transmitters PCB. Approx 3 cm square.

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.

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 equipment.


Winding the coil.
The first think to do is wind and mount the coil. The coil is simply a length of 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 jewellers screwdriver or knitting needle is ideal.
You will need between 4 to 6 turns and you may have to experiment here. 6 turns gave my prototype a transmission frequency of around 120MHz. A coil with fewer turns should reduce the frequency.

 

Figure 3. Winding the coil.

 
Figure 4. Mounting the coil on the PCB.

Once the coil has been wound, leave it on the winding former for now so that it doesn’t get deformed whilst your mounting it. Pop each end of the coil into the correct PCB hole stretching the coil as needed so that its windings are evenly spaced. Turn over the PCB and solder in both ends of the coil.
 

Figure 5. Coil centre-tap wire

Take a 5cm length of copper wire and bent a very small hook in one end.

 
Figure 6. Mounting the coil centre-tap

The hook in the centre-tap stops the wire falling through the hole.

 
Figure 7. Soldering the centre-tap

Solder the centre-tap wire to the approximate centre position of the coil. When it’s secure, turn over the PCB and solder the wire to the track and trim off the excess wire.

Next mount the remaining components except the transistors, in any order that you feel most comfortable with.
Lastly, you need to mount the transistors Q1 & Q2, and you need to be VERY carful that you insert them the correct way. Depending on which transistors you use, you may have to bend some of the legs around each other. If you need to do this, make sure that they don’t touch each other.

 
Figure 8. PCB Layout

Next solder in the wires from the 9 volt battery clip making sure you get the positive and negative the correct way around.

When it comes time to solder on the microphone you need to be carfull. On the base of the mic there will be two solder pads. If you look closely, one of the pads should be connected to the case; this is the Negative.
If you connect the mic the wrong way around it won't work and you will probably damage it.


Figure 8a. Connections to Microphone and Mic's internal circuit diagram,


Aerial.
You don’t need anything clever in the way of aerials for this transmitter. The longer the aerial wire is, the further the transmission range will be but for testing, just connect a 25cm length.
Make sure that the other end of the aerial doesn’t come into contact with anything; that includes any part of the circuit at anything that may be earthed.
When you’re done, you should end up with something that looks like the picture in figure 9.

 
Figure 9. Completed FM Transmitter.

First tests.
Ok, now for the tricky bit. Assuming you’ve connected everything together correctly, then depending on the transistors used, tolerance of the components, characteristics of your coil and position of the trimmer capacitor, when you connect the battery you will be transmitting audio somewhere on the FM band, probably between 80MHz and 150MHz.
Place your FM transmitter near an FM radio and SLOWLY start to tune from one end of the band to the other. As you tune the radio with one hand, keep gently tapping the microphone on the transmitter with the other hand. At some point you should hopefully start to hear the tapping. When tuning you need to experiment to find the exact frequency. When you find the frequency, make a note of it and keep going a bit further. Sometimes you can find a stronger signal a little further down the dial.
With the component values specified, both my trial units popped up at around the same frequency.
 

Figure 10. FM receiver showing 119.9MHz FM

I then “crushed” the coil slightly; almost certainly one or more of the turns are now shorting together (see fig.11)
 

Figure 11. Crushing the coil slightly to lower the frequency.

If you can’t pick anything up, using a jewellers screwdriver rotate the tuning capacitor quarter of a turn and re-scan the FM band on your radio. Repeat the process again if you still can’t find a signal.
 

Figure 12. Frequency has dropped to around 110.9MHz FM.

When tuning the transmitter don't touch any part of the circuit as you will cause the output frequency to drift around.

Now the microphone used has a built in audio amplifier (see Fig. 8a) and I kid you not, it can hear an ant blowing its nose at 50 meters. If you just speak softly close up into the microphone it will probably sound distorted because you will over load the input.
These are ideal room bugs !!

 Figure 13. PCB Foil at 200% scale.

If anybody requires an electronic version of the PCB foil please drop me a line.
The PCB was designed using DipTrace PCB software and there is a free version of this product available for download that can be used to modify / print the foil.

Happy transmitting.