Just for fun i wanted to try and make a spybug or an FM transmitter, after some googeling i found a lot of designs but they all have the same working principle which is frequency modulation: modulate a higher frequency with smaller one, in other words all those transmitters have a carrier and a modulating wave, so i looked for the simplest spybug that includes a microphone, and then i found a quite famous design from mec1995, so i got curious about that circuit, how a one transistor spybug would work? does it work well? does it have any range?
My nature of not letting stuff as they are, and trying to modify them led me to modify that circuit a little bit 😃.
I replaced C2 from the original schematic with a fixed capacitor of 15pF, and replaced C3 with 10pF because 4.7pF was too small, i also replaced the antenna (the 38cm wire) with a 40X7 mm; 40 turns spring that i stretched to 60mm.
Download Kicad project
|L||212nH||8 turns of 22 awg wire on a 6 mm drill bit|
|Q||BC548||BC548 or equivalent like 2n3094 or bc547|
|Ant||Spring||Extension spring i stretched|
|Bat||3V||3V battery with its holder|
- This circuit without the antenna will be tuned to around 89Mhz, the longer the antenna the smaller the frequency, you may need to play with it.
- All the parts i use, i salvaged them from an old fm radio 🤘 this project is perfect for recycling components.
How it works:
This spybug works by modulating a carrier frequency with a modulating wave. the carrier frequency is determined by the resonant circuit the inductor L and the capacitor C3. In order to modulate that carrier frequency there is a capacitor C2 and a transistor (which is controlled by the mic) in series with the resonant circuit.
To understand it imagine that the transistor is fully off (cut off region) then frequency is determined by LC3C2, but if the transistor is fully on (saturation region) the frequency is determined only by LC3, so the frequency can be modulated up to LC3 and down to LC3C2.
In this circuit our transistor is biased by R1,R2 and the mic so we have the half of the power supply in its base which is 1.5V, that means if the mic does not pick up any sounds the frequency of the transmitter is determined by LC3 and C2/2 because the transistor is halfway on (active region). If the mic picks up a sound (modulating wave) it varies the transistor’s bias thus the frequency of the transmitter. thats how we get modulated frequency in this circuit, i hope i explained it enough 🙏
The circuit is so simple that putting in a PCB will only complicate it, so every thing is soldered in the breeze.
The first thing i made is the coil, i calculated it to be 212 nH so to get that value i cut 19 cm of 22 AWG wire then i clean about 1cm of each end, and i wound it 8 times around a 6 mm drill-bit shaft, and i stretched it till it’s 8mm long,
I tinned the coil legs and soldered one leg to the positive pin of the battery holder.
I bent the collector and the emitter legs of the transistor towards the outside then i soldered the collector to the remaining leg of the coil.
I soldered R2:330Ω between the negative pin of the battery holder and the emitter.
I soldered the capacitor C1:1nF in parallel with the electret mic, then i added R1:4.7k to its positive terminal.
Here i soldered the remaining leg of R2:330Ω with the positive pin of the battery holder, then the base of the transistor to the positive terminal of the electert mic, after that i soldered the remaining leg of R2:330Ω to the negative terminal of the battery holder.
I made the antenna from a spring i had, i stretched it and straighten one of its ends to solder it on the collector leg of the transistor. Note that the antenna lowers the frequency of the spybug, you may wanna play with it.
Now the Spybug construction is complete only two steps left, the tuning and hot gluing the components in place.
Tuning the spybug:
To tune the spybug you just made, simply put it close to an Fm radio and start scanning for a feedback, when you hear a squealing it’s the frequency of your spybug, now to change it all you have to do is expand the coil L to increase the frequency or compress it to decrease the frequency.
With hot glue gun i squeezed a lot of glue into the coil first then between the components and i covered every thing with it.
After playing with this circuit i found it very instable and is has a short range, the frequency drifts off if you get your hands close to the antenna or touch the circuit. Unless you want to have fun, learn, experiment or improve your soldering skills there is no need to build it for real spying.