1 Transistor FM Spybug

by amgworkshop

62K75035

Intro: 1 Transistor FM Spybug

In this instructable i will show you how you can make the simplest spybug ever!
I was looking for a project to make for fun when i found this FM transmitter,quickly i got curious, it is a simple FM transmitter but can it become even more simpler? The answer is yes! I replaced the variable capacitor with a fixed one, re-calculated the tank circuit (LC circuit) to be able to tune it only by the coil instead of the variable capacitor.

PS: this circuit design is unstable, the resonant frequency drifts off if you get your hands close to the antenna or the circuit, the length of the antenna affects the resonant frequency, but it still a fun project to make.

Download the Kicad project & high res schematic

STEP 1: Tools & Components

Tools:

  • Soldering iron and solder
  • Cutting pliers
  • A ruler
  • 6 mm drill bit
  • Hot glue gun
  • Same sand paper

Components:

  • R1 : 4.7k resistor
  • R2 : 330 ohms resistor
  • C1 : 1nF ceramic capacitor
  • C2 : 10pf ceramic capacitor
  • C3 : 15pF ceramic capacitor
  • Q : BC548 or 2N3094
  • L : 212nH (see next step)
  • Mic : Electret mic
  • Bat : 3v battery and its holder
  • Ant: Ideally a piece of wire, 1/4 or 1/2 of the wavelength of the frequency, I just used a spring that i stretched

Almost all the components i used in this project i salvaged them from an old radio, it's a good opportunity to reuse old parts and keep our planet happy :D

STEP 2: Making the Coil

I calculated the coil to be 212nH so the resonant frequency should be around 89Mhz without the antenna.

The coil is very easy to make just follow this steps:

  1. Cut 19 cm of 22 AWG wire
  2. Strip about 1 cm of each end of the wire with sand paper (only if you're using enameled wire)
  3. Grab a 6 mm drill bit & wind 8 turns around its shaft
  4. Stretch that coil until it's 8mm long
  5. Tin both ends you stripped before

STEP 3: Soldring

I didn't made a circuit board for this circuit, it will only complicate it.

The battery holder supports every thing, the coil and the transistor supports the rest, the pictures explains how the component are soldered together, but here are the steps:

  1. Solder one of the coil's leg to the positive pin of the battery holder
  2. Bend the Collector & the Emitter pins of the transistor at 90° towards the outside
  3. Solder the Collector to the remaining leg of the coil
  4. Solder the Resistor R2:330Ω between the negative pin of the battery holder and the Emitter
  5. Trim the unnecessary wires, as in the picture
  6. Solder C3:15pF in parallel with the coil L
  7. Solder C2:10pF between the Collector & the Emitter
  8. Grab your electret mic capsule and solder C1:1nF in parallel with it
  9. Bend one leg of the resistor R1:4.7k and solder it to the positive terminal of the mic, then trim it
  10. Solder the remaining leg of the resistor R1:4.7k to the positive pin of the battery holder
  11. Bend and make the mic close to the Base leg of the transistor then solder it to the positive terminal of the mic
  12. Adjust and cut the remaining leg of the resistor R2:330Ω then solder it to the negative terminal of the mic
  13. Grab your antenna and solder it between the Collector and the coil L

Now your spybug should be ready for the next step.

STEP 4: Tuning & Hot Gluing

The spybug is almost finished, all what's left to do is to tune it and hold every thing with hot glue especially the coil.

  1. Get an FM radio, an analogue one is better than a digital in this step
  2. Put your FM spybug close to the radio or to its antenna (don't touch the circuit or the antenna)
  3. Slowly turn the tuning knob of the radio until you hear a squealing, it's the feedback sound like in the video

Now you know the frequency your spybug transmits on, if you want to change it you either

  • Increase it by stretching the coil L
  • Decrease it by compressing the coil L

To hold everything tightly, start by injecting enough glue into the coil and between all the components then slowly add glue until you cover all the components.

Congrats!! you just finished making a spybug, have fun! :D

More about this project

My website

My Youtube channel

If you have any questions feel free to ask

26 Comments

MohammedA415 7 years ago

Thank you but what the rang of the transmitter

jkrutsch-1 7 years ago

do you have any recommendations for sites or further reading to learn about the math involved when it comes to transmit distance and frequency?

jkrutsch-1 7 years ago

Thank you! It's all a new vernacular for me so being told operative words to use is very helpful in my searches. I appreciate it.

deveds 7 years ago

pls who can i make simple one but with 3.5 mm jack to work on mp3
like

but in 3v

RoiK1 7 years ago

very nice, good job!

rring883 7 years ago

Thanks for posting. I thought your presentation was very understandable. Maybe you mentioned it, but what is the range of this transmitter? I like that you provided photos, diagrams, and video.

amgworkshop 7 years ago

It's a couple of meters, but if you use a 9v battery instead of 3v with 22n capacitor in parallel and a longer antenna you would multiply it's range. Thanks!

MicX1 7 years ago

Thank you for your time capsule , you bring me back to 1985 ...

amgworkshop 7 years ago

Thanks! I hope you enjoyed the ride.

amgworkshop 7 years ago

Thank you very much! i appreciate it

jwzumwalt 7 years ago

Nicely done. I plan to build one for the fun of it :)

bmaverick 7 years ago

If you can clamp down the spring end to the transmitter into a frame and the other end to a mechanical adjuster, one can adjust the frequency like one of those old Health Kit Ham radios. A spring is perfect for an FM transmitter. For improved range, a 5/8 length antenna would really help. Copper or aluminum would be best for the spring being an antenna. Steel would work alright, but not for the small transmitter. Nice to see your instructions have a copper alloy spring. Years ago, I picked up one of those spy like camera and mic ink pens. It charges the battery via USB. It will hold a total of 3 30 second movies at low 640 resolution. It's more of a gimmick device. Nice to see your work for the mic in a FM transmitter.

amgworkshop 7 years ago

I didn't know about the spring been used to tune the radio, it's really an interesting and a clever solution, it got me thinking.
Thank you!

gxwarr 7 years ago

Good job! I think I will make this!
More Comments