Build a simple 3-stage FM transmitter on a breadboard from a design by dazaro3.
The dazaro3 Youtube channel has some of the simplest and most practical FM transmitter circuits around, with a bonus that he actually shows them working. He also sells kits and finished products at www.bug-transmitter.com/.
NB: I finally bought one from dazaro3 to say “thanks” - see LEAP#281 FM/ThreeStageTransmitter for details.
A Breadboard FM Transmitter Bug even demonstrates how it is possible to build a working transmitter on a breadboard.
This project is simply a reproduction of that circuit to find out how easy it is to get working.
My first attempt was a complete flop - oscillation was very temperamental, and when it did transmit it was running far to high in the 200MHz range. Two problems:
- the first breadboard I used was quite old, and wasn’t making good contacts - especially with small ceramic caps with very thin leads
- the coil was not quite right - I think the inductance too low
Switching to a newer breadboard fixed the connection issues. Getting the coil right required a bit of trial and error.
Getting the Coil Right
I’ve learned to hate making coils, because no matter how precise the isntructions and how studiously I follow them, I always seem to end up with results that vary greatly from the original plan.
I recently got hold of a handheld frequency meter, and this made the whole process much less frustrating.
With the first L1 coil I made, the circuit was oscillating but up in the 200MHz range, indicating the inductance was too low.
After a few trials (varying the dimensions, tuns and the wire), I settled on a coil that along with a 33pF C4 produced a stable transmission at 83.35MHz. This coil is 4-turns of solid-core AWG18, length=8.5mm, diameter=5.5mm.
That’s quite a bit different than the 44nH predicted by a simplified air coil calculator. I suspect the variation may be largely due to the fact that I’m using unusually heavy AWG18 solid-core wire.
It is surprisingly good! With a 165cm AWG30 antenna connected, I can easily walk from one end of the house to the other and pickup a good signal.