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Project Notes

#280 FM/BreadboardTransmitter

Build a simple 3-stage FM transmitter on a breadboard from a design by dazaro3.

Build

Notes

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.

The LC resonance calculation implies the coil has an inducatance of 110nH.

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.

Performance

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.

Construction

Breadboard

Schematic

BreadboardTransmitter_layout

Build

Credits and References

About LEAP#280 RadioFM
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.