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Project Notes

#279 FM/SingleStageTransmitterKit2

Build, tweak and tune another simple single-stage FM Transmitter Kit

Build

Notes

I picked up another FM transmitter kit because it was too cheap to ignore.

It turns out to have a typical 1-transistor circuit, similar to LEAP#208 SingleStageTransmitterKit.

Kit Details

Some details according to the product info available from the seller:

  • Frequency range: 88MHz-108MHz
  • Working voltage: 3V (with CR2032 button cell)
  • PCB materials: FR-4 glass fiber board
  • PCB specifications: 48*26mm thickness: 1.6mm

Tuning

After a quick build, the device was transmitting slightly high on the FM band (I could get some interference but not a good signal at the upper limits ~108MHz).

Taking a look at the air-coil inductor, seems it should have an inductance of around 66nH:

Measure Value
diameter (d) 3.5mm
length (l) 3mm
turns (n) 5
Inductance 65.9nH per IN3OTD’s calculator

The simplified formula:

L = (d^2 n^2) / (l + 0.45d) [μH]

gets close to the same result: 66.9nH

Assuming that’s about right, the supplied C4 value of 30pF predict a resonant frequency of 113MHz, which would explain and confirm why I can’t get anything in the standard FM range.

Around 90MHz is pretty clear for me, so I tried replacing C4 with 47pF.

This worked! I’m getting a signal at 84.15MHz, from which I infer the coil inductance might be closer to 76nH.

Performance

OK, I guess you shouldn’t expect great things from such a dinky 1-transistor circuit. At least it works!

With 30cm antenna wire, reception beyond 5m was getting a bit dodgy. Frequency stability was reasonably good; once locked in I didn’t need to re-tune much.

Here’s and SDR capture of the device broadcasting on 84.15MHz. All the peaks in the waterfall are harmonic noise from the device.

broadcast_capture

Construction

The schematic shown here has component values as per my final build, not as provided in the kit. Two details changed:

Ref Kit Value Actual Value
C4 30pF 47pF
L1 4T 5T approx 76nH

Breadboard

Schematic

Build

Build

Credits and References

About LEAP#279 RadioFMTransmitter
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.