Build, tweak and tune another simple single-stage FM Transmitter Kit
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.
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
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 92MHz, from which I infer the coil inductance might be closer to 64nH.
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.
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 64nH |
- DIY kit Simple FM wireless microphone FM transmitter board parts electronic training DIY kit radio fm microphone transmitter - from seller on aliexpress
- kit instructions
- LEAP#208 SingleStageTransmitterKit - another similar kit
- name
- Air core induction calculator
- LC resonance calculator
- Single-layer Coil Inductance and Q - IN3OTD
- S9018 NPN transistor data - parts.io
- ..as mentioned on my blog