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:
|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.
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.
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|
|L1||4T||5T approx 76nH|
Credits and References
- 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
- 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