In a previous post I’ve wrote about a stable and simple miniature FM transmitter circuit, anyway it was a good one.
Now another circuit using the C9018 high frequency transistor, based on a different spin of the common base Collpit’s oscillator.
Single transistor FM transmitter circuit diagram
The circuit is rather simple, uses only one transistor and few passive components.
Here the RF feedback is taken from the centre tap of C2 and C3, close to original Collpit’s oscillator.
You can calculate the effective capacitance of the tuned LC circuit by determining the series capacitance of C2 and C3.
If both C2 and C3 are equal, the effective capacitance will be the half.
Part list and construction
Here’s the component list,
- Q1 – C9018 NPN transistor
- R1 – 470 Ohm, 1/4 watt
- R2 – 10 kOhm, 1/4 watt
- C1 – 2.2 nF ceramic disc capacitor (222)
- C2 – 30 pF ceramic disc capacitor, (30)
- C3 – 30 pF ceramic disc capacitor, (30)
- C4 – 10 pF 30 pF ceramic disc capacitor, (10)
- C5 – 1 nF 30 pF ceramic disc capacitor, (102)
- C6 – 100nF 30 pF ceramic disc capacitor, (104)
- L1 – 16 turn on 3 mm diameter
This circuit uses only 10 parts including the coil.
The coil is winded on a 3 mm drill bit, 16 turns of 0.4 mm (28 S.W.G) enamelled copper wire.
It’s very easy to construct the circuit on a strip board, just few components to solder. Always try to minimise the track length.
My prototype without the audio source connected, small enough to fit on a matchbox. Fix the inductor with some wax for better frequency stability.
This circuit runs on a 3.3 Volt regulated power supply.
Testing the transmitter
This FM transmitter circuit performed well in terms of frequency stability, almost zero drifting after about 4 hours of continuous operation.
But still digital FM receivers can’t tune to this transmitter, as it’s not a PLL based design.
There’s no antenna connected to the transmitter, but you can easily receive clear signals from 10-12 meter distance. After that, the signals starts fading rapidly.
So, that’s all for this FM transmitter project, if you have any suggestion or question, just drop a comment.