If you’re looking for an incredibly simple(4 component), no inductor no IC oscillator this is it. It works great for Audio Frequencies. It is a little unstable, a tradeoff for simplicity. RF frequencies are hard to get working, but possible.
Step 1: Step 1: Build It
Building is very strait forward. Five components generate an audio tone! (hopefully I did not mess up the schematic =])
DO NOT forget the decoupling capacitor (across the power supply).
Resistor and capacitor values are not critical the just affect frequency.
Use a pot as a variable resistor to make a variable frequency oscillator.
Step 2: Step 2: Mod's (FM Band)
With a standard 2N2222A, 2N3904 or another standard NPN transistor frequencies of up to 200MHz are capable. With a simple tweak or to an FM broadcaster can be made.
Using the same base oscillator replace the speaker with an A.F. output transformer ($2.69 at radio shack). I used a 22uF cap and 10K resistor. (or keep the speaker and just swap the capacitor and resistor for an FM Mic)
The most important part is to remove the decoupling capacitor (or it will shunt the RF).
It is quite difficult to get this thing to oscillate. The first two times I built it I used a separate lead to ground the transformer. It would not
work on the Bread Board bus. In the photo I grounded it to the bus but had to put in a decoupling capacitor with a separate lead to positive.
Once again it would not work on the Bread Board bus. (it was a 68pF cap and apparently didn't shunt the signal). I used a nice Telescopic Antenna (like from an 88MHz RC car) I got on clearance at the shack. It will broadcast at different frequencies simultaneously though.
The sound quality was surprisingly good, but the transmitter is so hard to get working. It's so finicky I'm reluctant to put it on a PCB
It would make a decent tracker because it can generate a beat frequency without input. It will however broadcast this on the entire FM spectrum.
Step 3: Specs & How It Works
The wave form is close to square (quite noisy). Duty cycle will depend on charge capacitor and emitter path resistances
without an o-scope i could not test RF.
I have yet to test the max range, but it went from the basement to the corner bedroom on the secant floor (approx 30m or 98 ft.)
and that is through two floors and two walls.
Since it takes ≈.6V to turn on a transistor it stays off until the capacitor (charging though the resistor) reaches .6V
Then the transistor turns on and the capacitor discharges though the load (speaker or transformer)
It takes less power to sustain the on state so when the transistor turns off it will have less than .6 volts.
and take time to turn on again
It is a relaxation oscillator similar to using a zener diode, neon lamp, or the emitter collector junctions on a bjt however it has a lower voltage requirement and no specialty components.
Power: 1.5-40V (collector-emitter voltage open base 2N2222A is 40 volt) theoretically lower than 1.5v is possible
Range: ≥100m (98 ft.)
Frequency: 1-200 MHz (dependent on transistor used)