I just built the one you referenced (
http://members.shaw.ca/roma/ under
Circuits, #23,) and it works properly with a 2N2222A.
Note that the transistor is 'backwards', ie, the emitter is connected to the
1.5k from V+. Also, I had to crank up my power supply to about 8.5v before
it would start oscillating. Lower than that, it didn't oscillate.
I put the base and emitter on my scope, and it looks like the transistor is
acting like a zener, holding off the current until the voltage gets to about
8.2 volts. At that point, the base starts getting more positive. When the
base gets to .6V, the voltage shoots up briefly, and the emitter voltage
drop to 0.
My guess as to whats happening is that the emitter-base junction is leaking
charge at 8.2V, which is charging up the base. When the base-collector
junction gets forward biased (base up to .6V,) current flows, causing an
avalanche breakdown of some kind in the emitter-base junction. This quickly
discharges the capacitor, so the current drops enough to stop the breakdown
and restart the charging cycle. Its like a UJT oscillator.
I think we just set a new record for lowest parts count - three parts
not counting the LED. An electrolytic capacitor, a current limiting
resistor, and a single standard BJT transistor, not a UJT or PUT.
I just got thru clipping together one with some alli clips. I used a
BC337-25. a 470 ohm resistor, and a 330 uF capacitor. I cranked it up
to 10V, but no cigar. Not even a flicker. When I got to 12V, it
started to slowly flash, longer than 1 second per flash. When I got
to 13V, the flash rate was much faster, somewhere in the 6 to 10 Hz
range. Above that and it stopped flashing and just glowed steady. So
it apparently needs a stable voltage that is very dependent on the
transistor used. Also I found that if I touched the base, it does
weird things like stop oscillating or speeded up. Actually the flash
is more like a throb or pulse, with bright and dim phases, but no
sharply defined on or off. The LED comes on bright quickly, and
tapers off slowly to the dimmest point where it repeats the cycle. I
guess the best way to describe it is a sawtooth wave.
Another warning: do not attempt to use the transistor for its regular
purpose after using it in this circuit. Reverse biasing the E-B
junction to its avalanche mode will cause the transistor's beta to be
seriously degraded. If you do use a transistor for this, it's
probably advisable to cut off the base lead to prevent it from being
used as a regular transistor.
So there you go, two independent verifications that the circuit does
indeed work, and we have set a new low in parts count. The resistor
might be eliminated but at risk of excessive current. I think it
should be left in. Three parts is about as low as it can get with
discrete parts, even with a UJT or PUT.
--
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