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2n2222 and voltage dividers

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Ian.M:
Answer 3:  Use two transistors* and apply positive feedback to make a Schmitt trigger.

Here's one of many ways of doing that:


LTspice sim attached.

* As a transistor inverts in a common emitter configuration, and has less than unity voltage gain as an emitter follower (common collector) and less than unity current gain in a common base configuration, you cant apply DC coupled positive feedback to a single transistor.

Tom18991:
Thanks both, very helpful. Ian, I was playing with pairs of transistors to bump the gain and make the on/off sharper, but will try your approach without delay :)  Is the value of R3 in your circuit tied to the value of R2, or is that just a coincidence?   Also, given R3 being there, would I be right to think this combo inverts the output?

I also tried using a 74xx gate to sharpen the on/off, but that requires a new voltage regulator, etc., so would rather avoid...

Ian.M:
  Coincidence.  R1, R2 were copied from your original circuit - I'd probably use higher values, except if it made choosing R5 difficult.

R3 was chosen to provide sufficient base current to saturate Q2 with any reasonable load you'd use it for.

It depends on your definition of inverts.   Voltage-wise, its non-inverting: Input high=LED OFF, same as direct connection to the LED cathode.

One further note - R5 permits a few uA to flow through the LED when its meant to be off.  This might *just* be noticeable with a high efficiency LED in a very dark room.

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