Hope this shows up ok:
This works, I think this is how:
When the pushbutton is closed the circuit is pulled low, the base goes
low, and the transistor conducts. When the pushbutton is open the base
goes high, opening the transistor, opening the circuit, even after the
pushbutton returns to the NC position. R2 has to be about double R1.
With this setup I do have a power-on problem...it trips every time. I
tried the RC method to try to keep the base low for a period of time after
power-up but could not get it to work. suggestions?
Andrew Holme said:
Since I need to latch the existing circuit I was going to take the
NOR-Latch output and drive a PNP (2N3906) transistor that I put in
series with the existing pushbutton. The NOR latches high and
'opens' the transistor.
This got me thinking. When the existing circuit goes high can't I
just drive the transitor directly from that, essentially creating a
self-latching circuit with a single transistor (emitter feeding back
to base) and a couple resistors? No external power needed.
I tried this and it seems to work. I'm not sure this is a good ideas
since when the exisitng circuit is 'closed' my transistor is all
sitting at 0 volts.When the circuit opens it goes to 5V and my
transistor only then gets 'powered up". If this is a good idea ?
I don't understand. You'll have to draw me a diagram: ascii would be