Re: Latching a N.C. pushbutton

From: Andrew Holme (andrew_at_nospam.com)
Date: 01/06/05 

Date: Thu, 6 Jan 2005 22:45:02 -0000

jtm wrote:
[snip]
> 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.

That's impossible.

> 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 fine.

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