Re: Latching a N.C. pushbutton

From: jtm (jtm_at_yahoo.com)
Date: 01/07/05 

Date: Fri, 7 Jan 2005 11:23:12 -0500

Hope this shows up ok:

       +5V
 |
>
 < 5.6K
>
 | R1=5K
 |----^^^---
 | |
>| |
   |--------
         /| |
 | |
 |----^^^---
 | R2=10K
 |
 o|
  |NC Pushbutton
 o|
 |
 |
       ===
 G

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" <andrew@nospam.com> wrote in message
news:crkfbm$pel$1$830fa79f@news.demon.co.uk...
> 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.
>
>