Re: Latching a N.C. pushbutton

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

Date: Fri, 7 Jan 2005 12:28:28 -0500

this should display better

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

"jtm" <jtm@yahoo.com> wrote in message
news:UIyDd.3638$7k5.2564@fe37.usenetserver.com...
> Hope this shows up ok:
>
> +5V
> |
> >
> < 5.6K
> >
> | R1=5K
> |----^^^---
> | |
> >| |
> |--------
> /| |
> | |
> |----^^^---
> | R2=10K
> |
> o|
> |NC Pushbutton
> o|
> |
> |
> ===
> G
>
>
> This works, I think this is how:
> When the pushbutton is closed the circuit is pulled low, the base goes
> low, and the transistor conducts. When the pushbutton is open the base
> goes high, opening the transistor, opening the circuit, even after the
> pushbutton returns to the NC position. R2 has to be about double R1.
>
> With this setup I do have a power-on problem...it trips every time. I
> tried the RC method to try to keep the base low for a period of time after
> power-up but could not get it to work. suggestions?
>
>
>
>
> "Andrew Holme" <andrew@nospam.com> wrote in message
> news:crkfbm$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.
>>
>>
>
>
>