Tal. Here's a circuit I use alot. It is called a "missing pulse detector". I
use it for anything from detecting an interruption in AC mains, to momentary
drops or interruptions in DC levels. You just have to condition the input. For
your application, an additional NPN open collector inverter on the input with
limiting resistor, and select your ~1ms timing RC and you will get a /output.
Uses a 555 timer chip.

http://www.pic101.com/data/mp4a.gif
It comes from an old Signetics databook (1972)
Formulas, if you don't have them handy can be found at:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html


Good luck - Rick

Tal wrote:

> Hi
>
> I need help or advice how to monitor the presence of 36vdc and get an alarm
> when it disappear more than 1ms.(even if it's return to normal state after
> the alarm is on)
> I have 2 source of voltage 5vdc and 36vdc that I can use. the reason is
> sometimes (I suspect) the 36v is disappearing and need to prove it for sure
> before taking any steps.
>
> Regards
>
> Tal

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