# Off turns ON

Discussion in 'Electronic Basics' started by Bart, Oct 6, 2007.

1. ### BartGuest

Point 1:
I wanted to build a simple device to tell if a single leg of a 240Vac
3-phase machine is dropping out momentarily. My first thought is a simple
voltage divider (1/4watt resistors in series) across any two legs, pulling a
small voltage off one of the smaller resistors, to a bridge rectifier, then
to a flip-flop. I haven't figured out the rest but my idea was if the truth
table on the flip-flop became odd (not even) momentarily it would light an
LED from an SCR triggered by the output of the flip-flop.
Point 2:
I've encountered the dilemma of turning something on (an LED) if something
else turned off (even momentarily) and never figured it out. This is for
troubleshooting purposes, like say, I wonder if a 24Vdc powersupply fails, I
want my LED to light up. Or if I lose a 5Vdc incoming voltage, I want my LED
to light up. I know I'll have to build different boards for each scenario
and power them with AA or 9V battery but I'm not sure where to start. I'll
enclose each in a small sealed box with leads coming out and alligator clips
on the end. If I suspect a 24Vdc problem, I pull that box out of my toolbox
and hook the leads up to the + and - of the 24Vdc powersupply and then wait
for the machine or whatever to fail, then I can go check my device and see
if the LED is on, proving I lost 24Vdc at least momentarily.
Any help is greatly appreciated,
Bart

2. ### John FieldsGuest

---
After you full-wave rectify the phase you're interested in, use that
signal to trigger a retriggerable one-shot with a period slightly
longer than one-half cycle. Then, if the one-shot times out, use
that signal to trigger an RS flip-flop which will light the LED and
keep it lit until you manually reset it.
---
---
Use a comparator set to trigger at whatever voltage you consider a
power supply failure to be, and then use the comparator to trigger
an RS flip-flop which will light the LED and keep it lit until you
manually reset it.

Would you like schematics?

3. ### BartGuest

Tried to email you.
Thanks very much for the help.

4. ### BartGuest

I've tried emailing you twice at concerning
your offer for schematics. Is this the correct email address? Is there some
other way of contacting you?
Regards,
Bart