FW: [Elecraft] OT Need circuit help please

[Dan Barker]

Well, that was fun!

I don't have a lot of component values hanging around, but I got your
circuit running as a 100ms one-shot. It doesn't recover too fast (cap
discharging through a 1 MegOhm resistor). Diddle with the values to get the
250ms you need, and if you need it to "arm" more quickly between uses (power
must be off around 10 seconds with the component values I could find before
it'll run close to full duration - if you fire it a second time while the
cap is still partway full, the duration is much shorter.

Circuit.

You need a transistor (I chose a pretty silver one<g>) to pass the +12 to
the load. Power supply goes to the emitter and the load (relay) is driven by
the collector.

The base needs to be (on the junk-box transistor I chose, and most) within
0.6V of the supply to shut off the transistor. So, we set up a voltage
divider between the supply and ground. I used 44K at the top and 1 megohm to
ground. This will shut off the transistor using a 12 volt supply. Use maybe
a 33K if you're on 14 v. Play around with it. You want the voltage on the
collector near zero at your max operating voltage.

OK, now we have three components, a pass transistor and a voltage divider
holding it shut off. At initial power on, we need to sink some current
through the 44K resistor to drop the base below supply minus 0.6, so
parallel a 200 mfd electrolytic accross the 1 meg resistor. As it charges,
it pulls more current through the 44K and the voltage drop exceeds 0.6 and
the pass transistor switches on. The one-shot has an instantaneous onset,
and stays level for 100ms or so, and falls back to zero (as the cap charges)
very quickly (looked almost square on the 'scope - maybe 10-20 ms).

Only after removing the power does the cap start to discharge. So, you can
short it with a paper clip (on the bench to play more often) or if your
application doesn't need it to recover, it'll drain through the 1M after a
while.

If you need it to recover faster, you'll need more than 4 parts.

Dan / WG4S / K2 #2456

<snip>
12VDC when applied to a relay activates it for maybe 1/4 sec and then drops
out.
...
I was hoping for a RC circuit.
</snip>