How do I add a capacitor to a laser alarm system?

Hey, I'm trying to build a laser tripwire but can't figure out a way to put a capacitor in it so it will only stay on for a couple of seconds at the longest. I'm trying to drive a 9 volt motor so I need parts that can handle the voltage. Thank's in advance!


seandogue6 years ago
I think what you want is a preset count-down timer and a latching relay circuit.

By preset, I mean that you predetermine the amount of time you want the device to stay in the ON position until the timer expires

Basically, the detector throws a pulse when the detection is made

The pulse activates the timer, which does two things
1) turns on a relay for the motor
2) starts the count-down timer

When the time expires, the count down timer sends an interrupt to the holding circuit for the relay (A "release" signal) which turns off the latching relay
evilgenius32 (author)  seandogue6 years ago
Thanks for the idea, I was looking in to doing it that way but it seemed too complicated for me. Do you have any simpler ideas that a noob like me could do? If not no worries, I can just do a little tinkering and get something to work.
Well, now that I think about it, you could probably just get away with a 74121 with an RC that is comparable to the time you want the motor on, then apply its output to the opto relay (they require small input currents) and feed the pulse to its input or through a Schmidt trigger first.
There are smart relays that can do it all.
You still Have to provide 12 VAC power
And wires to your motor and 2 jumpers.
The relay has a timer adjust knob.
I use it in my dark room exposures.
It is pricy.