Question about how to build a timed switch

[Igor Carajo]

Hi,

I'm not a EE or an electronics expert and I'm hoping that somebody
will be able to help me. Did you notice when you get in your car and
you put the key in the ignition, you might get a "fasten seatbelt"
light in your dashboard for several seconds and then it goes out?
Well, I need some sort of timer that will do exactly that: close a
circuit when you give it power, for 10 seconds or so, then open the
circuit. If you disconnect power from the switch, it resets and next
time you give it power, it closes the circuit again. This component
will work with 12 volts. Thanks in advance for any help.

 

[Sir Charles W. Shults III]

You can purchase time delay relays in many catalogs or from many mail order
houses. The other solution is to make a capacitor charge up on application of
power, switch on a transistor, and disable your relay (which would normally be
on when powered up).
So, the default condition when power is applied is "capacitor not charged,
transistor conducting, relay on" and as time passes and the cap charges, the
transistor is turned off. Or, you could go the digital route and build up a
counter/timer circuit, but that is way too complex for such a simple task.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip