Off Delay Circuit Needed

**pblakeney** · 11-02-2009, 07:39 PM

Hi, I am new to mp3car.com and think this is a great site. I have searched other forums for what I need and see something similar but I was not sure if what I need is quite like what is in the other forums.

I am looking to energize a relay immediatley on power and then when power is removed I want the relay to stay engaged for another 30 seconds or thereabouts.

I did see a solution that involved a NPN transistor with a cap connected to the base to ground and the trigger also connected to the base with the relay being placed on the collector or emitter. I like this, it seems simple but I was wondering if the relay would immediatley energize when power is applied at the trigger input or would it only energize once the cap has charged up?

If I understand correctly the relay will stay energized once power is removed from the trigger input because of the capacitor and will deenergize based on the cap discharge rate.

Will this circuit work, I can see if I can find the circuit and post it here if you need to see it. Or is there some other circuit that might work better. Thank you in advance!

**Bugbyte** · 11-02-2009, 09:06 PM

I'm no expert on this stuff but I *think* the cap will charge quickly when you apply power so you'll get a very quick on but it drains slowly, so you'll get a delayed off.

Post the circuit. Someone here can confirm it for you.

**pblakeney** · 11-03-2009, 01:33 AM

Thanks for your reply. The circuit that I was asking about is located at the follwing link. I also attached the same circuit in a word document.

**Bugbyte** · 11-03-2009, 11:50 AM

Oops, I edited your picture to fix it and lost the doc. Reattach if you need to.

Again, I'm no expert but I think that circuit will charge very quickly and discharge much slower. There's not too many parts in it - go ahead and build it!