delay off circuit? (not for psu)

**scott_fx** · 02-15-2005, 06:16 PM

Hey everyone,

I am going to be installing a rearview cam that will go on automatically when i put the car in reverse but i want to also be able to touch a momentary (qtouch) switch and have the cam turn on for 20-30 seconds. Does anyone know how to do this? I want this switch to be independent of the computer so this will have to be a hardware based solution.

Thanks,
`S

**rando** · 02-15-2005, 06:19 PM

You should be able to do that with a simple 555 timer circuit. A microcontroller is another option.

**scott_fx** · 02-15-2005, 06:27 PM

Originally Posted by rando

You should be able to do that with a simple 555 timer circuit. A microcontroller is another option.

Thats what i'm looking into right now, but trying to find what i need is a llittle confusing. I'm pretty new to building with ic's but i'm not that bad at it. Do you know of any sites that have a good schematic and write up?

**scott_fx** · 02-15-2005, 06:34 PM

i think i found it...20 seconds but it uses a 4013 flip/flop ic... still looking for a simpler 555 ic schematic
http://www.discovercircuits.com/PDF-...ffwtimeout.pdf

**mushin** · 02-15-2005, 09:15 PM

That looks pretty straightforward, I doubt you're going to find much simpler than that.

**cbergeron** · 02-15-2005, 09:41 PM

Yep, Rando's right. You should use a 555. Look up "monostable one-shot multivibrator" on google and you should get some simple circuits like that. About 8 years ago I built a simple circuit to put my power windows down using a single touch button. It was an old Camaro with power windows, but they were slow and I hated having to hold both buttons down while they slowly came down (I lived in Florida and the car had Glass T-Tops). Anyway, long story short, using a 555, and a Capacitor/Resistor combo (for timing) you can create a simple circuit to do this.

Kind regards,
CB

**cbergeron** · 02-15-2005, 09:44 PM

Ok, here's what I found via google:

http://www.uoguelph.ca/~antoon/circ/monovib.htm

You change the delay time by multiplying R1 and C1.

Kind regards,
CB

**cbergeron** · 02-15-2005, 11:24 PM

Edit:

You'd probably be wise to clean up the input supply at 5-15v by putting the vehicle power through a 7805.

Also, you'd have to add a simple transistor inline between the pin 3 output and the relay, because the 555 can't source enough current to power a decent relay.

Finally, you _might_ need to debounce the switch to prevent false triggers. I experienced that only one time though, using the circuit I described (it was a long time ago ~1997). If you power the circuit from an ACCessory line (I would assume), the worst case scenario would be a 20 (or whatever) second rear-view camera view.

Have fun!
-CB