Time Delay Relay?

I need a "normally-open, timed-closed (NOTC)" relay, as described in vol_4, chpt_5, pg 3. I did find one circuit (linked below) I think would work, but I'm not sure if it works how I want. Can someone explain how the circuit below works in layman's terms?? Is it as simple as using the "+12 Volts" wire as the input trigger?

http://ourworld.compuserve.com/homepages/Bill_Bowden/page2.htm#delay.gif

The function I need is to delay a CONSTANT alarm input for a few seconds, and reset/kill the circuit when the input signal is removed. My goal is to fit 3 sirens (1 weak, 1 very loud, 1 interior "pain generator") on my car's alarm, but to keep it quiet when I arm/disarm/false trigger the alarm. Everything else I found were references to time-delay circuits that use a PULSED trigger/input (555 timer), or very complicated circuits that do not denote pulsed or constant input.


PS: Designing such a circuit for me is out of my league atm. That would be like asking most of you rebuilding an engine or designing/fabricating an entire turbocharger kit from scratch (which I've done).

When +12 volts is applied to the circuit, current flows through the 47k resistor and charges the 100 uF capacitor. The capacitor acts as the delay element. When the charge on the capacitor gets to the trigger voltage, set by the transistors, current through the transistors will activate the relay.

As stated on the web page, the delay is about 7 seconds and can be increase by increasing the capacitor value.

So, in theory, when you use a trigger the circuit by applying 12 volts, the relay will activate after about 7 seconds.

I was thinking thats how it worked, thanks! Not knowing enough about transistors, I wasn't sure, it just seemed too simple. I just need a few seconds in case I set the alarm off, while I fumble for the remote.

My next issue is a substitue for the 2n3053. I'm putting an order together for Digikey, but they don't carry it. I'm curious if a TIP31 would work instead as mentioned in the link below...oddly enough it referes to another project by the same author. The amperage specs would say its overkill, but I'm using it for another project - so I'd have extra. Otherwise I've found reference to a 2N2219A, which digikey carries too.

http://sci.tech-archive.net/Archive/sci.electronics.components/2005-02/0757.html


Sorry for being such an electronics n00b, the classes offered at my high school were rediculously basic - and long ago!

I would give the TIP31 a try if, they seem pretty similar except the much larger current and power capabilities of the TIP31.

The 2n2219a looks like it might be a good replacement as well. Most of the DC characteristics are in the same region if not slightly better that the 2n3053.

If you end up having problems then the 2n3053 is available from http://www.mouser.com for $0.76 a piece. If you are not familiar with them they are very similar to DigiKey and a very good source of parts.

Thanks for your help - midwest FTW.

I've ordered from mouser in the past, but I have several specific sensors, smd LED's, etc I'm buying from digikey.

You're welcome. Good luck with the project!

Just wanted to update this with my success. Digikey delivered my package Monday (ordered it Friday, standard shipping!), got it all together and it worked great. Its a very nice little circuit to use.

Oddly enough, I then needed a circuit to delay closing a relay, which happens to be the circuit directly below it, 2nd on the right in the picture. That too worked great. Now my electric valve stays open for .1-10+ seconds after being told to close, just the way I want. My only lasting problem, I still can't figure out the formulas they provide, some terms are far out there. I guess I'm doomed to guess and check unless I go back to school...

What formulas are you having trouble with???