# Electronics help needed (delay circuit) **URL:** \ **Category:** Factual Questions\ **Created:** [October 3, 2011, 4:23pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444 "2011-10-03T16:23:36Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [October 3, 2011, 4:23pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444/1 "2011-10-03T16:23:36Z")
Occasionally I build models that are illuminated, LEDs in the cockpit or whatever. The other day, as I was showing my latest rocketship off, switching the lights on and off for my friends to admire, it occurred to me that what I really wanted was a circuit controlled by a momentary switch. Tap the switch, the lights go on for 15 seconds or so, then go off on their own. Where can I find a circuit (or a circuit diagram) to do that? What is a circuit like that called? The lights are 5 LEDs, 20 mA each. I’m familiar with a soldering iron, and can attach wires to an IC, and that’s about the limit of my capability. I’d prefer something solid-state, for reliability. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [October 3, 2011, 5:51pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444/2 "2011-10-03T17:51:10Z")
What you want is a “time delay de-energize” although it can be called other things. The classic way of making one of these is to use a 555 timer. If you search for 555 timer circuits you’ll probably find quite a few. Here’s one example. See the “negative recovery” circuit here: [http://home.cogeco.ca/~rpaisley4/LM555.html#20](http://home.cogeco.ca/~rpaisley4/LM555.html#20) You’ll need to add your own LED driver to the output, which would just be a transistor and an appropriately sized dropping resistor in series with your LED. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 3, 2011, 6:13pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444/3 "2011-10-03T18:13:44Z")
You can also make this circuit with a N-FET, a capacitor and a resistor. Something like this: [http://cl.ly/0E3b0c0S2e2H1O3i1W1L](http://cl.ly/0E3b0c0S2e2H1O3i1W1L) You will need to play with the values of C1 and R1 to get the timing the way you want it. The FET is pretty non-critical, but a “logic” FET will give longer possible on-times. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 3, 2011, 6:38pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444/4 "2011-10-03T18:38:25Z")
> [@engineer\_comp\_geek](#): > > What you want is a “time delay de-energize” although it can be called other things. The classic way of making one of these is to use a 555 timer. If you search for 555 timer circuits you’ll probably find quite a few. > > Here’s one example. See the “negative recovery” circuit here: > [http://home.cogeco.ca/~rpaisley4/LM555.html#20](http://home.cogeco.ca/~rpaisley4/LM555.html#20) > > You’ll need to add your own LED driver to the output, which would just be a transistor and an appropriately sized dropping resistor in series with your LED. The 555 was the first thing that popped into my mind. But another thread sometime back pointed out that there are inexpensive micro-controllers available which would simplify this for a whole control panel. ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [October 3, 2011, 8:00pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444/5 "2011-10-03T20:00:42Z")
Thanks, those look do-able. And I think I’ve even got a 555 around somewhere, heaven knows why. 🙂 ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [October 3, 2011, 11:18pm UTC](https://boards.straightdope.com/t/electronics-help-needed-delay-circuit/598444/6 "2011-10-03T23:18:09Z")
I concur with the 555 timer or the RC circuit but if you want to get really fancy and control a bunch of LEDs in all sorts of ways, then you can’t beat the [Arduino](http://en.wikipedia.org/wiki/Arduino) route. You could do all kinds of cool things such as [controlling an RGB LED](http://www.instructables.com/id/Arduino-RGB-led-controller/) such that they change color!