# Cicruit design question **URL:** \ **Category:** Factual Questions\ **Created:** [September 14, 2002, 2:59pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427 "2002-09-14T14:59:49Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![cmosdes](https://avatars.discourse-cdn.com/v4/letter/c/a587f6/32.png) [@cmosdes](https://boards.straightdope.com/u/cmosdes)\ **Post date:** [September 14, 2002, 2:59pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/1 "2002-09-14T14:59:49Z")
I need to design a circuit that will light up one of 4 light bulbs based on which one of 4 switches gets pressed first. The light bulbs can be small, like for a flashlight or something. I’m not talking about driving a 120W household light. This is going to be kind of like a Jeopardy thing, where there will be 4 people each holding a button or switch or whatever, and the first person to press their button should have their light turn on. Ideally, this will prevent any of the other lights from going on. I know I can do this using digital logic in the following way for a 2 contestant type situation: L1 = first light L2 = second light A = first contestant switch B = second contestant switch L1 = A\*!B\*!L2 L2 = !A_B_!L1 In other words, I’d use 4 inverters and a 3 input AND. If I needed more power than the AND gates can supply, I’d tie the output to a pass gate or something. I’m wondering if anyone has any better ideas. I’m sure there must be a better way to do this other than using, for the 4 contestant case, 4 7-input AND gates (for each AND gate I’d use two 4-input AND gates) and 8 inverters. There is a bit of a size constraint so the design should be relatively small. Thanks! ---
**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\ **Post date:** [September 14, 2002, 3:12pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/2 "2002-09-14T15:12:51Z")
Here is the circuit : [![](http://www.4qdtec.com/putz/snap.gif) ](http://www.4qdtec.com/putz/snap.gif) Here’s the description [http://www.4qdtec.com/putpr.html#snap](http://www.4qdtec.com/putpr.html#snap) Hope that helps ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [September 14, 2002, 3:16pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/3 "2002-09-14T15:16:02Z")
To do it w/o logic gates I would think a good way would be to use a bunch of relays. you press the button and energize your relay which cuts power to all buttons (through another relay) and lights your light (the relay must stay on after the current in the button switch is removed and turn off when the light circuit opens). you will most likely need some diodes too. I don’t know if this is a practical design but it should work ---
**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\ **Post date:** [September 15, 2002, 11:11pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/4 "2002-09-15T23:11:32Z")
You could also use some SR latches (e.g. the [74LS279 quad SR](http://www.national.com/pf/DM/DM54LS279.html) ); all 4 Si inputs are tied to the judge’s reset button, and each Ri input is gated with the AND of (contestant i’s switch) and (all latch outputs Qi). The Qi outputs control the lights (Qi low=\>light i on). The first contestant to hit the switch sends his Qi low, locking out all the others. This should be 1 4-input AND, 4 2-input NANDs (inverting since the latch input is actually R’) and 4 SR latches: 2 14-pin DIPs and 1 16-pin DIP. (The switches shouldn’t need to be fully debounced, because of the latches, but a low-pass filter on the input might be a good idea anyway.) ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 20, 2002, 2:09pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/5 "2002-09-20T14:09:18Z")
**Omphaloskeptic** , I’ve drawn out a schematic, and I’d email it to the address you posted in your post that isn’t here anymore, except that the address isn’t here anymore because the post isn’t. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 20, 2002, 2:27pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/6 "2002-09-20T14:27:30Z")
Never mind, I just uploaded it: [relay schematic](http://www.geocities.com/cornflakesdammit/jeopard1.jpg) If you get a “page unavailable” message, just click in your location bar and hit enter, at least that worked for me… ---
**Author:** ![cmosdes](https://avatars.discourse-cdn.com/v4/letter/c/a587f6/32.png) [@cmosdes](https://boards.straightdope.com/u/cmosdes)\ **Post date:** [September 21, 2002, 4:04pm UTC](https://boards.straightdope.com/t/cicruit-design-question/128427/7 "2002-09-21T16:04:16Z")
**cornflakes** - I can’t get to the schematic you uploaded even if I hit enter. I know there is a trick to getting to webpages there, but I don’t remember how to do it. I was the one with the e-mail address that was there and now isn’t: [billclay101@hotmail.com](mailto:billclay101@hotmail.com) Thanks again!