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multiple power switches
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multiple power switches

by ahdavidson on Thu Jul 29, 2010 10:41 pm

I have a little project in mind to make an Arduino controller for an old traffic signal that landed in my classroom. I'd like to control the green, amber, and red lights separately, under program control. They have been wired together with a single AC power cord.

The Power Switch Relay seems ideal for that application.

My question is whether there might be any clever tricks that could be employed to use just one relay, or do I need to get one for each lamp, and make three power tails out of the big yellow housing?
Last edited by ahdavidson on Sat Jul 31, 2010 1:17 pm, edited 1 time in total.
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Re: multiple power switches

by adafruit on Thu Jul 29, 2010 11:02 pm

if you want to control them separately, you will need three relays unless there is some low-voltage relay control board inside. if there is no control board, then the 3 powerswitch tails will certainly do the job with ease

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Re: multiple power switches

by ahdavidson on Thu Jul 29, 2010 11:30 pm

Nope, nothing inside except a wiring terminal block:
IMG_0978.png
terminal block
IMG_0978.png (647.06 KiB) Viewed 3308 times


My idea is to put a RTC on the Arduino and then hang this big traffic signal in my classroom and program it to control the lights to warn the kids when there are five minutes left before the end of the period, etc. I could add a wave shield for sound effects, too.

I might even use a couple of XBees to make myself a wireless magic wand to do stuff to the lights while I'm teaching!
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Re: multiple power switches

by adafruit on Thu Jul 29, 2010 11:43 pm

ha the RTC + power switch tails is definitely the way to go :D you may want to go with the chronodot since it is more precise

but yeah, you will need a relay for each light - they are beefy 120V!

post a video when its working!

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Re: multiple power switches

by richms on Fri Jul 30, 2010 2:06 am

There was a thread about someone doing something similar for the github offices.

http://www.urbanhonking.com/ideasfordozens/2010/05/the_github_stoplight.html

Basically they did everything wrong that was possible with respect to the high voltage wiring in it.Relays just sitting inside it, no insulation because "the paint didnt conduct" etc.

If you stick with the pre made relays then you will be fine, but one thing.

If you are taking multiple 3 pin mains plugs out of the lamp, DO NOT common the neutrals together inside the lamp housing. I saw that on someones design for something similar, and it basically means that you will have voltage sitting on the other neutral pins if they are unplugged. All the ground wires to the casing (unless its a plastic traffic lamp), but each cables live and neutral to just one lampholder.

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Re: multiple power switches

by oPossum on Fri Jul 30, 2010 2:20 am

richms wrote:Basically they did everything wrong that was possible with respect to the high voltage wiring in it.Relays just sitting inside it, no insulation because "the paint didnt conduct" etc.


Well, at least they didn't use electrical tape.

Remember it's called electrical tape because it conducts electricity.
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Re: multiple power switches

by ahdavidson on Fri Jul 30, 2010 9:59 am

Thanks for the pointer to the GitHub project (the comments are the most educational part about that post) and the advice about not using a common neutral.

AC power is one of the reasons I bought three of the Power Switch Relays. I would like not to kill my students, just inspire them!
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Re: multiple power switches

by adafruit on Fri Jul 30, 2010 11:30 am

hmm yes the github project could have really benefitted from PSTs
the best thing you can do is go to the HW store and get some 3 prong plugs and wire up each one so that they work with an extension cord. then swap in the PST's and connect them to a outlet strip
one of the nice things about the PST is that it prods one in the right direction by requiring a proper plug instead of just wires :) dont forget to bolt ground to the outside of the metal case!

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Re: multiple power switches

by ahdavidson on Thu Aug 05, 2010 5:46 pm

So, let me make sure I have this right:

From each PST, run the white & black power lines to only one bulb. Don't tie all the white ones together, keep each PST's lines isolated from the others'.

Then connect all the ground lines from each PST together to the lamp case. Can that be done at one of the inside mounting screws for the terminal block that goes into the case?
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Re: multiple power switches

by adafruit on Thu Aug 05, 2010 5:57 pm

you should put plugs on each lamp, you can get wiring plugs at the hardware store for a few $.

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Re: multiple power switches

by ahdavidson on Thu Aug 05, 2010 6:14 pm

Right, I knew that's what you meant, just checking on the logic of the connections from the plugs.
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Re: multiple power switches

by adafruit on Thu Aug 05, 2010 8:57 pm

you can connect a single Grounding wire to the case. thats easy, there are these hardwarestore plugs for grounding. dont know the name of them.

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Re: multiple power switches

by Volkemon on Sun Aug 22, 2010 10:46 am

@oPossum - Funny. You did make me look, however :)

I have a traffic signal or two. (One of the few advantages of the 2004 hurricanes!)

Instead of the the Power Switch Relay (a fine product) I would like to use Relays such as these:
http://www.farnell.com/datasheets/571705.pdf

Aside from needing a transistor (or two..) on the output pin of the Arduino (relay has close to 500mA coil draw!!) and a diode across the coil to absorb reverse current flow, it looks to be an easy and economical solution. ( Less than $1 each from Newark, before shipping)

I am very familiar with household high voltages and their proper handling. The low voltage/Arduino is the 'new' area for me.

EDIT: For local sourcing, I found this also: http://www.radioshack.com/product/index.jsp?productId=2062480 Little more money, but I can have it by walking across the street....Hmmmm.....
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