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I have a retired stoplight - with real lightbulbs. I am looking for a simple way to wire it to have it automatically move RED-GREEN-YELLOW cycle just as the ones on the street. I have manually switched it but was looking for something more "automatic" that I can put together as a learning project with my 15 year old.

I saw this answer: Most Simple Stoplight circuit

But I am not sure about the power requirements for the circuts - the bulbs are actual bulbs as used and are 120V and I think 100 watt.

Ideas or suggestions?

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Oh the wonders of having questions written in American. When I saw this I thought that by stoplight you meant the brake light on a car, not making what would be called a "Traffic Light" in English. As Winston Churchill said "Two nations divided by a common language." –  Ian Mar 15 '11 at 11:58
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@Ian, I'm always surprised at the difficulty the British have with our language. ;) –  kenny Mar 15 '11 at 14:12
    
@kenny, It is a wonder that we let you have anything when you look at the way that you have broken the language that we lent you :-) –  Ian Mar 15 '11 at 15:07
    
@Ian, we followed the rules of the English language, as they are codified in [document not found]. –  Nick T Mar 15 '11 at 15:35
    
@Ian I was unaware that "American" was a language. But this does pose an interesting question: If in British English "Traffic Light" is the equivalent of American English "Stop Light," and British "Stop Light" is American "Brake Light," then is there a usage or meaning of "Brake Light" in the UK? –  JYelton Mar 18 '11 at 14:26
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3 Answers

Funnily enough, I was just making a stoplight myself.

I found a microcontroller (see: arduino for a cheap, premade dev-platform) with a triac for each light to be the cheapest, simplest way to achieve what you want.

A triac is a bit like a diode switch that is able of switching mains voltage, and when coupled with an optical isolator, can be driven (safely) by low-level logic voltage (from a microcontroller, ttl, or anything else you like). They cost about 99 cents each (plus $2.15 for a typical optoisolator) so its a very cheap solution. Here is a prototypical TRIAC control circuit, you could require additional capacitors/resistors depending on load type and the characteristics of the particular triac, so see their datasheets. The input resistor is set at the right value for the diode inside the optoisolator to receive the correct current from your logic source enter image description here

The more expensive solution is to use a solid-state relay, which will accomplish approximately the same thing for a bit more.

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thanks, sounds of interest! –  Mark Schultheiss Mar 17 '11 at 20:36
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In order to drive that load, you would need relays or power transistors.

You cannot drive relay coils directly from uC, you would need transistors to get more current (uC can supply not more than 20mA).

Another option is to use high-voltage BJT/FET transistors, but you also cannot drive them directly from uC.

So all you need is 3 power transistors or relays, and 3 low-power transistors.

This way you can switch nearly unlimited load.

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Well, since your main criteria is simplicity, I'd say use the classic method. A 1 RPM motor, a plastic shaft and rotor, some un-etched circuit board, and two wipers--one onto the center of the rotor and one off the rotor's edge. Commutate the mains onto the board with the rotor, leads come off the edge of the board to the lights. Divide the board into sections: Half of the board goes to the red light, 1/8th to the yellow, and the rest to green.

Cut the copper away from any mounting points and cover/box for safety.

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I saw a similar "toy" 12-volt implementation made exactly as you describe, except that it used an old can with tape providing the insulated parts. It impressed me when I first saw it as a kid, and I remember it to this day as one of the first "engineering" projects I ever saw. –  Mark Harrison Mar 16 '11 at 0:51
    
hmm good idea for "simple" –  Mark Schultheiss Mar 17 '11 at 20:40
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