dadret - You may or may not like this answer, but here's how I got around it ....
For this application, anything from 5 to 15 volts would be fine. Be sure you match the voltage requirements of the relay to the supply voltage. if you use 12 volts for the circuit, use a 12 volt relay. The op-amp can be a LM741 (available at Radio Shack). If you get an LM1458 (also available at Radio Shack), it has two amps on one chip, allowing you to reduce the number of components by combining circuits on one board. You could even use an LM324 which has 4 amps on one chip. The potentiometer could be a small PC mount version. Once it is set, it can be left alone. You could use one potentiometer for all the reference voltages to all op-amps as the current draw is minimal and they all need the same reference voltage. The reference voltage just needs to be set below the incomming voltage from the traffic light controller (to trigger the comparator). If you know the output voltage of the traffic light controller, just set the reference voltage (controlled by the potentiometer) a little below this. For instance, if the output of the traffic light controller is 1.5 volts, you could adjust the potentiometer to give you a reference voltage of around 1 volt. The basics behind the comparator are this. When the voltage on the positive input goes higher than the voltage on the negative input, the output goes high (or puts out a positive voltage). Thus, when the output of the traffic light controller is low (or off), the positive input voltage is lower than the reference voltage (the negative input) and the output of the op-amp is low. When the output of the traffic light controller switches to high (or on), the voltage on the positive input of the op-amp is higher than the reference voltage on the negative input and th output of the op-amp goes high (or on). All resistors can be 1/8 or 1/4 watt unless noted.
Mark.
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dadret wrote:Sounds like to be on the safe side I should get out one of my old MRC power packs. I'm concerned about Mike R. response as I was planning on running up to 10 lights from one controller and apparently I can't do that. I really don't want to buy four more controllers. Mike - I'd like some details on how you set yours up. I can live with all of the lights changing at the same time.
Walthers provides no information in their catalog about the capacity of the controller, but if it is like most devices of this type it has very limited output. They want people to buy lots and lots of them, so they are usually designed for only one or two sets before becoming overloaded and going up in smoke. You must check the documentation that comes with it, if they provide any, to determine how many signals you can use. If nothing else, you'll know when you reach the threshhold because it will start to overheat and smell bad.
jeffrey-wimberly wrote:Use the AC output of a DC power supply. Not all MR electronics have a rectifier and connecting DC power to it could burn it out. When in doubt go with the manufacturers recommendation.
I must respectfully disagree.
Some devices use AC power (motors and lights for the most part)
Some devices use DC power (electronic circuitry, other kinds of motors, lights, LEDs)
Feeding DC into an AC motor can damage it, but feeding DC into electronics with AC inputs generally does no damage since these circuits require DC anyway and contain rectifiers. Lightbulbs don't care one way or the other BTW (except for brightness).
In short - don't feed AC into a circuit requiring DC but vice versa is usually not a problem unless motors are involved. Still it's best to be safe if you don't know for sure.
jeffrey-wimberly wrote:When in doubt go with the manufacturers recommendation.
This is sage advice.
Karl
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Something else to keep in mind ....
If I read your intent correctly, you plan on running numerous traffic lights from the one controller. The output on these controllers is about 1.9volts at a very low amperage so no dropping resistors are required to operate the LED signals. The down-side to this is the fact each output will only power 2, maybe three LEDs. Try to add any more and the entire set will either dim considerably or not even power up.
To solve this problem, I ended up using the Walthers timer strictly as a trigger to control an OpAmp which in turn controls minature relays. This way the relays can control how ever many signal sets you could possibly want .... although this way, EVERY traffic light on your layout will all change at exactly the same time.
At least by using a Walthers controller at every intersection, they may be out of sequence enough to look more realistic.
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The controller no doubt has a rectifier circuit to change the AC input to DC, so you should be able to use a DC power supply with no problem.