LiftMaster Safety Beams

I'm hoping that someone can appreciate what I'm trying to accomplish. My son and I have been discussing safety and one of the things we discussed was the safety sensors on the garage door that cause it to reverse when something is in its path. He wants me to use a spare set of sensors I purchased to help him build a science fair project to demonstrate this in class using a simple electrical circuit with a light connected say via a relay so that each time the beam is broken the light goes on or off and a buzzer will sound.
Here's the problem, I'm pretty good with electricity and wiring but I don't get the wiring for the sensors. There are only 2 screws on the transmitter and 2 screws on the receiver with nothing identifying a voltage or polarity. I also don't see how a connection is being broken or made when I was expecting to see a contact closure.
For the sake of keeping dad hero image thing going, help.
Many thanks
JE
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depending on the sensor in question for a photovoltaic cell the power is dc and polarity does not matter.
One is simply a focused light and the other is a resistor. When light falls on the reciever it goes either high or low resistance and the control circuit detects that.
You should be able to see whats happening with a voltmeter.
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I have them functioning but still not able to detect when the beam is broken. I have tried the volt/ohm meter but still not seeing a change.
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On 22 Dec 2005 16:41:42 -0800, snipped-for-privacy@hotmail.com wrote:

JE. To make this simple Imagin the space between the 2 sensors as being invisable wires. When you break the wire the ccurrent stops. This is sort of the principle on the sensors ability to detect motion or blockage. Answer to 2nd question is there is on the lift master sensors a white w/black stripe and a solid white if you can't find that look for printing on each wire and connect those together black stripe to black stripe and white to white. That makes the circut complete to make the sensors work properelly. Hope I helped anthony --anthonys door systems snipped-for-privacy@yahoo.com
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