How do garage door sensors work ?


What I need to know is how the "electronic eyes" work. Do they send a signal back to the opener ? Are they just a voltage loop that is opened when something crosses the beam ? I believe thay are powered by DC voltage.
The reason I ask is I am trying to build a timing clock for a guy and his soap box derby car, and he got a pair of sensors for us to use and timing spots to start and stop the digital stop watch, I need to build.
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John65stang wrote:

Typically it is a IR emitter and sensor set up. If beam is interupted, sensor generates signal. From this you can rig up anything you want.
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Tony Hwang wrote:

Actually, it's the reverse, at least for the sensor itself. When light energy hits the receiving sensor, it generates a voltage which is then detected. If he wants to use what he has,he can use a VOM to measure it, then rig up an apporpriate circuit. However, I'd check Radio Shack, as they may have cheap kit or already built hobby type solutions for this.
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Why don't you use a transponder setup for radio control car/boat/airplane racing? Here's one that is used for rc cars; http://www.rclapcounter.com /.
Rich http://www.garagedoorsupply.com

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Be aware that most photoelectric sensors work by pulsing the LED. They do this to get a higher brightness out of it (example: 50% dutycycle at twice the power). As such if you use it to try to measure time there will be a certain "quantizing" error in your measurements. The ones I'm familiar with worked from 60hZ to the hundreds of Hz region. Don't know if thats enough of an error to influence a soapbox derby outcome. (Can a person holding a stopwatch be more accurate than 1/60th of a second?)
THere are of course special analog sensors that are used for time critical measurements but I doubt they would be used on a garage door.

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