# radio shack invisible beam entry alert



## groovie ghoulie (Nov 12, 2008)

I have a old radio shack invisible beam entry alert, Cat No. 49-311. It projects a beam of infrared light, when the beam is interrupted, the sensor triggers the built-in alarm or an optional security device. It includes a single 12 volt output and has both NO and NC relay contacts. My ? is... this is only momentery while the beam is broken, I'd like to use this for a spider drop, where people would be walking and break the beam and the spiders will drop. how can I make it so it would be activated for like 5 sec as opposed to just the time the beam is broken? Hope this makes sense. Thanks in advance!


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## Otaku (Dec 3, 2004)

You have some options here. I recommend building either a 555 timer circuit that you would set for the time that you need, or a simple R/C timer. The 555 would be triggered by the NO side of the relay, and would require a separate power source (9 volt battey). The R/C timer can be triggered by the 12VDC output pulse from the IR beam sensor. This type of timing circuit can use a potentiometer to set the desired time.

For info on 555 timer circuits go here:

http://home.cogeco.ca/~rpaisley4/LM555.html

This guy knows much about using 555's.

For an easy-to-make R/C timer, go here:

http://www.goldmine-elec.com/pdf/G4567.pdf

I've used this R/C circuit in several applications. Your choice kind of depends on the prop you're controlling and how it's powered. Drop me a PM if you need any help.


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