Hila Research Centre
Break Beam Detector
Sound an alarm when the light falling on a photoresistor is blocked!
Voltage (+V) can vary from 4 to 9 volts. Experiment by substituting different values for resistors R1. Try different values for C1. You can use a speaker from an old set of headphones.
Use a flashlight for a light source. If you would like to try creating a beam using lenses and high intensity leds follow this link to the Hila light beam detector.
Link to Resistor Colour Codes
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Project from Hila Science Camp: http://fox.nstn.ca/~hila