Light Sensitive Alarm Project

The same technology that detects smoke can be made to detect light.

There are all kinds of applications for light-sensitive alarms, most of them security-related. A basic alarm isn't too difficult to build; all you need are some basic supplies and a small solar cell.

Basic Function

A basic light-sensitive alarm will use a photovoltaic cell to detect the light and convert it to an electrical signal. That electrical signal will trigger a relay, which will route power from a battery to an alarm.

Optical Smoke Detector Function

An optical smoke detector works by projecting an optical beam into a small chamber. When smoke enters that chamber, the optical photovoltaic sensor catches the change in intensity of the light, converts it to electricity and triggers the alarm.

Testing the Detector

Opening the smoke detector case and placing a digital multimeter on the output wires from the photo-sensor cell will enable you to test the detector. Introduce some smoke into the chamber, and read the sensor output in millivolts.

Conversion

Cut the sensor's lines, and extend them through the case. Connect the lines to the output terminals of a photovoltaic solar cell. Size the cell so that it produces the required voltage in the presence of your test light level. You may want to begin with an oversized photovoltaic cell, then adjust light sensitivity by shielding part of the cell with an opaque cover.

References

• "Fundamentals of Heat and Mass Transfer, 6th Edition"; Frank Incropera; 2006
• Electronics Manufacturers: Smoke Detectors
• Fire Fighting Enterprises: Optical Beam Smoke Detectors: Getting the Best from Beams