Versatile Optical Trigger
Approx construction cost US$16
A Top-Tenner beginner’s project for light-controlled electrical equipment.
This is a circuit that is flexible enough to cater for many different applications. In its basic form, the Versatile Optical Trigger switches a load on or off, depending on the amount of light falling on its sensor. It can be set to respond in reasonably bright conditions or in dim light. It can be adapted to work either way round, switching on when the light gets brighter, or when it becomes dimmer.
Applications for the basic circuit include switching on a porch lamp at dusk, briefly sounding a buzzer when someone’s shadow falls on the sensor (or when the cat leaves the house by the cat door), or to switch on a lamp in a cupboard when the door is opened.
With the addition of a second circuit board, there is an optional extra stage to this project, called the Differentiator. This changes the response of the trigger. Instead of being sensitive to light level, it becomes sensitive to the rate of change of light level.
More details on this construction project can be found in the January 2001 issue of EPE Online, the world's first web-delivered electronics and computing hobbyist magazine.
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