Latching light detector is frugal with power and parts

Posted on Mar 6, 2013

This idea demonstrates three uses for the humble LED. The circuit in Figure 1a forms a simple light detector that latches and turns on an LED when the ambient light exceeds a preset limit determined by potentiometer P1. LED D1 is both the indicator and the light detector. All junction diodes exhibit some degree of photosensitivity. Light-emitting diodes are photovoltaic in that they generate a small voltage in response to light with suitable spectral content. Provided that they have light loads, some LEDs can generate more than 1V in adequate light conditions. The Q2-Q3 differential pair acts as a comparator.

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