Use LEDs as photodiodes
Raju R Baddi, Raman Research Institute, Bangalore, India; Edited by Martin Rowe and Fran Granville -November 18, 2010
The simple circuit in Figure 1, which can be powered with a 3.6V nickel-cadmium rechargeable battery, lets you use an LED to detect light. The circuit consumes practically no quiescent power. Two LEDs act as photodiodes to detect and respond to ambient light. When ambient light is present, the upper LED, a small, red, transparent device covered with a black pipe, has a higher effective resistance than the lower, large, green LED. The voltage drop across the input of the NAND gate is less than its threshold voltage for logic 1, making the output of the NAND gate low. When the ambient light goes off, the voltage drop across the reverse-biased green LED increases, forcing the NAND gate’s output high.
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Place the circuit so that sufficient light falls on the green sensor LED. Doing so avoids any voltage buildup near the junction that could be close to the NAND gate’s threshold voltage. The NAND gate’s power consumption rises sharply at the threshold voltage. When the gate’s input voltage is within the defined limits for the logic state, its power consumption is extremely small.