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Return to BlogAutomatic Dark Detection Circuit
Posted Feb 09, 2012 at 11:54 am
Figure 1
The dark detection circuit is very simple that it was designed with only a phototransistor providing a control on a single LED which may be used for pumpkin lanterns.
The phototransistor used is an infrared-sensitive type with dark lens which will respond to sunlight and incandescent lights and not to fluorescent or discharge lamps. It only has an order of 1mA of saturation current wherein less than 1mA of current output will be taken from conditions much less bright than sunlight. A Darlington pair is being used to increase the sensitivity that is enough to detect not just direct sunlight.
The phototransistor turns partially ON upon detecting daylight. This turns ON the first 2N3904 diode and pulls the base of the second 2N3904 to ground that prevents the LEDs from turning ON. The base of the second 2N3904 will be pulled high through the 5K transistor because no current can flow through the phototransistor during dark out and this will cause the LEDs to turn ON. The current consumption varies from 0.5-1mA on daytime and approximately 35mA when the two LEDs are driven at nighttime.
Larger load resistors are used on super bright types of LEDs.
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2 years ago: I like the circuit; can you add something that would detect a person approaching the area so that a path can be lighted?