Evil Mad Scientists have put together a nice Darkness Sensing LED Circuit that senses darkness using a phototransistor to detect light.
“When light falls on the phototransistor, it begins to conduct up to about 1.5 mA, which pulls down the voltage at the lower side of the resistor by 1.5 V, turning off the transistor, which turns off the LED. When it’s dark, the transistor is able to conduct about 15 mA through the LED. So, the circuit uses only about 1/10 as much current while the LED is off. One thing to note about this circuit: We’re using a red LED. That’s because the voltage drop across the transistor allows less than the full 3 V across the LED. The full three volts is really only marginal for driving blue LEDs anyway, so two-point-something really doesn’t cut it.”
Via: Make
Darkness Sensing LED Circuit
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So it appears to be a light sensor just wired differently.
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That is a nice, simple, useful circuit.
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Its so nice Thanks a lot
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I am looking for some help. After seeing this example I have a need to add another option to the darkness sensing circuit. The new feature is motion detection while in “Darkness”. Any ideas? The end state of my project is to detect motion in darkness and turn on a single LED or four LEDs. Once motion is nolonger present the LEDs will turn off.