A dark detecting circuit for your jack lantern

Posted on Feb 5, 2014

Here`s an inexpensive electronic circuit that you can build to put in your Jack-o`lantern. It provides power to drive a few LEDs at night, and automatically turns them off during the daytime. It`s a simple and automatic dark-detecting circuit that you can use to for your very own photosensitive pumpkin. While the minimalist circuit is marvelously

A dark detecting circuit for your jack lantern
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compact and simple, it is limited both in terms of sensitivity, LED driving capability, and extensibility. It can drive a single red or yellow LED from a lithium coin cell but that`s it and it requires fairly bright light (e. g. , direct sunlight) to turn off the LED. So our new dark-detecting circuit is only almost as compact or simple, but is much more sensitive, and is capable of driving several bright LEDs for your Jack-o`lantern. We`ll get started with the basic circuit construction, using two LEDs for eyes, and then look at how to modify the circuit to use more or different types of LEDs. The photosensitive element is a phototransistor. We use type LTR-3208E here; it`s an infrared-sensitive type with a dark lens. Being IR sensitive it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps- it really will come on at night. The typical "saturation" current of this type of phototransistor is only of order 1 mA, which means that under conditions much less bright than sunlight, we get much less than 1 mA of output current. To increase the sensitivity of our pumpkin enough to detect indirect daylight lighting- not just direct sunlight- we use the phototransistor as one element of a Darlington pair. When daylight is detected, the phototransistor turns on (just a little bit), which turns on the first 2N3904, which pulls the base of the second 2N3904 to ground, and preventing the LEDs from turning...

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