(If your LEDs are superbright types, you might want to cut down on brightness and power usage by using larger load resistors.) The photosensitive part of the circuit is independent of what’s actually driven by the second 2N3904, so you can change the load applied there easily. As drawn here, the circuit draws 0.5 – 1 mA (depending on ambient light) when it’s daytime and up to about 35 mA when driving the two LEDs at night. When it’s dark out, no current can flow through the phototransistor or the first 2N3904, which allows the base of the second 2N3904 to be pulled high through the 5 k transistor, which turns on the LEDs through that transistor. 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 on. 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. 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. Being IR sensitive it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps– it really will come on at night. We use type LTR-3208E here it’s an infrared-sensitive type with a dark lens. The photosensitive element is a phototransistor. Battery holder with 2 AA-size alkaline batteries.A phototransistor (we use type LTR-3208E).Here’s a list of parts for the basic design (with two LEDs): 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. 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. 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. While the minimalist circuit is marvelously compact and simple, it is limited both in terms of sensitivity, LED driving capability, and extensibility. In that project– basically an LED throwie with a sensor– a phototransistor controls a single LED. Our pumpkin project is closely related to the minimalist dark-detecting LED circuit that we showed previously. It’s a simple and automatic dark-detecting circuit that you can use to for your very own photosensitive pumpkin. It provides power to drive a few LEDs at night, and automatically turns them off during the daytime. Here’s an inexpensive electronic circuit that you can build to put in your Jack-o’lantern.
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