That’s because the voltage drop across the transistor allows less than the full 3 V across the LED. One thing to note about this circuit: We’re using a red LED.
So, the circuit uses only about 1/10 as much current while the LED is off. When it’s dark, the transistor is able to conduct about 15 mA through the LED. 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. The circuit diagram looks like this please ignore the messy handwriting. Here.) From this, we add on the phototransistor, which senses the presence of light, and we use its output to control the transistor, which turns the LED on. Our starting point is the simplest LED circuit: that of the LED throwie, which has an LED driven directly from a 3V lithium coin cell. The black package blocks visible light, so it is only sensitive to infrared light– it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps– it really will come on at night. The LTR-4206E is a phototransistor in a 3mm black package. As we mentioned, the last three cost about $0.30 all together, and much less in bulk. We got the LEDs and batteries on eBay, and the other parts are from Digi-Key, but Mouser has them as well. It casts a visible beam, visible for about twenty feet in a well-lit room. This LED is red, blindingly bright at 60 candela, in a 10 mm package. On the bottom (L-R): the LED, an LTR-4206E phototransistor, a 2N3904 transistor, and a 1 k resistor. Here are our components: On top: a CR2032 lithium coin cell (3 V). Adding a light-level switch like this can significantly extend their lifetime. Throwies normally can last up to two weeks. But, one fun application is to make LED throwies that turn themselves off in the daytime to save power. What can you do with such an inexpensive light-controlled LED circuit? Almost anything really. You can build it in less than five minutes or less (much less with practice). To our LED and battery we add just three components, which cost less than thirty cents altogether (and much less if you buy in bulk).
Photo transistor switch circuit how to#
In this article we show how to build a very simple– perhaps even the simplest– darkness-activated LED circuit. (Our own solar circuit collection is here.) Look here for some op-amp based photodetector circuits with LED output, and check out some of the tricks used in well-designed solar garden lights, which include gems like using the solar cell itself as the sensor. The time-honored tradition is to use a circuit with a CdS photoresistor, sometimes called a photocell or LDR, for “light-dependent resistor.” (Circuit Example 1, Example 2.) Photoresistors are reliable and cost about $1 each, but are going away because they contain cadmium, a toxic heavy metal whose use is increasingly regulated. Here’s a simple problem: “How do you make an LED turn on when it gets dark?” You might call it the “nightlight problem,” but the same sort of question comes up in a lot of familiar situations– emergency lights, street lights, silly computer keyboard backlights, and the list goes on.