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Every day, lots and lots of electricity and energy is wasted from people accidentally leaving electronics on, or flashlights. This invention is the perfect solution to that problem. The flashlight only turns on when it senses that the room is dark enough to need a light. It reduces energy and electricity waste, and makes sure you never leave your flashlight on. This is also a very simple circuit, so it is good for beginners, and can be modified to suit your needs.

Step 1: Materials

For this project you will need:

  • A breadboard
  • CR2032 Battery Holder
  • CR2032 batteries
  • Small PCB (optional)
  • Infrared photo transistor
  • White LED
  • NPN transistor
  • Resistor

Step 2: Wire the Breadboard Circuit

Wire the breadboard circuit according to the pictures.

Step 3: Soldering the Circuit

First, solder both resistor's ends to the right pin and middle pin of the transistor. Then, solder the photo transistor's leads to the left and middle pins. Finally, solder the LED's leads to the transistor's left and middle leads.

Step 4: Adding the Battery Holder

Now, add the battery holder to the circuit. The side with the photo transistor on it is the positive side.

Step 5: Electrical Taping

Finally, all you have to do is wrap electrical tape around the flashlight to keep everything in place, and you are done!

Step 6: The Finished Product

Congratulations! You have made your own light sensing flashlight! From here, there are many things you can do with this very simple circuit.

<p>I have a question... can I use dark colored infrared photo transistor? It's the only one I have in my collections...</p>
Interesting. I found this same circuit in a rechargeable solar light I bought in a dollar store a while back, and I ripped it out and installed it on a paintball mask as a hands-free flashlight! I never could find out what the unmarked transistor was, though. Thanks!
<p>You're welcome!</p>
Wow, this simple circuit has thousands of uses, thanks.<br>The battery is 3 volts right?
<p>Thank you! Yes the battery is 3 volts.</p>
You can use a flashlight 2 AAA batteries and add this circuit, great
<p>What if you make it light up in light and turn off in dark?</p><p>Then we'd have the Metro last light watch.</p>
what if you put it in your pocket?
<p>Because it is dark in your pocket, it lights up.</p>
that's what I was thinking. so putting it in your pocket makes it die. maybe having a switch and the light sensor would work better.
<p>Correct, that would make it a lot more efficient. That feature could be included in a future revision of the circuit.</p>

About This Instructable

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Bio: I am a passionate inventor who is always thinking of new ideas and making them a reality. I do not attend MIT, that is just ... More »
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