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I'm trying to build some system that would detect light and turn a 12v led strip ON. The only power that I have available is a 9v battery.

I found some Arduino compatible boards but it seems overkill to use an Arduino for something that simple.

So I searched for a simple photo-resistor and found this one. However I am not sure it that will work.

My goal would be to wire the battery to the photo-resistor and the lamp in series and the photo-resistor would act as kind of a potentiometer effectively dimming the led strip as the light goes down.

However, I am not too sure about the values announced by the photo-resistor with a maximum of 150v dc (my 9v battery is way below so it should be no problem but is it actually enough then?) and a maximum of 100mW for which I am not sure if it shall be enough to power the le strip.

Small note:

  • I do know that 9v is too low and thus the led strip is already not at its max brightness. I tested the leds and battery and it is working with sufficient brightness for my usecase.
  • I have some size constraints and that is why I am looking for a solution that will be as small as possible
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  • \$\begingroup\$ Do you want to turn the LED strip on or off if there is light? Did you consider that the LED light may affect the photoresistor if they are not optically separated? \$\endgroup\$ – Marco Zollinger Aug 15 at 15:31
  • \$\begingroup\$ The use case is very weird I know, but if should "follow" the light. So far I think the best way would be a voltage regulator and an ATTINY85 to read a photoresistor and using addressable leds so I can power it with the 9v but then control them with a pin of the ATTINY. Does it makes sense? \$\endgroup\$ – Georges Legros Aug 18 at 13:32
  • \$\begingroup\$ Yes that sounds good to me. You want to use a voltage divider with your photoresistor to read the voltage with the ATtiny. I think most adressable LEDs use 5V though, and if you have more than a handful, they require quite some current... I guess you know that your 9V battery will not last very long. Other way to do it is to use a transistor driven by the photoresistor to switch a non-adressable LED strip. In both cases it is important to mount the photoresistor away from the LEDs, else it will see its own light and stay on. \$\endgroup\$ – Marco Zollinger Aug 19 at 12:02
  • \$\begingroup\$ Yes the leds need to be up for about 20 to 30 minutes maximum. I found some led strip that will take 5v input and are addressable. The idea is to shut down the leds when the luminosity of the room is below a certain threshold so I guess it can be done this way. Also, you're right, the photoresistor will be mounted in a place where the leds cannot affect them. Thanks! \$\endgroup\$ – Georges Legros Aug 20 at 9:35
  • \$\begingroup\$ Just to be sure, I will be using a L7805 as a voltage regulator to step down from 9 to 5v. I know I should be using capacitors in order to stabilize the output but in this case I don't think it is critical. \$\endgroup\$ – Georges Legros Aug 20 at 13:40

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