Hi Guys,

I need to test some flashlights for how long the batteries will last so I need a LDR circuit that will turn off the power instead of turning it on. I just plan to buying some cheap 1.5v clocks and interrupt the voltage with an LDR circuit. My question is (before I but the components) is: if I flip the transistor in this diagram will it turn clock off when the light goes out or do I need to do it with a relay?

Thanks for any answers you might have,
Kyle

 

Sorry, my spelling it horrible:
Hi Guys,

I need to test some flashlights for how long the batteries will last, so I need a LDR circuit that will turn off the power instead of turning it on. I just plan on buying some cheap 1.5v clocks and interrupt the voltage with an LDR circuit. My question is (before I buy the components): if I flip the transistor in this diagram will it turn clock off when the light goes out or do I need to do it with a relay?

Thanks for any answers you might have,
Kyle

 

Just reverse the position of the LDR and 50K resistor. You can also just replace the 50K resistor with a 100K pot so you can set the light or dark turn on point. Keep in mind this is just a single transistor circuit for detecting light or dark. There will be transition between the transistor turning fully on or off. Here are a few examples of your circuit.

Ron

 

Thanks, I'll order the components and start testing.
Thanks again,
Kyle

 

This could be a good project for an Arduino.
You could measure the light level as well as having a display that shows the time, battery volts..... Whatever else you need.

 

We have a lot of equipment for measuring light including a Sekonic C7000 that will give us spectrometer readings but oddly enough nothing to actually test how long the light stays on...