Light Sensor

Thread Starter


Joined Jan 19, 2012
I am building a project that indicates moisture in the soil and when it detects the low resistance in the dry soil turns on an LED. (I already have a circuit for this.)

I was wondering if anyone had any good ideas how I could use a simple sensor to pick up that light off the LED to signal a pump to activate until the light turns off, in which case the pump turns off.

Anything would be helpful, I am not quite sure how to really go about it, or what components I would need, but it doesn't sound too hard to accomplish. (Examples are always nice.)



Joined Apr 16, 2010
Sure. Use a Light Dependent Resistor (LDR) sometimes called a photocell and often using Cadmium Sulfide. When the LED comes on, the resistance drops dramatically in the LDR, which can be used with an op-amp to bias a transistor which can operate a relay.
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Joined Dec 26, 2010
Why do you want to work your pump switching circuit from the light coming from the LED, rather than taking an electrical signal from the circuit?