How to use an LDR as a crude motion detector

Thread Starter


Joined Nov 28, 2009
I wish to control a transistor and relay by use of a light dependent resistor. *That is simple enough for a "nite light" application, by having the LDR as part of a voltage divider, but I would prefer if I could switch the transistor when the LDR shows a short-term change in resistance, such as when a person walks past in front of the ldr. *This could function as a crude motion detector. *Anyone have an idea of a circuit to do this? *The simpler the better. *THANKS.


Joined Nov 7, 2013
Don't have a circuit, but it seems you need a circuit that only activates with a rapid change. That can be done with a series capacitor in the sensing circuit.

Something I have done is to use two LDRs in tubes looking the same direction.

Then used a circuit that only responded to the different light levels between the two.

I'd have try and find the diagram.


Joined Aug 7, 2008
A crude check showed a 2:1 change in R of a hi Z LDR when light path interupted. Maybe capacitor coupled into a FET amplifier driving a 555.
Lots of variables to play with. ' Might breadboard if interested.


Joined Nov 7, 2008
This application requires a microcontroller! A cheap 8 pins (pic12f683) has all the periferals you need already on board. 16 bits timer to set precise time interval, 10 bits analog to digital converter to read in microsecs the voltage across your LDR and the possibility to make calculation and statistics on the ADC reading, and the result can be easily sent to your PC via serial com or directly to a LCD. All this for few cents. Programming could be an issue for you but learning is a good investimemt and it will pay back.


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