reversing an LDR

Thread Starter


Joined May 6, 2008
For my school project I am creating a solar tracker. I'm just proving a theory and not actually making a large scalled version.
The tracking method that I decided on was using a light sensor. But LDR's decrease in resistance as light is lost. I'm going to have the LDR inside a small tube so that even if the sun (or in my case a small light) is slightly passed the tracker, the LDR's will have no resistance and therfor cause the tracker to move. However I need to reverse the LDR so as it increases in resistance as the light increases.

How would I go about this?
Sorry If I wrote it wrong because I probably confused myself.



Joined Jul 17, 2007
Hi Jason,
Why would you wish to increase the resistance when the light is growing stronger?

As it is, when the light gets stronger, the resistance grows less, so current through the resistor increases for a given voltage across it.

That should give you a good hint as to what you might do.

You really should have posted this in the Homework Help section.

We can give you nudges in the right direction if you're "stuck" - but we can't do your homework for you.

What have you researched thus far?

Show your work.


Joined Apr 20, 2004
You might use two CDS cells with a partition between them, If not points at the sun, one will be in partial shadow. The increased resistance can be a signal prompting a movement until both cells have equal illumination.

I am going to move his to the Projects forum.


Joined May 16, 2005
If the LDR is used in a voltage divider, the change in resistance will result in a change in voltage between the two resistances. How this change relates to the change in light will depend on whether the LDR is closer to Vcc or closer to ground. You can pick the one needed to get the response you want.