Use of LDR and Transistor to swtich current between branches

Thread Starter


Joined Oct 21, 2011

I've tried to design a circuit using a Light Dependent Resistor in parallel with another branch that is connected to a transistor. When it is dark, the LDR resistance is high, and the current should pass through the branch parallel to the LDR and hence through the transistor. When it is light, the LDR resistance is low, and the current should pass through the branch with the LDR and not the branch with the transistor.

The problem is, I can't make it so that current only flows through one branch. At the moment, it either flows through both branches regardless of the light level or only the branch with the LDR.

The schematic is below.

Any suggestions would be appreciated.
Thanks for helping me out.


Joined Sep 9, 2010
Try putting the LED and its resistor above the transistor instead of below it. In other words, let the transistor control the path to ground instead of the path from Vcc.


Joined Dec 26, 2010
Here are a couple of versions of your circuit. The simpler one will work only if the LDR goes to quite a low resistance in bright light.
The second one should be better if the LDR has more resistance. Changing RADJUST alters the level at which the LEDs change over.


Thread Starter


Joined Oct 21, 2011
thank you very much to the both of you for your help! Especially Adjuster, thanks for the drawings! I tried the second one as my LDR has a high resistance, and it works perfectly!

Thanks once again for helping me out.