I bought a photoresistor at Radio Shack today and noticed something peculiar about it. When exposed to light, the resistance decreased. In the dark, however, the resistance increased dramatically. Isn't this the opposite of how a photoresistor is supposed to function?

I thought that photoresistors work in the opposite manner: when exposed to light, the resistance should increase, and when in the dark, the resistance should decrease to a very small value.

Why does my photoresistor operate opposite how normal photoresistors should?

 

I thought that photoresistors work in the opposite manner: when exposed to light, the resistance should increase, and when in the dark, the resistance should decrease to a very small value.

You thought wrong...your photoresistors works right.

Ken

 

Think of it as a partially photoconductive device.

 

Here is a link to the wikipedia information on photoresistors.

hgmjr

 

Really, it doesn't matter.

You can use a photoresistor on either side of a voltage divider to drive the base of a transistor to control much greater currents/voltages.

For a dark detector, you can place the photoresistor on one side of a voltage divider.
For a light detector, you can place the photoresistor on other side of a voltage divider.

You can use a 10k resistor or a pot for the other side of the voltage divider.

 

Here's why I'm confused. I built the circuit in the photo but it still performs the opposite way I'd expect. I've varied R1 from 1K ohm to 51k ohm and observed no significant difference. I can't figure out why the LED still turns on only when the photoresistor is exposed to light. Shouldn't it now turn on only in the dark?

 

You might be better off using the photoresistor between the base and the positive supply. You could then introduce another common emitter stage if you needed the LED to illuminate in the absense of light.

hgmjr

 

That's odd.

Are you sure that's an NPN transistor?

 

Are you absolutely convinced that you have the pinouts correctly identified?

hgmjr