How does a CdS cell work?

[CStroman]

I'm going to make it turn on an LED when my bedroom light is turned off, and then I just wondered, "how does it work?"

 

[pm]

The resistance of the trace through the cell changes depending on light. In the dark it's in the 5-20 Mohm range (depending on the type) and in the presence of a decent amount of light, the resistance drops to 50-150kohm.

As far as the specifics of how it operates, I can make an educated guess: The combination of cadmium and sulfide materials produces a semiconductor with a bandgap that is the wavelength of optical light. Incoming light, excites, or gives enough energy to electrons tightly bound to the atoms in the valence band to move into the conduction band and essentially gives the material the ability to transmit electricity. In essence, the material is tuned to optical light such that it switches from a material that doesn't conduct electricity very well at all to one that conducts electricity moderately well.

 

[blahblah99]

pm, yeah, i believe that is how those cells work.


depending on your electronics knowledge, how simple / complex do you want to make this? do you want it to just turn on/off or varying brightness with the light level?

The simplest method is to feed the cell into a comparator like the LM393 at the + input, and a voltage reference (the point where you want the led to turn on). Feed the output of the comparator through a 4.7k resistor into a transistor like a 2n3904. Place a resistor in the emitter to limit the current, and the led in the collector to power. Thats basically the simplest circuit I can come up with right now. An op-amp, a transistors, your led, and some resistors. of course, your gonna have to work out the values 😉


if you need a schematic for that and the values i can work figure it out for ya.

 

[CStroman]

Maybe I should just take the parts from a night light. That'd probably be the least work.

 

[CStroman]

Never mind. I decided to just hook up a switch.