Photocell Help

[Cooper Mor]

I am building a circuit so that when it gets dark I have 8 LED's come on. I Bought a five pack of 5 Cadmium-Sulfide Photocells from RadioShack (part # 276-1657). So I have a battery pack that runs to the switch that runs to a photocell that runs to the LEDs that connect back to the battery pack. But the problem is the LEDs come on in the light, how can I make them come on in darkness? My circuit looks like the attached picture.

 

[Reloadron]

Your photocell is a LDR (Light Dependent Resistor) The more light striking the photocell, the lower the resistance and eventually your LEDs light. With your circuit as it is you can't change the light/dark and how it works. Also, even as it is, it is not a very good circuit.

You don't mention the supply voltage (battery voltage) or anything as to the LED specifications like the forward current or forward voltage?

You would do well to build what you want (dark = LEDs On) using either a transistor or a comparator. This would allow you more control, especially a comparator with hysteresis as to LED On and LED Off at what light levels striking the LDR sensor.

Ron

 

[Cooper Mor]

What kind of a transistor? Like what is a RadioShack part # that would work in this situation?

 

[Reloadron]

The reason I asked the LED specifications was so I could better suggest something, the same is true for the battery voltage. Yes, Radio Shack sells both the 2N2222 and the 2N3904 NPN type switching transistors if I recall correctly or their equivalents, and yes, one of those might work. Likely the 2N2222 but I can't give you a circuit unless myself or anyone trying to help knows the specifications for the LEDs. Also the battery voltage. Both are important. Both are needed. You will likely also need a few resistors off the shelf at Radio Shack. LEDs work based on current flow. They have a rated forward current and a rated forward voltage drop at that current. Till you can provide more information this is as good as it gets.

If you read this page and scroll down to Using A Transistor Switch With Sensors you will get an idea of what the circuit would look like for either light or dark. Oh yeah, you may want a 10K Ohm pot from Radio Shack also.

Ron