basic photosensors

badlandsghost · Post by **badlandsghost** » Sun Nov 11, 2007 3:35 pm

I picked up wally world light sensitive led nightlight the other day, and have been playing with it for my halloween beasties mk2 project, but I'm still kinda fuzzy on the theory/application side of things. net searches haven't helped much either. anybody here have a basic primer on these things and the best way to use them?

Thanks
J

en'til Zog · Post by **en'til Zog** » Mon Nov 12, 2007 1:08 am

If you use a 4023 to make your LEDs 'blink' and a large cap around those LEDs to make 'em do it slowly, I think you can put a teeny solar cell array in there to bias the thing into the "OFF in daylight mode. I'll work on that. Prolly need a 3 to 6 volt teeny surplus array for drving a watch or maybe a CDS photoresistor.

Hm....

Oh, and be really careful with that 110VAC night light. I'd remove the 110V part and just use the LED and CDS cell bit at low voltage. FYI

badlandsghost · Post by **badlandsghost** » Mon Nov 12, 2007 4:57 am

en'til Zog wrote:Oh, and be really careful with that 110VAC night light. I'd remove the 110V part and just use the LED and CDS cell bit at low voltage. FYI

Sorry, didn't mean it that way, I already dumped the outlet connections etc, I've just been playing around w/ the circut board/led/solar sensor, seeing what points light it up w a 9v and resistor

Thanks' for the info,

J

macfrank · Post by **macfrank** » Mon Nov 12, 2007 9:41 am

Most of those solar powered night lights have more than just the CDS cell controlling it, since the solar cells will stop producing enough power to recharge the batteries long before it's dark enough to turn the light on.

Since the CDS cell is a resistor, night light could be using it together with another resistor to measure voltage. As the light changes on the CDS, the voltage will go up or down. Look at the CDS cell; one pin may be connected directly to either the + voltage or ground, and the other pin will probably be connected to a resistor. The other end of that resistor will be going to either V+ or ground. If you measure the voltage at the joint between the resistor and the CDS, you should see it vary with light. Depending on how it's wired up, you can increase or decrease the sensitivity by changing that resistor.

badlandsghost · Post by **badlandsghost** » Tue Nov 13, 2007 6:49 am

Actually its (was

) wall powered, but one arm from the cell runs to a brn/blk/org/gold resistor, which runs into one of the 3 (-) terminals on an IC, and the positive output arm from the IC then runs to the LED (+) arm. And the other arm of the of the sensor is also tied in there as well
(hope that makes sense)

So if I hook the 9v up to it to light the led, then I check the resistance at that point, and it shows the variable resistance, then that would be the point to start trying different values in. correct?

Believe it or not, I think I know what you mean,
thanks
J

macfrank · Post by **macfrank** » Tue Nov 13, 2007 9:35 am

badlandsghost wrote:
So if I hook the 9v up to it to light the led, then I check the resistance at that point, and it shows the variable resistance, then that would be the point to start trying different values in. correct?
J

You want to look at the voltage (not resistance) at the junction between the CDS and resistor, since you're basically looking at a voltage divider, where one of the resistors is variable.

The 10K resistor can be replaced by a small potwentiometer (or a multi-turn trimpot the center pin goes where one end of the resistor used to be, one of the other pins goes to the former CDS/resistor junction) and you can adjust it until you get the sensitivity you need. Then take the pot out of the circuit, measure the resistance and replace it with a fixed resistor near that value.