Photo-resistor circuit question.?

Question

I want to build a circuit where I have a 12v power source (AC adapter) to a pre-built LED circuit assembly that is made to accept 12v as a power source. However I would like to be able to use a photo-resistor to where the LED turns on when its dark (like a nightlight). I've searched the internet but I can't find a schematic for this type of application. Please help. Thanks.

Answer 1

There is an app note on Maxim Semi's website about a light sensor here: http://pdfserv.maxim-ic.com/en/an/AN1909...

(You should replace R1 with a potentiometer so that you can adjust the light threshold)

It will turn your photoresistor into a digital signal.  You should then tie that signal into a solid-state relay, which can turn on or off the power for your led circuit.

One note of caution: many AC adapters are unregulated, meaning that your 12V adapter might output 15V, and drop closer to 12V as more current is drawn.  I strongly suggest using a regulator to power the photoresistor section of your circuit.

Answer 2

You won't find many circuits using a photo-resistor for something like this.  Most people would use a phototransistor or photodiode.

I also don't think you're going to do it without any support components.  You could make it light up in the light with just a photo-resistor and an additional current-limiting resistor, but that's not what you, or much of anyone else, would be looking for.

So, photo-resistor, dark = high resistance, light = low resistance.  The simplest secondary component to use would be a logic inverter.  The cheapest thing to use would be a transistor.  So, let's look at a simple BJT circuit using an NPN transistor:

Attach a 1k resistor from the +12V line to the base of the transistor.

Attach the photo-resistor between the base of the transistor and the ground, or negative terminal, of your power supply.

Attach the positive (longer) leg of the LED to the +12V line.

Attach the other leg of the LED to the collector of the transistor.

Attach a current limiting resistor, or a potentiometer between the transistor's emitter and the ground.

This last component sets the LED brightness.  Be careful here, it's easy to burn out an LED or the transistor if you pass too much current through.  A proper design would take the transistor parameters into account, which I haven't done here.  Just make sure it can handle 25-30 mA minimum current.  A potentiometer would allow you to control the brightness, once you found one you liked, you could measure that setting and sub in a resistor.  A first pass guess at a good limiting resistor would be something between 500 and 1k.

Answer 3

Check out "voltage comparator" circuits.  A comparator compars two voltage inputs and generates a hi or low output based upon one input voltage being higher than the other.

THe link  shows how to use a comparator using two potentiometers (volume controls) as voltage dividers . THese allow you to adjust both input voltages by adjusting the potentiometers.

To use a comparator with a photo resistor (cds Cell)  you will replace one of the potentiometer voltage dividers with another voltage divider made up of a photo resistor and a fixed value resistor.

Depending upon which input of the comparator(+ or -) and which side of the voltage divider the cds is placed you can make a circuit that will sense either dark or light.  

The second link shows a simple schematic of a cds light detector. Remember you can change how the circuit works by swapping inputs of swapping cds and its resistor positions ( the cds either pulls up or pulls down the voltage when hit by light)