*Automatic LED Light*
By: 2manytoyz
20 March 2007

I've been caught a few times in my garage during a power outage.  It suddenly becomes quiet, and pitch black.  Lots of things to trip over, and while I have plenty of flashlights, they're playing hide -n- seek at the moment.

As a simple remedy, I bought an emergency light from Harbor Freight.  It provides good general lighting, if only for a few hours.  But there are plenty of other areas where it would be nice to have some task lighting, even when there isn't an emergency.  In my house, I have those green nightlights, which allow me to navigate through my house without turning on any lights.  I wanted something similar, but wanted it to run on my 12V alt power.

Now I can have a bright white LED light anywhere in my garage or house.  When the lights are off, it comes on automatically.  It runs all night, no matter if the power is off or not.  I can walk through my house, lights off, and check on all systems.  This circuit draws little power on or off.  Puts off just the right amount of light for a specific area. 

I can't claim credit for the circuit design.  Been around for a while in various forms.  Found this as a 9V version, and found it works well on 12V also.  All measurements are based on a 12V source.  Note that the measurements are for the entire circuit, not just the LED.


The parts are common and readily available.  I bought a pack of 5 Photoresistors at Radio Shack.  Had all the other parts on hand already.  The bright white LEDs I use are from Digikey (P/N CMD333UWC).

As you can see, there really isn't much too this.  When the Photoresistor is covered, the transistor allows the current to flow through the LED.  I mounted the components on a prototype circuit board also from Radio Shack.

Put it together on a breadboard to test it.

Current draw with the LED off is 0.14 mA.

Held my finger over the Photoresistor, the LED turns on.

Actual reading was 19.20 mA.  That's over 50 hours of constant light from a 1 Amp battery.

I put the LED & Photoresistor on leads so I could position them where needed.  They could have also been mounted in a project box.

I will eventually dress this up in a box, and put connectors on the wires.  This was a quick -n- dirty build just to test the circuit.


I put this in my garage, near my battery, charger, and inverter.  The single LED easily illuminates the shelf where the hardware is mounted.  While I can see everything in that specific area, it doesn't cast off enough light that my neighbors could see my garage lit up during an outage, through the clear panels along the top of my garage door.  I plan on building a few of these, and putting them in key locations I want to check on without having to turn on lights.


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