LEDs and their Alter-Egos

What we know already

LED (Light Emitting Diodes) emit light when current is passed through them. They only conduct current in one direction (Like diodes). If you hook up positive and negative to them backwards, they won't conduct enough current to glow and they won't burn out either.

Some fun first off

Multi-meters can power LEDs! 

Theory first:

1. LEDs are silicon based devices, as such need to have a minimum voltage put into them first (Usually about 0.6 Volt), before they'll conduct current and start to glow.
2. Multi-meters can test diodes/LEDs resistance by putting out a voltage (Through the meter probes) into the LED/diode, causing the LED/diode to turn on and conduct current.
3. In the case of a diode, this just means you can measure its electrical resistance, and therefore know if it works.
4. In the case of LEDs, however, this will cause the LED to emit light.

How to do it:

1. Any meter which has a diode range can do this. The diode range is just a picture of a diode (Image above). Find the option on your multi-meter, and select it.
2. Get an LED (Any colour, any size, two wire, preferably not a special flashing one),
3. Separate the LED wires a little,
4. Put your meter probes on the LED wires (In my experience: Connect the red positive probe to the wire going to the LEDs smallest internal electrode, and the black negative probe to the wire going to the LED's largest internal electrode). Note: If you get the wires the wrong way around, just swap them back. The LED doesn't suffer any damage from having the wrong polarity of wires applied to it.
5. Tah-dah! The LED now lives up to its name and emits light! :D 

Remember this little test in the future, its a great way to test an LED's: 1. Alive/Dead state, 2. Colour, and 3. Whether it's a special flashing LED.

An LED's Alter-Ego

LED's not only emit light, when current is passed through them. They also emit voltage when light is passed into them. To test this, we do something similar to the above process, with a few exceptions.

Theory first:

1. LED's emit voltage proportional to the strength of the light - the stronger the light, the higher the voltage. Note: The current you'd get from LED's emitted voltage would be VERY small, so you cannot power anything big, directly. Rather, use a transistor amplifier stage or the analogue input of a micro-controller (Arduino, Parrallax, Propeller etc) - as any of these use very little current to turn on.
2. When obtaining electricity from an LED, as compared with putting voltage in, there is no minimum light threshold before voltage will be emitted - the smallest amount of light will emit the smallest voltage.

How to do it:

1. Any meter which has a DC volt range is what we want. I'd recommend the 20v range - as the output from the LED will be 1.5-ish volts (1500mv-ish). Find the option on your multi-meter, and select it.
2. Get an LED (Any colour, any size, two wire, preferably not a special flashing one),
3. Separate the LED wires a little,
4. Put your meter probes on the LED wires (In my experience: Connect the red positive probe to the wire going to the LEDs smallest internal electrode, and the black negative probe to the wire going to the LED's largest internal electrode). 
5. Point the LED toward a light source and tah-dah again! You should now be getting some voltage readings. Clouded sky = about 1.6v, Dim lit loungeroom = 1.4v, Sunny sky = UNTESTED.

Note: If you get the wires the wrong way around, in this case, the test will work just the same, except your meter will tell you its backwards - no harm done.

I find its always great fun to discover somethings hidden functionality. Especially when it's something as common as an LED.

Applications for LED Alter-Ego

1. Sun tracking circuit: Using two LEDs to signal servos and their transistors to point a solar collecting device toward the sun.
2. Dark room, got lit alarm: Use one LED's emitted voltage to set off a light/siren (Via a transistor amplifier first, ofcourse) when someone enters a room, and turns on a light.

If you think of some extra applications for this ability - let me know in the comments section below :)