The use of LEDs (light emitting diodes) in props and effects is a common and easy lighting effect. If you have never used an LED before, it might seem complicated. However, it is no more complicated than a common flashlight. With a couple of key facts, you will be able to use LEDs in your effects.
First off, LEDs run on DC current, so that means you must either use batteries or a DC power supply to power them. We'll start with batteries, but we have some simple suggestions for alternate power supplies later in the article.
In a flashlight, the bulb is rated for a certain voltage. The flashlight is designed to use a certain size battery of a certain voltage. If the voltage is too low, the bulb will be dim or not light at all. If the voltage is too high, it will cause the bulb to burn out prematurely or even immediately. An LED behaves in just the same way.
LEDs come in a variety of voltages, but by far the most common is 3 volts (3V). That is convenient, because two common household batteries wired in series yield 3 volts. That is, "AA", "AAA", "C" and "D" batteries all output 1.5 volts and when you use two together in series, they yield 3 volts (1.5 + 1.5 = 3).
One difference between a flashlight bulb and an LED is that with the flashlight bulb it does not matter which way it is wired. In other words, either the positive (+) current or the negative (-) current can be wired to either pole of the bulb. With an LED, the positive current MUST be wired to the anode. The anode is denoted on LEDs with either a flat spot on the anode side of the LED or it has a longer lead (the longer of the two wires sticking out is the anode).
Using this information, you can perform a simple test. Take two "AA" batteries (or other 1.5V batteries) and tape them together end to end. The positive nub protruding from one battery should make contact with the negative (flat bottom) of the second battery. Taping them together is simply for convenience. Now use an alligator lead or just a piece of wire to connect the anode of the LED to the positive nub on the top battery and connect a second wire to the negative terminal of the battery to the short wire (cathode) of the LED. If it everything is set-up correctly, the LED should light. If it doesn't make sure all your connections are good, the batteries are good, the LED is not damaged and that you are correctly connecting the anode to the positive current.
Now you have lit one LED. But what if you want to light more? There are two ways you can light multiple LEDs off of one power source. You can wire them in series, which means that each LED is wired directly to the next LED and the positive current is attached to the first LED's anode and the negative current connection is made to the cathode of the very las LED. When you do this, the voltage of each LED is added together and you need more voltage from your source. So, if you have four LEDs wired in series, combined they would require a voltage of 12V (3V + 3V + 3V + 3V = 12V). However, the problem with wiring in series is that if the connection breaks anywhere in the siring or if one of the LEDs burns out, none of the LEDs will light. Also, if you want to light a lot of LEDs from one power source, the voltage requirement climbs quickly.
You can also wire the LEDs in parallel. That means that, in effect, that you have a separate two wire connection to every single LED. In this way, you can use the same 3V power source to light as many LEDs as you like. Wiring this way also ensures that if one LED goes out, the rest will remain lit. The more LEDs you wire in parallel, the more quickly the power source will be drained. If you will use many LEDs, then you will get longer life from a pair of "D" cell batteries than from a pair of "AAA" cell batteries. When wiring in parallel, it isn't actually necessary to run two separate wires to each LED. Instead, you can run a single positive wire and a single negative wire for the length of the circuit and make connections for each individual LED to tap power from the wires.
What if your power source has higher voltage than your LEDs? You can use a higher voltage power supply, but you must restrict the current to avoid damaging the LEDs. This is done by simply installing a resistor in the positive connection between the power source and the LED. The resistor is similar to crimping a water hose. Using a resistor in a circuit is like crimping the hose to reduce how much water flows. The size of the resistor will depend on the voltage demand of your LEDs and the voltage supplied by the power source. The calculations to determine the correct resistor size are fairly easy. However, that exceeds the scope of this article. There are several on-line calculators that allow you to enter the specifications and they tell you what size resistor to use.
Using an Power Supply other than Batteries
You can use AC household current to power your LEDs, but it has to be converted to DC first. This is what power supplies do for common electronics like cell phones, cordless phones and all types of computer related equipment. You can use an old power supply for powering you LEDs. Just note the voltage output of the power supply and then use a resistor to modify the voltage to suit your needs. You will need to create a little box to contain the wiring. You can cut the DC plug off of the power supply and wire it permanently into the box, or you can wire a matching receptacle into the box to allow easy disconnection of the power supply.