One of the best ways to learn about LEDs is by tinkering with them. While encased in lamps, LEDs might be tiny, sensitive, and hard to understand, but when out in the wild they are affordable, easy to use, and rather simple devices. This makes them ideal for experimentation and a great way to learn the basics of electronics.
The previous mention of do-it-yourself LEDs covered the Throwie, the simplest of the LED circuits. It combines a battery and an LED into a easy to understand device that anyone can make in minutes. The next step up from that two-part project is adding a resistor to the mix. This will allow you to adjust your circuit so that the proper amount of power is going to the LEDs which will, in turn, ensure they get the maximum lifetime possible as well as the optimum brightness. Without a resistor, it’s easy to send too much power to the LEDs and shorten their lives significantly, or simply break them.
To figure out the resistor you need in your LED circuit, some basic math will be required. The formula is based on Ohm’s law, where V = voltage across the resistor, I = the current through the resistor, and R is the resistor value. Here are three ways of stating Ohm’s law:
I = V/R V = IR R = V/I
It’s this last one (R = V/I) we want to use:
Resistor rating = (battery voltage - LED forward voltage) / LED forward current
This means you need to know the voltage of your power supply, the forward voltage of your LED, and the forward current in amperes (amps) that the LED draws. Both the forward voltage and forward current will be provided by the LED manufacturer (if you bought the LED from a retailer, they may include the specs along with the packaging or have a link to the manufacturer’s data sheet on their website).
Note
The forward current for LEDs are specified in milliamperes (mA), so you’ll have to divide your mA number by 1000 in order to get the amount of amps.
A common red LED has a forward voltage of around 2.0 volts (V), and a forward current of 20 mA. So the math for a single red LED powered by a 9 volt battery would be calculated like so:
(9.0V - 2.0V) / (20 mA/1000) = 350 ohms
Because people tend to opt for the nearest higher rated resistor, you could opt for a 390 Ohm one (the color code would be Orange White Brown), but 220 ohm resistors are more common, so you could put two of them in series for 440 ohm. Then, try the same circuit with a single 1 kilohm resistor. You should see the LED get less bright than with the two 220 ohm resistors. Figure A-1 shows a superbright green LED with a 1K resistor.
Note
This LED circuit is shown on a solderless breadboard, which allows you to quickly wire up components. The rows and columns are connected to each other in such a way that the resistor and left leg of the LED are tied together, and the yellow wire and the right leg of the LED are tied together. Similarly, the resistor is connected to the red wire, and the yellow wire is connected to the black wire. If you trace the circuit starting from the red wire (the battery’s positive lead), the circuit travels in this path: from the positive lead, through the resistor, then through the LED, and back into the negative (black) lead by way of the yellow wire.
Note
Because the superbright has higher forward voltage (around 3V) and higher forward current (around 80 mA) than a red, it would only need a 100 ohm resistor. However, if I did that, it would be too bright to take a decent picture, hence the 1K resistor.
But what if you wanted two LEDs in a series? That’s entirely possible, you’d just need to adjust the formula. In this case, the formula will change to:
(9.0V - 2.0V - 2.0V) / (20/1000) = 250 ohms
Twice the LEDs means twice the voltage is required, so you’ll have to change your resistor in order to compensate for the difference. Pretty easy, right? Figure A-2 shows the circuit wired up.
But what if you want to test out the deleterious effects of overvolting an LED? The math makes it clear that this is pretty easy to do, you just need to offer too much power to the LEDs or not enough resistance. Figure A-3 shows a fried LED.
Warning
If you decide to try frying an LED, don’t hold it in your hand, and do so in a well-ventilated area. One way to avoid holding it in your hand is to use a breadboard to hold the LED and connect the battery further down the breadboard. You can safely throw the LED away after it’s cooled down.
By playing with the resistance variable you’ll be able to see the LED in its full spectrum—from dim, to bright, to peak brightness, to burnt out. The LED’s lifetime will travel along this same asymptotic curve if you were to plot it out, with there being an inverse relationship between brightness and lifetime.
If you’re inclined the explore more hands-on experiments with LEDs, check out MAKE: Electronics by Charles Platt (O’Reilly/MAKE).
