Using LEDs & IREDs

Using LEDs and IREDs

(Light emitting diodes and infrared emitting diodes)

LEDs and IREDs are diodes, meaning they conduct in their forward direction only. If their maxiumum reverse voltage is exceeeded, they can be destroyed, and the maxiumum reverse voltage is sometimes only about 12 volts.

The amount of light emitted is proportional to the forward current. A typical amount of continous forward current that is safe for an LED is 10mA, although an LED can be pulsed briefly to much higher currents without ill effect. Some LEDs are much more efficient than others at converting current to light.

LEDs come in many colors, and even infrared, sometimes called IREDs. IREDs are bright and efficient, and the optoelectronics that detect light (phototransitors and photodiodes) respond best to IR.

LEDs are not resistors: the current through them is *not* proportional to the voltage across them. An LED may be off at 1.0V, dim at 1.5V, and burned out at 2.0V. You do not usually use them with a volatge source across them, as you would a light bulb. Instead you put a resistor in series with the LED, and you pick the resistor value as follows:

Each kind of diode (including LEDs and IREDs) has a Vf, forward voltage, which depends only weakly on forward current. We'll take it to be a contant. Ordinary Silicon diodes have Vf=0.6V. Most LEDs and IREDs have Vf=1.5V up to Vf=2.0V. For blue LEDs, Vf can be as high as 6.0V.

In the circuit shown, if you want LED and LED with Vf=1.5V to have forward current If=10mA, with a 6V battery as shown, you would choose R = (6.0V - 1.5V) / 0.010 A = 450 ohms.