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Optoelectronics
LEDs, photodiodes, solar cells and optoisolators.
LEDs, photodiodes, solar cells and optoisolators.
Optoelectronic devices turn electricity into light or light into electricity. Emitters such as LEDs make light; detectors such as photodiodes and solar cells respond to it; and an optoisolator pairs one of each so a signal can cross a gap with no electrical connection.
An LED is a forward-biased PN junction. When electrons cross the junction and fall into holes, they release the energy as light. The size of the semiconductor's band gap sets both the colour and the forward voltage: a wider gap gives a shorter wavelength (towards blue) and a higher voltage drop. Infrared LEDs, invisible to the eye, drive remote controls and optical data links.
An LED has no way to limit its own current, so it needs a series resistor.
The resistor takes the leftover voltage, so its value is R = (supply − LED drop) ÷ current. Real parts vary, so check the datasheet for the drop and the maximum current.
A silicon cell makes about 0.5 V however large it is; its current follows the light and its area. Panels connect cells in series to raise the voltage. A cell's efficiency is the fraction of the light energy it converts to electrical energy. Silicon is by far the most common material in power-generating panels. For using one at a station, see Batteries, solar and portable power.
An optoisolator (or optocoupler) puts an LED and a phototransistor in one package, facing each other across an insulating gap. A current in the LED makes light, the light turns the phototransistor on, and the signal gets across although the two sides share no electrical connection.
That isolation is the point. It keeps a control circuit separate from a higher-voltage circuit, breaks ground loops between a computer and a radio, and protects a delicate input from the other side's faults. A solid-state relay is the same idea: an optoisolator driving a semiconductor switch in place of a coil and contacts. Mains wiring still demands a part and a layout rated for it. Isolation is only as good as the device's rating.
A related device is the optical shaft encoder: a slotted wheel interrupts a light beam, and counting the pulses tells a circuit how far a knob has turned.