![]() ![]() This leaves us with a device that can control a switching element without any physical contact! Such a device already exists, and as you guessed, it is the optocoupler! Optocoupler Inputs and Outputs If we can put an LED and a phototransistor in a closed tube, the light coming from the LED (assuming it is properly driven, of course) will light up the ‘base’ of the phototransistor and render it conductive. Photodiodes and transistors are used in things like proximity sensors, which detect small changes in voltage or current across these devices depending on the amount of light that falls on them. Photodiodes work in a very similar manner they change their ‘resistance’ depending on the amount of light that falls on them. ![]() They look very similar to diodes and come in clear packages. This leads to some very interesting possibilities, the first of which is the phototransistor, basically a two terminal transistor without a base lead. This means that the photons of light are actually able to knock the holes and electrons around in a doped semiconductor material. But, if you carefully manage to decap a regular discrete transistor and apply a voltage across the collector and emitter leads, you’ll notice that a tiny current still flows! This is because of the light that’s falling on the base of the exposed transistor die. What is Optocoupler and How It WorksĪs we have already learnt about transistors, an ideal transistor will not allow any current to pass through it if the base pin is not triggered. They use light to pass signals between circuits. These components are called optocouplers or optoisolators or simply optos, and they perform the crucial function of passing signals between isolated sections of circuitry. What is more unusual is that these parts are generally found over isolation slots and gaps, which makes their purpose more cryptic. If you’ve ever taken apart any phone charger or a switching power supply, you’ll find some tiny black IC packages with an unusual number of pins, mostly four or six, both in SMD and through-hole variants. ![]()
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