Here I'll illustrate using optocouplers to perform voltage logic shifting between TTL and CMOS devices. See the following related pages: Fig. An optocoupler, also known as photocoupler or opto-isolator, is a device which can transfer an electrical signal across two galvanically-isolated circuits by way of optical coupling. Unlike transformers or capacitors, which can only transfer AC signals across the isolation barrier, optocouplers can. There are many different applications for optocoupler circuits, so there are many different design requirements, but a basic design for an optocoupler providing isolation for example between two circuits, simply involves the choice of appropriate resistor values for the two resistors R1 and R2. An optocoupler, as shown in Figure 1, consists of an input LED, a receiving photodetector and an output driver.
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