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An opto-isolator is a device that uses optical techniques to electrically isolate two related circuits, typically a transmitter and a receiver.
A common implementation involves an LED and a light sensor, separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and a corresponding electrical signal is generated at the output.
Among other applications, opto-isolators can help cut down on ground loops and block voltage spikes.
One of the requirements of the MIDI (Musical Instrument Digital Interface) standard is that input connections be opto-isolated.
An opto-isolator can also be constructed using a small incandescent lamp in place of the LED; such a device, because the lamp has a much slower response time than an LED, will filter out noise or half-wave power in the input signal. In so doing, it will also filter out any audio- or higher-frequency signals in the input. It has the further disadvantage, of course, (an overwhelming disadvantage in most applications) that incandescent lamps have finite life spans. Thus, such an unconventional device is of extremely limited usefulness, such as in science projects or crude alarm systems.
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