Optocouplers

             In electronics, an opto-isolator (or optical isolator, optical coupling device, optocoupler, photocoupler, or photoMOS) is a device that uses a short optical transmission path to transfer an electronic signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated—since the electrical signal is converted to a light beam, transferred, then converted back to an electrical signal, there is no need for electrical connection between the source and destination circuits. Isolation between input and output is rated at 7500 Volt peak for 1 second for a typical component costing less than 1 US$ in small quantities.

    The opto-isolator is simply a package that contains both an infrared light-emitting diode (LED) and a photo detector such as a photosensitive silicon diode, transistor Darlington pair, or silicon controlled rectifier (SCR).
The wave-length responses of the two devices are tailored to be as identical as possible to permit the highest measure of coupling possible. Other circuitry—for example an output amplifier—may be integrated into the package. An opto-isolator is usually thought of as a single integrated package, but opto-isolation can also be achieved by using separate devices.
    Digital opto-isolators change the state of their output when the input state changes; analog isolators produce an analog signal which reproduces the input. A common implementation is a LED and a phototransistor in a light-tight housing to exclude ambient light and without common electrical connection, positioned so that light from the LED will impinge on the photodetector. When an electrical signal is applied to the input of the opto-isolator, its LED lights and illuminates the photodetector, producing a corresponding electrical signal in the output circuit. Unlike a transformer the opto-isolator allows DC coupling and can provide any desired degree of electrical isolation and protection from serious over voltage conditions in one circuit affecting the other. A higher transmission ratio can be obtained by using a Darlington instead of a simple phototransistor, at the cost of reduced noise immunity and higher delay.
    With a photodiode as the detector, the output current is proportional to the intensity of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode. In photovoltaic mode, the diode acts as a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity. In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light. This optocoupler type is significantly faster than photo transistor type, but the transmission ratio is very low; it is common to integrate an output amplifier circuit into the same package.
    The optical path may be air or a dielectric waveguide. When high noise immunity is required an optical conductive shield can be integrated into the optical path. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an optoisolator or opto-isolator. The photosensor may be a photocell, phototransistor, or an optically triggered SCR or TRIAC. This device may in turn operate a power relay or contactor. Analog optoisolators often have two independent, closely matched output phototransistors, one of which is used to linearize the response using negative feedback.