Opto-isolator Principle

Opto-isolators are used for the transmission of signals between components while maintaining electrical isolation.

An opto-isolator is a small LED in close proximity to a phototransistor. The LED is driven by the transmitting circuit and an appropriate resistor is necessary, see the opto-isolator data sheet for assistance is sizing this.

The phototransistor drives the receiving circuit. Opto-isolators are most effective for binary switching operations. Analog signals need a more complex transmission method such as pulse width modulation. The opto-isolators in the have a connection to base that can be used to change the properties of the connection.

The bandwidth of signal transmission is dependent on the configuration used.

For higher output to input current ratios, use an opto-isolator with a Darlington transistor pair in the output. Opto-isolators do not output square waves.

For this reason, a comparator is needed on the output to ensure clean output signals. A possible configuration is shown below that uses the 4N27 and has a bandwidth of 20 KHz.

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An opto-isolator, also known as an optocoupler, is an electronic component that provides electrical isolation between two circuits by using light to transmit signals.

The principle of operation of an opto-isolator involves the following steps:


An opto-isolator consists of an input section and an output section. The input section typically comprises an LED (Light-Emitting Diode), while the output section contains a photosensitive device, such as a phototransistor or a photoresistor.

Input Circuit

The input circuit of the opto-isolator is connected to the control circuit or source that needs to be isolated. When an electrical signal is applied to the input circuit, it energizes the LED. The input circuit can be a low-voltage, low-current signal, making it compatible with sensitive electronic components.

Light Emission

As the LED is energized, it emits light, typically in the infrared spectrum. The emitted light intensity is directly proportional to the input current flowing through the LED.

Optical Coupling

The emitted light from the LED enters the output section of the opto-isolator, which contains the photosensitive device. The photosensitive device, such as a phototransistor, is designed to detect and respond to the incoming light.

Electrical Isolation

When the incoming light falls on the photosensitive device, it induces a corresponding electrical response. The response can be a change in resistance, voltage, or current, depending on the type of photosensitive device used.

Output Circuit

The electrical response from the photosensitive device is utilized in the output circuit of the opto-isolator. This output circuit can be connected to the isolated circuit, which is kept separate from the input circuit.

Isolation Benefits

The opto-isolator effectively isolates the input circuit from the output circuit, preventing the transfer of electrical noise, voltage spikes, or potential ground loops. It also provides protection against voltage differences and enhances safety in applications where isolation is crucial, such as high-voltage systems or situations where there is a need for isolation between sensitive control circuits.

The principle of an opto-isolator relies on the fact that light can be used to transmit signals across an isolation barrier, allowing communication between two circuits while maintaining electrical separation. This feature makes opto-isolators widely used in various applications, including power electronics, industrial control systems, telecommunications, medical devices, and more.