Optocoupler combine a LED and photo-transistor in the same case. The
purpose of an optocoupler is to separate two parts of a circuit.
This is done for a number of reasons:
- Interference. One part of a
circuit may be in a location where it picks up a lot of interference
(such as from electric motors, welding equipment, petrol motors etc.)
If the output of this circuit goes through an optocoupler to another
circuit, only the intended signals will pass through the optocoupler.
The interference signals will not have enough "strength" to
activate the LED in the optocoupler and thus they are eliminated. To
protect a section of the device. Typical examples are industrial units
with lots of interferences which affect signals in the wires. If these
interferences affect the function of control section, errors will
occur and the unit will stop working.
- Simultaneous separation and
intensification of a signal. A signal as low as 3v is able to
activate an optocoupler and the output of the optocoupler can be
connected to an input line of a microcontroller. The microcontroller
requires an input swing of 5v and in this case the 3v signal is
amplified to 5v. It can also be used to amplify the current of a
signal. See below for use on the output line of a microcontroller for
- High Voltage Separation.
Optocouplers have inherent high voltage separation qualities. Since
the LED is completely separate from the photo-transistor, optocouplers
can exhibit voltage isolation of 3kv or higher.
Optocouplers can be used as input or
output device. They can have additional functions such as Schmitt
triggering (the output of a Schmitt trigger is either 0 or 1 - it changes
slow rising and falling waveforms into definite low or high values).
Optocouplers are packaged as a single unit or in groups of two or more in
one housing. They are also called PHOTO INTERRUPTERS where a spoked wheel
is inserted in a slot between the LED and phototransistor and each time
the light is interrupted, the transistor produces a pulse.
Each optocoupler needs two supplies in order to function. They can be used
with one supply, but the voltage isolation feature is lost.
Optocoupler on an input line
The way it works is simple: when a signal arrives, the LED within the
optocoupler is turned on, and it shines on the base of a photo-transistor
within the same case. When the transistor is activated, the voltage
between collector and emitter falls to 0.5V or less and the
microcontroller sees this as a logic zero on its RA4 pin.
The example below is a counter, used for counting products on production
line, determining motor speed, counting the number of revolutions of an
Let the sensor be a micro-switch. Each time the switch is closed, the LED
is illuminated. The LED 'transfers' the signal to the phototransistor and
the operation of the photo-transistor delivers a LOW to input RA4 of a
microcontroller. A program in the microcontroller will be needed to
prevent false counting and an indicator connected to any of the outputs of
the microcontroller will shows the current state of the counter.
Input line optocoupler example
Optocoupler on an output line
An Optocoupler can be used to separate the output signal of a
microcontroller from an output device. This may be needed for high voltage
separation or current amplification. The output of some microcontrollers
is limited to 25mA. The optocoupler will take the low-current signal from
the microcontroller and it's output transistor will drive a LED or relay, as shown below:
Output line optocoupler example
The program for this example is simple. By
delivering a logic '1' to the fourth pin of port A, the LED will be
turned on and the transistor will be activated in the optocoupler. Any
device connected to the output of the optocoupler will be activated.
The transistor current-limit is about 250mA.