An
Optocoupler also known as Photocoupler or Optical Isolator is a
component that transfers electrical signals between two isolated
circuits using light. It physically and electrically separates a
low-voltage control circuit (like a microcontroller) from a high-voltage
or noisy power circuit (like a motor or AC mains). It is basically a
solid-state relay that optically interconnects two electrically isolated
circuits.
In this tutorial, I am going to talk about the PC817
Optocoupler which is one of the most common and inexpensive 4-pin
optocouplers.
Video: https://youtu.be/Rj9H0beMQq8
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Pin Configuration and Functions
The
PC817 optocouplers are key components in modern electronics and are
designed for "signal isolation" and "transmission" in a circuits. It
consists of two main parts inside a small, black, 4-pin DIP package:
- An Infrared LED on the input side
- And a Phototransistor on the output side
The
two components face each other but are physically separated by a
transparent, insulating barrier, providing complete electrical isolation
between the two sides. The device uses a combination of infrared LED
and phototransistor to not only protect sensitive electronic components
from "high voltages" and "electrical noise" but also to maintain signal
integrity and accuracy in a variety of applications.
Starting from the side with the notch or the dot and going anticlockwise:
Pin 1: Anode of the internal LED (+)
Pin 2: Cathode of the internal LED (-)
Pin 3: Emitter of the phototransistor (-)
Pin 4: Collector of the phototransistor (+)
How Optocoupler Works
To
fully understand the optocoupler's working principle, we will explore
it from four different perspectives, each highlighting a unique method
of circuit isolation. The main aim is to enable one circuit to activate
another while maintaining complete electrical isolation between them.
Setup 1
The initial configuration consists of two circuits assembled on a breadboard.
Circuit1
and Circuit2 are electrically isolated, meaning the operation of one
has no effect on the other. When powered, the LED on Circuit2
illuminates immediately. In contrast, the LED on Circuit1 is controlled
by a switch and only illuminates when activated. Activating the switch
in Circuit1 illuminates its LED, while the LED in Circuit2 always
remains illuminated, unaffected by the circuit on the left.
Setup 2
To control the right circuit's LED using the left circuit, we can connect them with a "transistor".
Activating
switch on the left side sends a current to the transistor's base,
enabling current flow from collector to emitter illuminating the LED on
the right side. Deactivating the switch cuts off the base current, and
turns off the LED.
However, this setup requires both circuits to
operate at the same potential (e.g., both at 5V) with a common ground.
And hence, it cannot be used for voltage level shifting or to control a
circuit with a higher voltage.
Setup 3
In the next setup, I'm using an IR LED and an IR Receiver to create my own optoisolator. Circuit1 (on the left) contains the IR transmitter and a switch, and Circuit2 (on the right) holds the IR receiver and an indicator LED that lights up upon IR signal detection. Flipping the switch turns on the IR LED. The IR receiver then detects this signal and responds by illuminating its indicator LED. While it is possible to design the infrared circuit manually, the PC817 optocoupler offers a fully pre-engineered solution with a compact built-in integrated circuit.
Setup 4
In
the final setup, we are going to use the inexpensive PC817 Optoisolator
to "optically interconnect" and "electrically isolated" the two
circuits.
When we apply a sufficient voltage higher than the forward
voltage (1.25v) across Pins 1 & 2 with a current-limiting resistor,
the internal IR LED turns on and emits infrared light. This light then
passes through the insulating barrier. As you can see, there is no
electrical connection betwee n the input and output. The infrared light
hits the base of the phototransistor, causing it to turn on and conduct
current between its Collector (Pin 4) and Emitter (Pin 3). So, we can
control a secondary circuit just by using a beam of light.
The
input side mandates a current-limiting resistor for the optocoupler.
Without it, the LED would try to draw as much current as the power
supply can provide, quickly exceeding its maximum rating and burning
out. The closest standard value would be around 330Ω or 470Ω.
A
resistor on the output side (the collector side) is not for protection
but for operation. It "pulls up" the voltage and converts the
phototransistor's variable conductivity into a variable voltage that
your microcontroller or next circuit can read.
Unless exposed to
light, the phototransistor acts as an insulator, blocking the flow of
current. So, when the Phototransistor is OFF, the LED is off and when
the Phototransistor is ON the LED is on.
Because of this setup,
voltage spikes and noise on one circuit will not destroy or disrupt the
other circuit. So, our circuits are protected. The two circuits can
therefore use different voltages and currents because of the separation.
We can expand the capabilities of this device by adding other
components such as a transistor to the output of Circuit2, allowing it
to control even higher voltages and currents.
Please
Note: The grounds on the input side and output side must be kept
separate to maintain the isolation. This is the entire point of using an
optocoupler.
Another critical parameter to consider when utilizing
an Optocoupler is the rise time (tr) and fall time (tf). The output does
not transition instantaneously as the input logic changes states.
One
of the PC817's outstanding features is its strong electrical isolation
capabilities, with voltage ratings up to "5KV". By creating this
barrier, it safeguards expensive microprocessors and logic circuits by
making the entire system much safer and more reliable.
Technical Specifications
Now let's have a look at the technical specifications of the optocoupler.
- Packaging: PC817 comprises of four pins and is available in SMT and DIP packages.
- Forward Voltage (Vf): The forward voltage of the PC817 optocoupler input diode is specified to be 1.25V. This parameter determines the minimum voltage required for the infrared LED to operate properly.
- Forward Current (If): Absolute max 50mA, typical use 5-20mA. Forward Current (IF) is the amount of electric current flowing through a semiconductor (like a diode or LED) when it is forward-biased and conducting.
- Collector-Emitter Voltage (Vceo): The maximum allowed collector-emitter voltage of the PC817 is 80V. This specification applies to safe operation in high-pressure environments to avoid electrical failure or damage to the optocoupler.
- Current Transfer Ratio (CTR): CTR is the ratio of output current to input current (Ic / If). A CTR of 50% is common. If you put 20mA into the LED, you can get up to ~10mA out of the transistor.
- Input Current: Typically limited to under 20mA for safe operation
- Isolation: input and output are internally protected with 5kV electrical isolation.
- Operating Temperature is between -30°C~100°C
- Optocoupler's Temperature Range during Soldering: 260 degrees. It is crucial to note that exceeding the specified temperature during soldering can damage the IC.
- Switching: The PC817 has a total response time (including rise and fall times) of 18 microseconds. This fast response capability is especially beneficial for applications requiring fast switching, such as pulse signal processing or high-speed switching circuits.
- Internal Resistance: PC817 has an internal resistance of 100 ohms and a maximum power consumption of 200mW. These factors manage the energy efficiency and heat load of the equipment.
- Reverse Current: PC817 comes with internal protection from reverse current. Due to the one-way current flow nature of IR, the PC817 protects the IR from any reverse current.
For more detailed information, please consult the datasheet available here: https://github.com/tarantula3/PC817-Optocoupler/blob/main/PC817A%20Datasheet.PDF
PC817 Applications
The PC817 optocoupler is commonly used in the following applications:
- Signal isolation and transmission
- Electrical isolation circuits, capable of withstanding up to 5kV
- Isolation of digital from analog circuits
- Level Shifting: It allows a 5V circuit to control a 12V, 24V, etc., circuit
- Microcontroller I/O switching circuits, effectively maintaining circuit continuity without disconnection
- Noise coupling circuits to keep the circuit in use without any interruption
- It helps in breaking ground loops
- Switching and zero cross: In AC/DC power control circuits the Optocoupler regulates AC loads by introducing frequency-induced pulses, enabling precise control within a defined range
- Driving relays, controlling motors or AC power switches
Is an optocoupler a suitable replacement for a relay?
The
key distinction between optocouplers and relays lies in their power
handling. Optocouplers are used for low-power signal isolation, and
relays for high-power load control. While there is functional overlap,
the specific load requirements determine the optimal choice.
Using an Arduino to Drive a 12V Motor
A
12V DC motor cannot be connected directly to an Arduino pin. The
motor's "high power demand" and "electrical noise" can damage the
sensitive microcontroller.
To mitigate this problem: The Arduino
controls the 12V motor through a PC817 optocoupler. A HIGH signal on the
Arduino pin activates the optocoupler's LED, turning on its internal
phototransistor to power the motor. A LOW signal turns it off.
The
PC817 electrically isolates the Arduino's 5V ground from the motor's 12V
ground, protecting the Arduino from motor-induced noise.
Why do optocouplers fail?
Common causes of optocoupler failures include:
- Voltage Surge: High-voltage surges can breach the optocoupler's isolation barrier, resulting in permanent failure
- Overcurrent: Exceeding the specified current on either the input or output terminal can damage the optocoupler's internal components
- Aging: The performance of an optocoupler degrades over time as its internal LED ages and loses efficiency
- Environmental factors: Harsh environmental conditions, particularly extreme temperature and humidity, can lead to performance degradation or failure of an optocoupler
- Assembly: During circuit assembly, ensure that all connections and solder joints are stable and intact
Thanks
Thanks again for checking my post. I hope it helps you.
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Video: https://youtu.be/Rj9H0beMQq8
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GitHub: https://github.com/tarantula3/PC817-Optocoupler/
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Blog1: https://diy-projects4u.blogspot.com/2025/12/PC817.html
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