Optocouplers Explained: How Phototransistor Isolation Works with PC817 Circuits

This video explains what optocouplers are, how they isolate two circuits using an internal LED and phototransistor, and how to design simple circuits with LEDs, IR emitters, LDRs and the PC817 optocoupler for safe, isolated signal transfer.

Introduction to Optocouplers and Electrical Isolation

Optocouplers or opto isolators are compact devices that allow signals to pass from one circuit to another without a direct electrical connection. The video explains that the device contains an LED on the input side and a detector such as a phototransistor on the output side. This arrangement provides galvanic isolation, so voltage spikes or noise on one side do not affect the other. The tutorial uses the PC817 as a concrete example of a standard optocoupler and shows how the device can be combined with other components to drive higher voltage loads and automate circuits safely.

Inside an Optocoupler: LED, Phototransistor and Pinout

The optocoupler version featured in the video is a four pin device with an input LED and a transistor output. Pin 1 is the anode, pin 2 is the cathode, pin 3 is the collector and pin 4 is the emitter. The LED emits light that is detected by the phototransistor, which then allows current to flow in the secondary circuit. The base pin is not accessible in this configuration. The video also explains that the LED and transistor are enclosed inside the package, so you cannot see the light or the semiconductor elements directly.

How Light Couples the Circuits: From PN Junctions to Photoconduction

The video digresses into a simplified explanation of PN junctions. When the LED is on, photons create electron movement across the depletion region in the phototransistor. This causes the transistor to conduct in the secondary circuit. When the LED is off, no photons are generated and the phototransistor remains off, blocking current. This light based control is what yields isolation while enabling signal transfer.

Simple Circuits Demonstrated: LDR and White LED

The first example uses a light dependent resistor LDR and a white LED. The LED is powered from a 9 V supply and is designed to draw about 20 mA with a forward drop around 3 V. The LED current is set by a series resistor of approximately 330 ohms plus 22 ohms in parallel, giving around 352 ohms. The secondary side includes a red indicator LED and an LDR facing the primary LED. The LDR resistance changes dramatically with light, providing a low resistance of around 70 ohms when illuminated. The resistor on the secondary side is chosen to allow about 20 mA through the LED by subtracting the drops for the LED and the LDR. The demonstration shows how ambient light can inadvertently trigger the circuit, which is addressed by blocking ambient light with tape. This is a good example of how optocouplers can transfer a signal while keeping the two circuits electrically separate.

Infrared Based Isolation Circuits

The video replaces ambient light triggering with infrared sender and receiver to avoid interference from visible light. The input on the primary side uses an IR LED with a forward voltage of about 1.2 V and chosen current. A red LED indicator is included on the primary side. The secondary side uses an IR receiver LED with a small forward drop and an indicator LED to show activation. Resistors are chosen to yield roughly 20 mA of current in both the emitter and receiver paths. The demonstration shows that the IR beam can reliably trigger the output circuit when the emitter is on and the receiver detects the light.

PC817 Optocoupler: Data Sheet and Circuit Design

The third circuit centers on the PC817 optocoupler. The input side uses an internal LED rated for 1.2 V and 20 mA. The secondary LED remains a 2 V red LED. The input resistor is computed to ensure the LED current is 20 mA, using a 270 ohm plus 22 ohm total of 292 ohms. The output side uses a 9 V supply with the collector tied to the positive and the emitter to the negative. A resistor is necessary to limit current, as the optocoupler is rated for a maximum 50 mA on the output. The video references the data sheet curve that relates collector current to collector emitter voltage and shows values such that 20 mA LED current yields about 2 V C-E voltage drop. Therefore the resistor must be about 250 ohms. The presenter uses a 100 ohm and a 150 ohm combination to form 250 ohms. When the input LED is activated, the phototransistor conducts and drives the output LED on the secondary side.

Takeaways and Further Learning

The video ends with a brief wrap up and pointers to related content on the Engineering Mindset channel. It emphasizes the isolation provided by optocouplers, introduces the concept of extending output capabilities by adding a transistor or other components, and invites viewers to explore more electronics engineering topics.