It appears you are referring to the **NET81**, which is most commonly identified as a **High-Speed Photocoupler (Optocoupler)**, often produced by manufacturers like *Cosmo Electronics* or *Sharp*. In electronics, these parts are used to transfer electrical signals between two isolated circuits using light. --- ### 1. Key Internal Components The NET81 consists of two primary electronic elements housed within a single 4-pin or 6-pin package: | Component | Function | | :--- | :--- | | **Infrared LED** | The input side. It converts the electrical input signal into light. | | **Phototransistor** | The output side. It detects the light from the LED and converts it back into an electrical current. | | **Dielectric Insulation** | A physical barrier (usually silicone or air) that prevents high voltage from jumping from the input to the output. | --- ### 2. Technical Specifications The "NET81" series is known for being a general-purpose, high-isolation device. Here are the typical electrical characteristics: * **Isolation Voltage:** Usually rated at **5000Vrms**, meaning it can protect low-voltage microcontrollers from 5000V spikes. * **Current Transfer Ratio (CTR):** Typically ranges from **50% to 600%**, depending on the specific sub-rank (A, B, or C). * **Response Time:** Fast switching speeds (around 4μs to 18μs), making it suitable for signal transmission. * **Package Type:** Most commonly found in **DIP-4** (Through-hole) or **SMP-4** (Surface mount). --- ### 3. Common Applications How the NET81 is used in electronic design: 1. **Switching Power Supplies (SMPS):** It provides feedback from the secondary (output) side to the primary (high voltage) side while keeping them electrically separated. 2. **System Isolation:** Used to protect sensitive I/O pins of an Arduino or ESP32 from high-voltage peripherals like motors or relays. 3. **Noise Suppression:** Because the signal is transmitted via light, electrical noise from a motor cannot travel back through the optocoupler to the control circuit. --- ### 4. Typical Pinout (DIP-4) | Pin Number | Name | Description | | :--- | :--- | :--- | | 1 | Anode | Positive side of the internal LED | | 2 | Cathode | Negative side of the internal LED | | 3 | Emitter | Negative side of the Phototransistor | | 4 | Collector | Positive side of the Phototransistor | ---