Optocouplers are compact in size, have a long service life, and can operate across a wide temperature range. They offer excellent anti-interference performance and provide complete electrical isolation between input and output, making them ideal for use in various electronic devices. Due to their non-contact operation and reliable isolation, they are widely applied in circuits such as isolation circuits, load interfaces, and household appliances. Below are some of the most common application scenarios.
1. **Switching Circuit Configuration**
In the circuit shown in Figure 1, when the input signal ui is low, transistor V1 is off, causing the LED in the optocoupler B1 to not conduct, resulting in high resistance between Q11 and Q12, which simulates an "off" switch. When ui is high, V1 turns on, activating the LED in B1, reducing the resistance between Q11 and Q12, effectively turning the switch "on." When ui is low, the switch does not conduct, allowing for a high-level conduction state. Similarly, in the circuit of Figure 2, when there's no signal (ui low), the switch is on, creating a low-level conduction state.
2. **Logic Circuit Construction**
Figure 3 shows an "AND gate" logic circuit with the logical expression P = A · B. The two phototransistors are connected in series, so the output P is high only when both inputs A and B are high. Using similar principles, other logic gates like OR, NAND, and NOR can also be constructed.
3. **Isolation Coupling Circuit**
The circuit in Figure 4 is a typical AC-coupled amplifier. By selecting an appropriate current-limiting resistor Rl for the LED in B4, the current transfer ratio can be kept constant, ensuring linear amplification of the circuit.
4. **High-Voltage Regulator Circuit**
As shown in Figure 5, a high-voltage regulator can be formed using an optocoupler. The drive transistor must have a high breakdown voltage (e.g., 3DG27 in the diagram). When the output voltage increases, the bias voltage of V55 rises, increasing the forward current of the LED in B5. This causes the voltage across the phototransistor to drop, reducing the bias voltage of the adjustment transistor, increasing its internal resistance, and stabilizing the output voltage.
5. **Automatic Hallway Lighting Control Circuit**
Figure 6 illustrates a hallway lighting control circuit. It includes four analog electronic switches (S1–S4): S1, S2, and S3 are connected in parallel to increase driving power and improve anti-interference capability. When powered on, the circuit drives a bidirectional thyristor via R4 and B6, controlling the hall lighting H. S4 and an external photoresistor Rl form an ambient light detection circuit. When the door is closed, the reed switch KD is opened by the magnet, keeping S1–S3 off. At night, when the owner opens the door, the magnet moves away, closing KD. The 9V power supply then charges C1 through R1. Once C1 reaches 9V, it triggers the LED in B6, turning on the thyristor and lighting the lamp. After the door closes, the charging stops, and C1 discharges through R3. After a delay, the voltage drops below 1.5V, turning off S1–S3, cutting off B6, and extinguishing the light, achieving a delayed shut-off function.
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