Several common optocoupler circuits
The optocoupler has small volume, long service life, wide operating temperature range and strong anti-interference performance. Non-contact and the input and output are completely electrically isolated, so it is widely used in various electronic devices. Optocouplers can be used in circuits such as isolation circuits, load interfaces, and various household appliances. The most common application circuits are described below.
1. Composition of the switching circuit 
In the circuit of Fig. 1, when the input signal ui is low, the transistor V1 is in an off state, the current of the light emitting diode in the photocoupler B1 is approximately zero, and the resistance between the output terminals Q11 and Q12 is large, which is equivalent to the switch being "off". When ui is high, v1 is turned on, the LED in B1 emits light, and the resistance between Q11 and Q12 becomes smaller, which is equivalent to the switch being "on". When the Ui is low level, the switch does not pass, so it is a high level conduction state. Similarly, in the circuit of Fig. 2, when there is no signal (Ui is low level), the switch is turned on, so it is a low-level conduction state.
2. Composition logic 
The circuit of Figure 3 is an AND gate logic. Its logical expression is P=A. B. The two photosensitive tubes are connected in series. Only when the input logic level A=1, B=1, the output P=1. In the same way, it can also form logic circuits such as "or gate", "NAND gate" and "NAND gate".
3. Isolating coupling circuit 
The circuit is shown in Figure 4. This is a typical AC coupled amplifier circuit. Appropriately select the luminous circuit current limiting resistor Rl, so that the current transmission ratio of B4 is a constant, which can ensure the linear amplification of the circuit.
4. Forming a high voltage regulator circuit 
The circuit is shown in Figure 5. The drive tube needs to use a transistor with a higher withstand voltage (the drive tube is 3DG27 in the figure). When the output voltage increases, V55
The bias voltage is increased, and the forward current of the light-emitting diode in B5 is increased, so that the voltage between the photosensitive tubes is reduced, the bias voltage of the adjusting tube is lowered, and the internal resistance is increased, so that the output voltage is lowered, and the output voltage is kept stable.
5. Forming the hallway lighting automatic control circuit 
The circuit is shown in Figure 6. A is four sets of analog electronic switches (S1~S4): S1, S2, S3 are connected in parallel (which can increase driving power and anti-interference ability) for delay circuit. When it is powered on, it drives bidirectional thyristor via R4 and B6. VT, VT directly controls the hall lighting H; S4 and the external photosensitive resistor Rl constitute an ambient light detecting circuit. When the door is closed, the normally closed reed switch KD mounted on the door frame is subjected to the magnet on the door, its contact is broken, and S1, S2, and S3 are in the data open state. At night, the owner went home to open the door, the magnet was away from the KD, and the KD contact was closed. At this time, the 9V power supply is rectified and charged to C1 via R1. The voltage across C1 rises rapidly to 9V. The rectified voltage is illuminated by S1, S2, S3 and R4 to trigger the LED in B6 to trigger the conduction of the two-way thyristor. VT also leads Pass, H lights up, to achieve automatic lighting control. After the door is closed, the magnet controls KD, the contact is disconnected, the 9V power supply stops charging C1, and the circuit enters the delay state. C1 starts to discharge R3. After a period of delay, the voltage across C1 gradually drops to the opening voltage of S1, S2, and S3 (1.5v), and S1, S2, and S3 return to the off state, causing B6 to be cut off and VT to be cut off. , H goes out, realizes the delay off function.
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