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1. Composition of the switching circuit
When the input signal ui is low level, 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 "off"; when ui is When the level is high, v1 is turned on, the light-emitting diode in B1 emits light, and the resistance between Q11 and Q12 becomes smaller, which is equivalent to the switch "on". When the Ui is low level, the switch does not pass, so it is high-level conduction. State. Similarly, when there is no signal (Ui is low), the switch is turned on, so it is a low-level conduction state.
2. Composition of logic circuits
The circuit is an "AND gate" logic circuit. The logical expression is that the two photosensitive tubes in the P=AB diagram are connected in series. Only when the input logic level A=1, B=1, the output P=1. Similarly, it can also constitute "or gate", "and non- Logic circuits such as "door", "or non-gate".
3. Composition of an isolated coupling circuit
The circuit is shown in Figure 4. This is a typical AC-coupled amplifying circuit. Appropriately select the current-limiting resistor R1, so that the current transfer ratio of B4 is a constant, which can ensure the linear amplification of the circuit.