Switching value: TTL (0/1) high and low level
The circuit on the left analyzes
DTR input 0, because the conduction voltage drop of the Schottky diode is very small, DTR0 is detected as a low level
DTR input is higher than 3.3v, and DTR0 is detected as a 3.3V pull-up resistor.
This circuit is used to detect low level
Analysis of the circuit on the right
Optocoupler isolation circuit, the conduction current of the optocoupler is 4~10mA, adjust the
resistance according to the input. The middle parallel resistor is used to limit the current, and the Schottky diode is used to resist the surge and avoid damage to the optocoupler.
Reference:
Talking about The design of the optocoupler circuit in the switching value acquisition circuit
The circuit design of switching value input detection and output (transfer)