It is a threshold switching circuit with abrupt input and output characteristics. This circuit is designed to prevent the slight change of input voltage (below a certain threshold) from causing the change of output voltage. When the input voltage changes from low to high and reaches V+, the output voltage suddenly changes, while the input voltage Vi changes from high to low and reaches V-, so the output voltage changes slowly. It can be seen that it is especially suitable for circuits that need a certain delay when starting.

Compared with the logic symbol of NAND gate, the logic symbol inside ic is slightly different. It adds a box figure, which represents an important hysteresis characteristic of Schmidt trigger. Hysteresis characteristic means that when the input terminals are connected in parallel as NOT gates, their input-output characteristics are as follows: When the input voltage V 1 rises to VT+ level, the flip-flop flips and outputs a negative jump; After a period of time, when the input voltage falls back to VT+ level, the output does not return to the initial state, but the input V 1 needs to continue to fall to vt- level, and the output will flip to high level (positive jump), which is expressed by the formula: vt+-vt-= △VT, and△VT is called the hysteresis voltage of Schmidt trigger. △VT is related to the supply voltage of IC. When the power supply voltage rises, △VT increases slightly, and the value of △VT is generally around 3V. Schmidt trigger is often used to shape the pulse waveform and make the rising or falling edge of the waveform steep. Sometimes it is also used for voltage amplitude discrimination, and it is also a very common device in digital circuits.