Schmitt trigger circuit composed of 555 and principle - Database & Sql Blog Articles

First, Schmitt trigger
1. The circuit consists of connecting the TH terminal (2) and the terminal (6) of the 555 timer as the signal input terminal ui, which constitutes a Schmitt trigger, as shown in Figure 10.20. The TRI collector terminal (7) of the 555 transistor is connected to the power supply VDD through the resistor R to become the output terminal uO1, and its high level can be adjusted by changing VDD; uO2 is the signal output terminal (3) of the 555. Figure 10.21 shows the operating waveform of the Schmitt circuit when ui is a triangular wave.

2. working principle

3. Main static parameters (1) turn-on level V +
During the rise of ui, the Schmitt trigger state is inverted, and when the output voltage uO is transitioned from the high level UOH to the low level UOL, the value of the corresponding input voltage is called the on level. In Figure 10.21, V + = 2VCC/3.
(2) Disconnect level V -
During the ui falling process, the Schmitt trigger state is updated, and when the output voltage uO is changed from the low level UOL to the high level UOH, the corresponding input voltage value is called the off level. In Figure 10.21, V - = VCC / 3.
(3) Hysteresis voltage ΔV
The hysteresis voltage is also called hysteresis voltage and is defined as ΔV= V +-V -
In Figure 10.21, ΔV=V +-V -=2VCC/3-VCC/3=VCC/3
If the voltage US is applied to the control terminal UCO (5), there will be V + = US, V - = US / 2, ΔV = US / 2, and the US is changed, and their values ​​are also changed.
It should be noted that the output level of the Schmitt trigger is determined by the input signal level. The meaning of the trigger is that when ui rises from low level to V + or from high level to V - The positive feedback process inside the circuit causes the uO to jump. Therefore, the circuit shown in Figure 10.20 is more accurate. It should be called an inverter with Schmitt triggering characteristics, because when ui=UiL, uO=UOH, ui=UiH, uO=UOL, and the realization is non-inverted. The logic function is not the trigger in the sense of the previous chapter - the flip-flop.

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