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A Schmitt Trigger for sensor level detection

We have seen hysteresis in sensors: we can use hysteresis in the comparator to eliminate noise. A Schmitt trigger is a comparator circuit that incorporates positive feedback. As a result, the comparator decision level for a low-to-high transition is higher than the comparator decision level for a high-to-low transition.

For a signal with some noise, a plain comparator circuit may result into some transition noise in the output signal which is similar to contact bounce. In figure 1 we can see the comparator output for a clean signal. What happens with a noisy signal can be seen in figure 2.

Fig. 1: Output of a comparator when the input is clean Fig. 2: Output of a comparator when the input is noisy

This response is quite logical because on a mV scale the signal does cross the decision level multiple times before it becomes “high” in the end. Just like contact bounce, a solution is to use a low-pass filter or software way of transition detection. However, a more appropriate solution for this problem is to use a higher decision level for a low-to-high transition and a lower decision level for a high-to-low transition. The comparator that does this is called a Schmitt Trigger and has the symbol of figure 3.

Fig. 3: Schmitt trigger symbol

The circuit is the comparator of figure 4 with a positive feedback loop. As a result, the comparator level on $V_{in+}$ is different for a high $V_{out}$ than for a low $V_{out}$. Based on these two positions, we can calculate the relation between the resistors and the two decision levels.

Fig. 4: Comparator with positive feedback

In figure 5 the effective position of the resistors is drawn for two situations: where the output is high and where the output is low.

Fig. 5: Effective resistor connection in comparator with positive feedback for two states

The final response and effect is drawn in figure 6 and figure 7.

Fig. 6: Input-output relation for Schmitt trigger

Fig. 7: Illustration of how a double decision level system (Schmitt trigger) results into a noise free response

Sensor Technology TOC

These are the chapters for the Sensor Technology course:

theory/sensor_technology/std_schmitt_trigger.txt · Last modified: 2018/10/16 06:17 by