I know that a Schmitt trigger is used to reduce noise in a circuit, but why does noise need to be reduced.

How does noise effect the overall circuit and what benefits do you achieve by reducing noise using a Schmitt trigger ?

  • \$\begingroup\$ Noise makes digital circuits not work. \$\endgroup\$
    – DKNguyen
    Feb 2, 2020 at 2:50
  • \$\begingroup\$ But why does noise not make them work ? \$\endgroup\$
    – Dan Khan
    Feb 2, 2020 at 2:52
  • 1
    \$\begingroup\$ Because if you expect everything below 1V to mean LO, and noise makes your 0.5V signal turn into 1.1V then your LO signal suddenly isn't going to be interpreted as LO anymore. \$\endgroup\$
    – DKNguyen
    Feb 2, 2020 at 2:52
  • 3
    \$\begingroup\$ Also, schmitt trigger doesn't reduce noise. All the noise is still there. It makes the circuit less sensitive to noise. There is a difference. If it reduced noise then other devices receiving the signal on the circuit without schmitt triggers would work even if they were sensitive to noise. \$\endgroup\$
    – DKNguyen
    Feb 2, 2020 at 2:57
  • 2
    \$\begingroup\$ @DanKhan Perhaps it's easier to understand if you think of this like an LDR being used to detect when the room light is above a certain level. If you had a circuit which measured the LDR resistance very precisely and activated something when the resistance was above \$X\$ and deactivated it when the resistance was below \$X\$, then slight variations in the room light due to 50-60 Hz AC oscillations might trigger it back and forth and back again if the light level was very near the "certain level." Adding a little hysteresis helps avoid this "chatter." \$\endgroup\$
    – jonk
    Feb 2, 2020 at 5:30

1 Answer 1


why does noise need to be reduced

Noise needs to be tackled because it creates problems.

One example below:
Assume the output signal is a pulse from a circuit which counts number of people entering a door. If the noise is not addressed, it will count wrong number of people. Connecting the orange pulse directly to a pulse counter will lead to wrong counts. Connecting the green pulse(Schmidt trigger output) will give more accurate results.

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There is one good note on schmitt trigger.


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