# schmitt trigger and noise

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 ?

• Noise makes digital circuits not work. Commented Feb 2, 2020 at 2:50
• But why does noise not make them work ? Commented Feb 2, 2020 at 2:52
• 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. Commented Feb 2, 2020 at 2:52
• 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. Commented Feb 2, 2020 at 2:57
• @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."
– jonk
Commented Feb 2, 2020 at 5:30