Brief Schmitt Trigger LM358 Op Amp Comparator Circuit

Schmitt triggers use a signal to determine whether the output is high or low, like regular comparators do. However, they also have a relatively small middle ground signal voltage range where the output stays in whatever state it was last put in to.

Schmitt trigger non inverting op amp comparator circuit using LM358 operational amplifier schematic diagram by electronzap
Schmitt trigger non inverting op amp comparator circuit using LM358 operational amplifier schematic diagram by electronzap

In the op amp schmitt trigger comparator circuit shown here…

  • Reference voltage: Usually a fixed voltage, that you want to compare a signal voltage with. In this example circuit, it is half of the supply voltage. So, with a 5V supply, it is 2.5V.
  • Signal: The adjustable voltage is applied to the non inverting input.
  • Feedback: Added to the signal voltage, the output voltage is fed back to the non inverting input. This is known as positive feedback. The output pulls the + (non inverting) input voltage close to what the output voltage is.
    1. A high output will pull up the signal voltage up a bit. That means it has to go down a bit extra lower than half way to drop below 2.5V, and set the output low.
    2. A low output will pull down the signal voltage a bit. That means that the signal will have to go up a bit more than half way to set the output high.
    3. That extra range that the signal has to travel to change states, is called hysteresis.
    4. Schmitt triggers are voltage controlled switches that have hysteresis.

Hysteresis helps avoid erratic switching that commonly occurs when there is a solidly set voltage where the output changes states.

Brief Circuit Schematics with Short Video List of Pages

Assorted integrated circuits (ICs) kit. Included is the LM358, NE555 timers and other commonly known ICs. I have previously covered some of the other ICs in YouTube videos. It is an Affiliate link ad that supports this channel.

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