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Schmitt Triggers ​

What It Does

Schmitt triggers are like more advanced comparators. They allow you to see if a voltage is above a certain level or not, and most importantly they compensate for noise.

Get clean outputs ​

Schmitt triggers are great when you need clean logic levels. Plain comparators can give noisy outputs when the two levels it compares are almost the same. Schmitt triggers compensate for this with something that is called "hysteresis".

  • The input has a noisy input simulation.
  • Notice the hefty fluctuations round 0V at the basic comparator.
  • Notice no fluctuations with the Schmitt trigger.

Inverting Schmitt trigger ​

You will see often Schmitt triggers that also invert. If you start building more complex circuits you will see why that is somehow more useful often. The 40106 is a common IC that has 6 build in inverting Schmitt triggers !

Formula

  • Vout = V+ when Vin < |Vth|+
  • Vout = V- when Vin > |Vth|-
Details
  • Vout is the output voltage of the Schmitt trigger
  • Vin is the input voltage to the Schmitt trigger
  • V+ is the positive supply voltage of the Schmitt trigger
  • V- is the negative supply voltage of the Schmitt trigger
  • Vth+ is the positive threshold voltage of the Schmitt trigger
  • Vth- is the negative threshold voltage of the Schmitt trigger
  • Lookup the threshold level in the IC datasheet.

With dedicated IC ​

  • Lookup the thresshold levels of your Schmitt trigger in the additional datasheet.

With comparator ​

When you just need one Schmitt trigger this building block is great. The configuration consists of a basic comparator plus a feedback resistor (see 100k in simulation), that creates that hysteresis effect to remove noise. What I also like about this one compared to the dedicated IC is that you can completely configure how the Schmitt trigger behaves.

  • Use larger feedback resistors to decrease the hysteresis.
  • Use smaller feedback resistors to increase the hysteresis.
  • I use often feedback resistors between 100K and 1M Ohm.

Warning

This circuit looks quite similar to the non-inverting-amplifier. Please notice the difference.

Non-inverting Schmitt trigger ​

Formula

  • Vout = V+ when Vin > |Vth|+
  • Vout = V- when Vin < |Vth|-

With comparator ​

Warning

This circuit looks quite similar to the inverting-amplifier. Please notice the difference.

With transistors ​

For transistor purists.

With dedicated IC ​

The non-inverting Schmitt trigger ic is more difficult to source at local shops in my experience, so you probably will use two inverting Schmitt triggers in series.