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Schmitt Trigger circuits compare the input voltage with different reference levels for turing On and Off.
HEF40106B Data Sheet from NXP.
Also known as a Schmitt Inverter. The 40106 is a good example.
The power supply must be between 3 and 15 Volts. 5 Volts is typical.
This chip is useful because it solves a common problem.
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A light sensor is connected to a normal NOT gate.
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When it gets dark, the street lights come on.
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When the street lights are on, it gets lighter so the sensor tells the lights to turn off again.
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Now it's too dark so they come on again and the lights flash on and off all night (pretty stupid design) and I have seen this in real life!
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This is solved by using a Schmitt NOT gate.
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When it gets dark, the lights come on.
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It gets a bit lighter but not enough to make the lights turn back off.
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The next morning, it gets a lot lighter so the lights turn off.
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When the lights turn off, it gets a bit darker but not enough to make the lights flash back on again.
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This is achieved by having different on On and Off reference voltages.

Comparing normal NOT gates with Schmitt NOT gates with a 5 volt power supply.
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When the input is falling, it must go below 2.2 Volts for the output to go high.
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When the input is rising, it must go above 3 Volts for the output to go low.
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This non symetrical behaviour is called hysteresis.
5 Volt Supply |
Normal NOT Gate |
Schmitt NOT Gate |
Low Input |
Less than 2.5 Volts |
Less than 2.2 Volts ( 43 to 45% of the power supply Voltage ) |
High Input |
More that 2.5 Volts |
More than 3.0 Volts. ( 55 to 60% of the power supply Voltage ) |
Compare normal and Schmitt inverters ...
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