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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.

• A light sensor is connected to a normal NOT gate.
• When it gets dark, the street lights come on.
• When the street lights are on, it gets lighter so the sensor tells the lights to turn off again.
• 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!
• This is solved by using a Schmitt NOT gate.
• When it gets dark, the lights come on.
• It gets a bit lighter but not enough to make the lights turn back off.
• The next morning, it gets a lot lighter so the lights turn off.
• When the lights turn off, it gets a bit darker but not enough to make the lights flash back on again.
• 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.

• When the input is falling, it must go below 2.2 Volts for the output to go high.
• When the input is rising, it must go above 3 Volts for the output to go low.
• This non symetrical behaviour is called hysteresis.

Compare normal and Schmitt inverters ...