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GCSE Logic Algebra Gates Latch NAND Array >Schmitt NOT< Truth Tables

Logic Schmitt NOT

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GCSE Combinational Logic

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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.

Schmitt Inverter or NOT Gate

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.

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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