Schmitt Trigger NPN BJT

NPN BJT Schmitt trigger using 2N3904 bipolar junction transistors learning electronics lesson 0029
NPN BJT Schmitt trigger using 2N3904 bipolar junction transistors learning electronics lesson 0029

Schmitt triggers don’t have a single input voltage where they switch output states.

For a non inverting output…

You have to raise the input voltage to a certain point (upper threshold) to change the output state from low to high. Whereas you need to lower the input voltage to a lower voltage (lower threshold), to switch the output from high to low.

There is a middle ground voltage range (hysteresis) where the output stays in whatever state it was last put in to.  The output won’t keep switching if the voltage keeps bouncing around between those 2 voltages as long as it stays well between them.

Schmitt trigger hysteresis graph diagram by Electronzap Electronzapdotcom
Schmitt trigger hysteresis graph diagram by Electronzap Electronzapdotcom

The NPN bipolar junction transistor (BJT) Schmitt trigger turns on and off suddenly when the input voltage passes through each of the 2 threshold voltages. Whereas a lone NPN BJT wired as a switch will fade on as the input voltage slowly rises above about 0.6V, and will fade off as the input voltage slowly  lowers below approx. 0.6V.

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