Schmitt trigger – NPN BJT

Table of Contents

Schmitt triggers are comparator circuits that use a signal voltage and hysteresis (a range of voltage where the output stays in the last state it was put into) to determine the output state. The output switches more solidly high or low than just a simple comparator.

Schmitt trigger hysteresis graph diagram by Electronzap Electronzapdotcom
Schmitt trigger hysteresis graph diagram by Electronzap Electronzapdotcom
  • Hysteresis:
  • Thresholds (2 of them):  The voltage points needed to be reached, in order to change the output.

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Diagram


Schmitt trigger 2N3904 NPN BJT demonstration circuit schematic diagram by Electronzap Electronzapdotcom
Schmitt trigger 2N3904 NPN BJT demonstration circuit schematic diagram by Electronzap Electronzapdotcom

 

Notice how the LED naturally wants to be on. When it is off, that means that the transistor next to it is conducting enough to drop the voltage below the LED’s forward voltage.

The transistor next to the LED normally wants to conduct because of the pull up resistor at it’s base. To turn it off, the transistor closer to the trimpot has to be conducting as that will lower the base voltage.

Video

Schmitt trigger circuit using NPN Bipolar Junction transistors BJTs by electronzap electronics


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