Resistor Capacitor delayed timed off Schmitt Trigger Circuit

RC time delay off schmitt trigger 2N3904 NPN BJT demonstration circuit schematic diagram by Electronzap Electronzapdotcom
RC time delay off schmitt trigger 2N3904 NPN BJT demonstration circuit schematic diagram by Electronzap Electronzapdotcom
  • Current will flow through the right side 1k resistor and LED, turning the LED on, unless…
  • Current flows through the left side 1k resistor and base of the NPN Bipolar Junction Transistor (BJT) on the right, unless….
  • Switch is closed, so that current flows through 220Ω resistor and left side NPN BJT. Turning on the left side transistor. Which holds the right side NPN BJT off, so that it doesn’t take current from the LED. LED is on.
  • Releasing switch means that the capacitor has to provide the current to keep the left side NPN BJT conducting, and thus holding right side NPN BJTs off. Nothing will change while the capacitor can provide enough current.
  • When capacitor voltage drops far enough that it can’t provide the current needed to keep the left side NPN BJT conducting, the left side transistor will suddenly stop conducting. The 22Ω emitter resistor keeps both emitters at the same voltage of the collector of the transistor that is conducting at any given time. When one transistor starts conducting, the emitters suddenly hit a voltage that turns the other one off.
    1. That will let the current from left side 1k resistor flow through the right side NPN BJT base.
    2. Which send the current from the right side 1k resistor to ground and away from the LED. LED turns off suddenly.

Video:

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