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Project Notes

#138 BJT Monostable

Test a 2-transistor monostable multivibrator.

Here’s a quick video of the circuit in action:



The stable state:

  • Q2 is on, biased through R2
  • Q1 is off, since R4 pulls-down the base
  • Voltage across C1 is VCC-0.7V, since Q1 collector is non-conducting and Q2 base will be at approximately one diode drop.
  • output is “low”

When input pulse received:

  • Q1 turns on
  • C1 voltage immediately shifted down by ~VCC volts
  • this pulls Q2 base negative, turning it off
  • output is “high”
  • C1 “discharges” via R2 with a time constant of 1000ms
  • output remains high until C1 pulls the base of Q2 high enough to turn it back on and reset the cycle

Increasing C1 or R2 will increase the “on” time accordingly.



The Schematic

The Build

Credits and References

About LEAP#138 BJTRC
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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

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