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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

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

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - 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
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.