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Project Notes

#075 Monostable 74LS122

Simple pulse trigger using 74LS122 monostable with external timing configuration.

The Build


The 74LS122 monostable multivibrator in this circuit has pulse width programmed by selection of external resistance and capacitance values R2 and C2. Pulse width is given as:

tW = K * R2 * C2
=> where K is nominally 0.45. If C2 is in μF and R2 is in kΩ then tW is in seconds

For 33μF and 100kΩ then pulse width is 1.485s. This is intentionally large so that a visible pulse produced.

Once triggered, the basic pulse width may be extended by retriggering the gated low-level-active (A) or high-level-active (B) inputs, or be reduced by use of the overriding clear.

In this circuit we have a pushbutton to trigger the pulse. LEDs are separately connected to output Q and Q complement, so they will toggle as the pulse goes high and low respectively.


The Breadboard

The Schematic

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.