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Project Notes

#490 555 Falling Edge Detector

Using the 555 timer, BJT inverter and AC-coupling to produce a fixed-width falling edge trigger pulse.


Here’s a quick demo..



This demonstrates a circuit for generating a pulse on a falling edge. It uses a 555 monostable circuit to generate a clean output pulse of fixed duration. The output pulse is active-low logic, but this can be inverted in a following stage if required.

The input edge detection is achieved with a PNP inverter being AC-coupled (via capacitor C1) to the 555 timer’s threshold input. The threshold is normally pulled low (R3), and a diode (D1) is included to absorb much of the falling edge pulse.


The scope trace below shows the pulse generated on the rising edge of the input. The output pulse is around 250ms, close to the calculated 242ms.

  • CH1 (yellow) - input signal
  • CH2 (blue) - RC filtered pulse
  • CH3 (red) - output






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.