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Project Notes

#489 555 Rising Edge Detector

Using the 555 timer and AC-coupling to produce a fixed-width rising edge trigger pulse.


Here’s a quick demo..



This demonstrates a circuit for generating a pulse on a rising 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 by AC-coupling the input (with capacitor C1) feeding 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

About LEAP#489 555 Timer
Project Source on GitHub Return to the LEAP Catalog

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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