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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

About LEAP#490 555 Timer
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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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