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NE555 as Monostable – Retriggerable

Retriggerable - Diode and Cap

Other solutions avoid to use the global reset.
Among the them here we expose two major ones: one with diode and filtering cap, one with bjt.

The goal still remains the same: the timing cap must be discharged at the new trigger input.

Let's begin with the diode and filtering cap: quickly note that now pins 2 and 6 are shorted, while pin 7 is unconnected because not necessary anymore: indeed the cap will be discharged through the filtering cap (to kill every DC signal) and the diode, which primary receives the trigger signal.

And here the captured waveform (with zooms onto the key points)

Retriggerable with Diode 1
Retriggerable with Diode 2
Retriggerable with Diode 3

The first shows to one-shot cycles: at left a complete one, while at right a retriggered one.

They are zoomed respectively in the second and the third picture.

As you can see, after the retriggering signal (yellow) the time ON differs from the fixed period; that's because with this kind of circuit is possible to have a valid trigger impulse without the complete discharge of capacitor.

So here why the behavior seems to deviate from the theoretical description.





Another solution, the last one we're going to show, does use a bjt as a switch for the cap discharging.

Retriggerable - BJT

A bjt is in parallel with the capacitor, and it's commanded by a switch.

The pnp configuration is required, and a common signal transistor can be used.

When the switch is pressed then pin 2 gets a valid trigger input while simultaneously the bjt is electrically closed, so the cap has a path to discharge.


The waveforms can be like the ones shown below:

Retriggerable with BJT 1
Retriggerable with BJT 2







Retriggerable with BJT 3
Retriggerable with BJT 4







In order:

  1. a complete one-shot cycle followed by more retriggered input signals;
  2. a zoom of the fixed time ON period;
  3. a zoom onto the retriggered zone, with an instant lost;
  4. a particular of the lost instant (yellow is input, and blue the output).

And that's all for the monostable multi-vibrator circuit with NE555!
Now explore by yourself, especially noting how the behavior is affected not only by the circuital topology but even by changing the value of external capacitor.

Hoping your curiosity has grown...

See you on next article.