Single pulse circuit needed

Other Hardware General Electronics
1

Hi all,

I need the following circuit WITHOUT ARDUINO but i can't firgure it out.

step 1 - push a button (momentary or hold).

step 2 - generate 1 single pulse of a fixed width.

The pulse must be a fixed length that is independent of the duration of the button press.

I found a circuit that should do the trick but it is without any component values. I tried to simulate it in yenka but that was no luck.

description:To produce a pulse of constant length, (no matter how long the button is pressed), the following circuit is needed:

2

Its a bad circuit, the capacitor will take out the protection diodes in
the second inverter. The issue is after the button is pushed the input to the
second invert will jump from 0V to 5V, but then decay back towards 0V
with time constant t = RC. But when the button is then released the input
of the second inverter will be forced towards -5V, and high currents will flow
into the protection diode on that input, damaging/destroying it or even
putting the chip into latch-up.

There are many safe pulse-generating circuits out there using a '555 or
monostable. (BTW if using a 555 substitute a CMOS version, much less
noisy on the supply lines).

You can also use an RC circuit and a schmitt-trigger XOR gate (not a CR circuit, note)

3

Thanks for your reaction on the circuit i posted.

I am experimenting with the monostable 555 but that circuit stays on until you reset it.

What i want is that it will be reset after a period of time no matter what you do with the button.

Do you know how that can be accieved?

4

It is easy enough to fix the problem with the original circuit (pointed out by MarkT), see attached. How long should the output pulse be and what 555 circuits have you tried? Post links or actual schematics if you want helpful comments.

pulse.jpg
pulse.jpg800×286 37.3 KB

5

Hi remington,

I've tried this circuit for example.

In the end I will use the circuit to drive a laser-module with the use of a transistor.

Like a sort of "gun". So each time you pull the trigger it will give a relative long pulse and you need to retrigger for the next pulse.

The thing with the circuit above is that the laser stays ON as long as you push the button.

I'd like to adust the pulse length from around 50ms-800ms (just long enough to see where you were pointing).

Thanks for your drawing. I will simulate it.

6

What you want is a 74LS121 this is a single shot edge triggered monostable. Being edge triggered the pulse width is totally independent of the trigger being held in any one state.

7

Yes, that is a problem with that 555 circuit. Quite a ways down on this page: http://www.bowdenshobbycircuits.info/page9.htm (under heading 555 timer Mono stable), the author shows how to isolate the push button from the input of the 555 with a capacitor, so that the 555 is edge-triggered and the output pulse length becomes independent of the button state.

8

Good link, jremington.

The one on the right:

9

Does the edge triggering mean that it's also triggered when you the release trigger....?

10

No, it only triggers on the falling edge when you press the button.

11

This did the trick!!!

Thanks guys!

timer.bmp (704 KB)

12

That first capacitor can be much smaller.