How to use logic circuits to reduce the pulse width?

Thread Starter

Nick Long

Joined May 12, 2020
75
Thanks for all your kind help. I have tried the ericgibbs's scheme with the clamp protection. It works well.
By the way, I am also curious about how to increase the pulse width (starting from the same rising edge) through logic circuits, except for monostable chips.
 

Thread Starter

Nick Long

Joined May 12, 2020
75
The pulse-width is a function of R x C.
Increase the values of R or C for longer pulse-width.
Thanks for your rapid reply. Sorry for my unclear expression. I mean changing a narrow pulse to a longer pulse with keeping the same rising edge, as shown in the figure below.
1672976926721.png
 

MrChips

Joined Oct 2, 2009
30,701
Thanks for your rapid reply. Sorry for my unclear expression. I mean changing a narrow pulse to a longer pulse with keeping the same rising edge, as shown in the figure below.
View attachment 284616
I understand now. You want a pulse stretcher circuit.
You can do it with logic gates or discrete components but in fact you would be designing a monostable multivibrator which is already available in a single package.
 

vu2nan

Joined Sep 11, 2014
345
Hello, my friends. Happy new year!

I want to design a logic circuit to reduce the width of a pulse waveform. The expected effect is shown in the figure below. The reduced pulse width does not need an accurate duration. It only requires a much narrower than the original width with an aligned rising edge. Of course, you can also realize this function using the non-logic circuit if you want.

Thanks.

View attachment 284417
Here's a relay-based solution.

The pulse width is a function of the time constant RC.

RC = Resistance of the relay coil x Capacitance C1.

123.png

When the push button switch is pressed, the capacitor charge pulse produces a single pulse output at the relay contact.

When the push button switch is released, the capacitor is discharged through the resistor R1 in order to be ready for the next pulse.

The value of R1 may be equal to the resistance of the relay coil.

Nandu.
 
Last edited:

Ian0

Joined Aug 7, 2020
9,667
Thanks for all your kind help. I have tried the ericgibbs's scheme with the clamp protection. It works well.
By the way, I am also curious about how to increase the pulse width (starting from the same rising edge) through logic circuits, except for monostable chips.
Take @ericgibbs 's circiuit, and add an OR gate on the input. Then take the output back to the spare input of OR gate.
You can then use deMorgan's laws to reduce it to two or 1 gate, or to accommodate negative or positive edge triggers and negative or positive output pulses.
 

MisterBill2

Joined Jan 23, 2018
18,167
There is another way to deliver a pulse of either longer or shorter than the single trigger pulse, it was invented by an individual who I worked with. He called it a "Flop Shot" . It was implemented with a CD4013 dual flip-flop IC. The Q output was fed back to the R (reset) input thru a high value resistor, with a capacitor also connected from that R input to common. A short positive pulse on the set or trigger input results in the Q output going high until the capacitor charges and the Reset becomes high enough to assert the reset, at which time the Q high output drops. The benefit is the other FF is available for other logic use, and the CD4013 was much more available than other devices at the time. Component availability is important when designing things for production runs.
 

Ian0

Joined Aug 7, 2020
9,667
These are the classic two-gate monostables, which can take input pulses that are either shorter or longer than the output pulse. The inverter or buffer on the output can be a spare gate of the same type as the first gate.
The single-gate circuit on the left works only with input pulses shorter than the output pulse. It requires a negative trigger. Using an OR gate produces one with a positive trigger.49C66077-2800-483A-849E-07E8DB8B7DFC.jpegBecause of the regenerative action, Schmitt triggers are not necessary.
Pulse width is not that accurate as it depends on the threshold level. If you need accurate timing use the 74HC123. Input protection resistors can be used on the second gate as discussed above.
 
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