hardware debounce circuit

ian field

Joined Oct 27, 2012
6,536
Hi ,

I found these two hardware debounce circuits .

View attachment 132591

View attachment 132598

what is the best one ?

The one with 100k resistor will discharge the cap slowly and allow more time for glitches at the switching threshold.

The best H/W debounce is 2 gates wired as a bistable and latched by a single pole changeover button - many PCs use them for the power and/or reset button, Turbo XT clones had a latching type.
 

ian field

Joined Oct 27, 2012
6,536
thanks for your reply ..

i want to use this 1*4 keypad

View attachment 132600



does it need debounce circuit ?
There are minimalist debounce circuits, some even with no semiconductors. I'd probably still go with a non retrigerable monostable. There is the 558 (easily confused with another part) quad 555 (556 is the dual) - but I can't remember whether its retriggerable or not.

A simple capacitor will iron out glitches - as long as you include limiting resistors so the charge/discharge current doesn't sputter the contact pads.
 

AlbertHall

Joined Jun 4, 2014
12,343
If you are reading the push-button with a microcontroller then no debounce is needed. Read the switch at a period greater than the expected bounce time, say 50mS, and take the value read as the switch state. If the button has just been pushed and is bouncing when read then you will pick up the switch closure this cycle or the next - maximum 50mS delay very similar to what you will get with an external RC debounce circuit. You could get a faster reaction with the changeover switch suggested by Ian Field but that method is more complicated.
 

ian field

Joined Oct 27, 2012
6,536
If you are reading the push-button with a microcontroller then no debounce is needed. Read the switch at a period greater than the expected bounce time, say 50mS, and take the value read as the switch state. If the button has just been pushed and is bouncing when read then you will pick up the switch closure this cycle or the next - maximum 50mS delay very similar to what you will get with an external RC debounce circuit. You could get a faster reaction with the changeover switch suggested by Ian Field but that method is more complicated.
I've heard of micros with built in debounce, but there must be a few around that you still have to put the routines in the code yourself.
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

Are you using a microcontroller or not?
If you are, then use debounce code, but if not, you probably need a hardware debounce circuit.

When using a microcontroller most people use code because it does not take much code and it takes no additional hardware.
 
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