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74HC14 for debouncing - question about the type of capacitor

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bodzio_stawski:
Hello!

I am trying to analyze some layout solutions aimed at reducing the button reflection effect (especially with 74HC14 IC).

I will use one of the most common system solutions from the internet:




I am aware that eliminating debouncing by software solutions is the most flexible solution to the problem anyway. However, insisting on this circuit, I am thinking about the type of capacitor.

A schematic like that suggests that when the breaker is closed, the capacitor will discharge through R2. The capacitor, by maintaining some potential on the line for a while, will mask any bouncing of the switch.

My question:

The use of a capacitor, as well as the nature of the disturbance it is supposed to eliminate, would suggest that it should be a polarized capacitor with quite a large capacity.


Meanwhile, I often find diagrams with a non-polarized capacitor. Even without a capacitor discharging resistor, with which it lives in symbiosis and determines its discharge rate. If we can afford the circuit for reducing debouncing effect, why not use, for example, tantalum capacitors instead of 100nF ceramic, which is sensationally often seen on ready-made PCBs?

retiredfeline:
HC being CMOS has high imput inpedance so you can raise the value of the charging resistor and bring the capacitor value down to sub-µF. You just need enough time constant to tide over contact bounce, not the entire depression.

jfiresto:
+1. I would favor the first circuit. Its CMOS inverter allows you to use a small C and a large R2 to produce the desired RC switch debouncing time, for example, a 100nF capacitor and a 220K resistor.

RoGeorge:
Assign some values of those RC in the schematic to be clear.

Never seen uF big caps for debouncing.  Beware the same circuit can be used to generate a Reset signal, and that is where usually uF caps will be seen.  For a reset circuit the RC constant time has to be much longer than in a button debounce, because it has to give time for the power supply to stabilize its voltage before the reset will be de-asserted by the reset circuit made out of the RC + trigger Schmitt.

For a casual button debouncing (not the reset button) debouncing time constants are in the range of ms to tens of ms when done in software.  The RC constant doesn't need to be that long.  Usually caps in nF to tens of nf range are enough for hardware debouncing.

Without values of the RC parts is hard to tell what would be best.

bodzio_stawski:
Ok, so on request, I provide examples of values, at the same time following the suggestion of jfiresto.



By lowering the capacitance of the capacitor and increasing the resistor R2, the result would be T = 22 ms.
So in this case it looks that we can simply save money thanks to the smaller ceramic capacitor.


--- Quote from: RoGeorge on August 14, 2022, 07:44:46 am ---Never seen uF big caps for debouncing. 

--- End quote ---

As I took a closer look, the larger (uF) capacitors are rather used together with the 74LS14 instead of the 74HC14 - that's right.


But I am still wondering, according to the topic, about the type of capacitor. I still can't understand why sometimes ceramic capacitors are used, and other times polarized capacitors are placed - as below:


(from here): https://www.researchgate.net/figure/Switch-interface-Figure-4-Switch-interface-equipped-with-debouncing-circuity_fig2_344781796

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