Push button de-bouncing, nothing works. [Please, only answer if you have actually done it (recently or often enough) to know for sure!]

Question

I see a lot of text on the internet and here telling us how to de-bounce a switch, in my case a push button. But I have tried all these circuits and I find that nothing other than a 555 monostable worked for me so far.

This is why I ask that you only answer if your knowledge is practical, not textbook, not hear-say, only if you have done it either recently or often enough that you have experienced the difference between theory and practice!

I have tried RC with inverter and that doesn't work except with Schmitt-trigger inverter (and I guess then Schmitt trigger buffer too), but even that not 100% reliably (I would say one extra triggering in about 100.)

My test setup is without a storage scope, just a 74LS161 counter and 4 LEDs hooked up.

My preferred solution is an S-R-latch, with 2 NAND gates (74LS00), but I had no luck whatsoever with that, regardless whether I tied the switch to high or to ground.

One thing nobody seems to mention is the darned make-before-break variety of push-button switches. Those are a total non-starter. But now that I just came home with 2 more buttons, tested and found them to be break-before-make, hooked them up to my DYI S-R-latch and I still get salves of up to more than 16 counts! Rarely, but I can't count a set of 16 without at least one double trigger.

Why am I the only one who has such bad luck? I show you my breadboard photos if you don't believe me. I put it on YouTube.

This is why I don't trust any theoretical answers, I built every circuit in the book(s) and can testify for several of them they totally and utterly fail right out the barn. I think most people who write about this don't actually have the experience.

So far RC with Schmitt-trigger is the only thing I could get working at 99% reliability, but not better, and I don't want the monostable, I really want the S-R latch (push - S, release - R).

UPDATE: people want to see schematics, so here is what I am now targeting. I have built all other circuits I could find. I will try the RC again from one of the answers. But for the SR-latch it's this:

^{simulate this circuit – Schematic created using CircuitLab}

I have three different types of double throw buttons, all I can get my my hands on, and one is MBB, doesn't work, the other is one of those with a little lever, like a morse code switch, supposedly extra precise, but nope, it bounces back and forth. The other is a button like the first but BBM -- at least most of the time.

I find that the double throw buttons are all bad. The third one is the best, but still it has horrible bouncing around, because when I hold it just right it can even do a MBB action. So, that may be the problem I am having. I just can't get a decent switch.

This means, for me the SR-latch trick is out, a no-go.

I need to use a single throw button then and try again with RC. I will work off the other answer below and report back.

UPDATE: I see a lot of negative ratings, but I don't care! I am here representing the experimental truth of the matter and not text book. Yes, we are on breadboards but these things do not work.

It probably has to do with the fact that the double throw buttons are all bad, at least in micro-switch format, they bounce around like crazy, make-before-break and apparently also bouncing between the two sides. The only circuit that I can use to do an push-release action with <10% glitch rate is the one I have described here:

Is it possible to use just a capacitor to debounce a button?

Answer 1

Your problem is not knowing input impedance chances with bias voltage on TTL.

TTL floats to a logic "1" input with Iin =0 uA at 2x Vbe drops or ~1.3V . This is unlike CMOS.

• The impedance is infinite when floating at this voltage. But at 0.4V the input impedance is 1K due the common base input bias current.
• To ensure the TTL has a safe margin or a valid "1" input > 2V the input impedance is > 100K with 20uA max at 2.7V.
• So to make it work, I biased the input towards the tipping point of input current between 1 and 0 so that the target voltage is lowered and transition time is extended during switch bounce.
• this is somewhat nonlinear Ohm's Law so I won't explain beyond this for now.
• Naturally, a Schmitt Trigger works better but still not good enough unless you use a much larger Cap.

For a 10ms bounce time and T=RC= 10ms so C=10ms/3.3k = 3uF

^{simulate this circuit – Schematic created using CircuitLab}

This is one way to do it with short twisted pair wires.

Decoupling cap MUST be near IC and much larger to absorb the transient.

some Hypothetical causes for failure.

• Using TTL instead of CMOS. (requires a different solution.)
• Not using a decoupling cap
• excessive long sig/ground wires.
• Must be Break Before Make (common type) switch.

It is much preferred to use 2 momentary switches to gnd or toggle 1 switch using TTL due to the large input current.

So learn to use CMOS instead. Input Impedance is > 10M but beware of ESD even if protected.

Answer 2

This always works.

The two push buttons can be replaced by a SPDT switch.

^{simulate this circuit – Schematic created using CircuitLab}

Here is a simulation

Answer 3

You have no series resistors on your LEDs, which is naughty and eats up noise margin, especially if the LEDs are red color.

You do not show any bypass capacitors. You NEED a bypass capacitor quite near each chip, 100nF ceramic is a good value.

Quite likely your problems will go away when you fix those things.

Answer 4

If you really have LEDs connected between logic signals and ground without current-limiting resistors, that's your problem! LSTTL does not have a lot of high-side drive to begin with, and the bare LED will completely overload it, killing your valid logic levels.

Basically, a red LED has a forward voltage drop somewhere in the vicinity of 1.7 V, and it will easily pull a high LSTTL output down to this level. However, the minimum V_IH for a TTL input is 2.0 V — which means that you're operating with a negative noise margin. It's no wonder that the counter behaves in a flaky manner.

If you really want to monitor signal levels, buffer your LEDs with 'LS04 inverters, and connect the LED between the 'LS04 output and Vcc with a 330-ohm series resistor. The LED will light up when the 'LS04 input is high, as desired.

^{simulate this circuit – Schematic created using CircuitLab}

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And just to demonstrate that I do indeed have "real world experience" in this area, here are a couple of photographs of a project I did back in the mid-1970s when I was still in high school. It's a "breadboard helper" that includes (left to right):

• A variable-speed 555 oscillator (note the metal can!)
• 8 DIP switches to ground
• 8 buffered digital monitors
• 2 debounced pushbuttons

The PCB was hand-drawn with a Sharpie pen. The circuit for the debounced pushbuttons is very simple, and you can see that the switch construction is pretty much the cheapest possible. Always worked perfectly.

^{simulate this circuit}