debounce problem

[ironbot]

Hi,
I have a plan to install a bumper switch in the neck of the robot and just set it up on breadboard to test. I used standard hardware debouncing by using an SN74AC04 hex inverter and 10k resistor and 10mf capacitor and a microswitch.

For test program I attached the zero interrupt of pin 2 and it works, but not well enough: it sometimes makes a mistake. Seems to me not a fast enough debouncing. If I press the switch repeatedly around 2 times a second, it makes mistakes sometimes.

My question: is there any way I could guarantee its operation, or a better way of implementing such a limiting switch, as it is to sit with a dc motor and just imagine if the bot is turning the neck and limiting switch won't work!

[Grumpy_Mike]

and 10mf capacitor

I bet you haven't, I suspect you have a 10uF (micro Farad ) capacitor.

If this is not giving you long enough debounce then make it bigger. However, it would be good to see the schematic of what you think is standard.

[tuxduino]

Have you considered adding a software debounce ?

[Grumpy_Mike]

Have you considered adding a software debounce ?

It is a lot more tricky on an interrupt pin, because the delay is in the ISR which is never a good thing.

[ironbot]

True, can't do software debounce for I'm on ISR, no delay() !

I drew what I call 'standard', sorry if not very clean drawing, and yes, I made a mistake, 10uF for sure!

Edit:
Sorry for I forgot: the resistor is connected to +5v.

[Far-seeker]

I drew what I call 'standard', sorry if not very clean drawing, and yes, I made a mistake, 10uF for sure!

Well as Grumpy_Mike pointed out, you should try a larger capacitor.  I don't know what you have available, but the delay of an resistor-capacitor circuit will be R*C, in seconds.

Edit: So in your present circuit you have 10*10^3 * 10*10^-6 = 0.1

[tuxduino]

Have you considered adding a software debounce ?

It is a lot more tricky on an interrupt pin, because the delay is in the ISR which is never a good thing.

Next time I need to read more carefully, I didn't notice the OP was using an interrupt 

I was wondering whether a simple state machine could be embedded inside the isr to provide a form of software debounce nonetheless...

[Far-seeker]

I was wondering whether a simple state machine could be embedded inside the isr to provide a form of software debounce nonetheless...

IMHO that would be more trouble than it's worth, unless it was somehow impossible to get a working RC circuit.

[ironbot]

Thank you all, like always my mind got clear by refering to Arduino Forums!

[JoeN]

Has anyone ever used a hardware debounce solution like the Motorola MC14490?  Are those effective?

[MarkT]

No,  the problem is the inverter!  It _must_ be a schmitt-trigger input. a 7414 not a 7404 for instance.

If you were using CMOS you could add hysteresis with a resistor feedback network, but that's tricky with TTL,  I think that 74AC04 must go.  Try a 74HC14 then you can use a much smaller capacitor (0.1uF and a 100k pull-up for instance gives 10ms time-constant)

The existing 10uF is being shorted into the switch each time it closes creating a tiny spark, which may eventually erode the contacts if the switch is small.  Adding 100 ohm or so in series with the switch will prevent such damage.  With a 0.1uF cap this damage is negligible.

[Nantonos]

I drew what I call 'standard', sorry if not very clean drawing, and yes, I made a mistake, 10uF for sure!

It is probably worth reading a bit more about debounce circuits. In particular your trigger is the wrong part, and a second resistor is advisable.

[dc42]

Is there a reason you are using an interrupt? I normally poll pushbuttons either in a tick interrupt or from a task that is scheduled to run every few ticks. This makes it very easy to debounce the switch in software. I only use an interrupt when I want the button to wake up the mcu from sleep mode, and once it has woken up I disable the interrupt and revert to polling.

[dhenry]

I drew what I call 'standard',

There is a missing resistor there.

Seriously, I have had this exact type of circuit resetting the mcu every time the button was pushed. And that mcu in question here was known to deal with interference.

I suggest that 1) you put back that missing resistor; and 2) if you have to go down with it, use a LOW-Quality (and I am not kidding here) button with high on-contact resistance. With a high-quality button, you run a substantially higher risk of resetting the mcu.

[dhenry]

The existing 10uF is being shorted into the switch each time it closes creating a tiny spark, which may eventually erode the contacts if the switch is small.

The most insightful comment in this whole thread.

MarkT is absolutely right here.