Push button contact debouncing

I would go for a 10 millisecond or so time constant, minimum. That means the product of resistance and capacitance is at least .01, with resistance in ohms and capacitance in farads.

One combination would be 10k and 1uF.

Thanks John. If I replace the 4093 with a 4069 or 4049, does the circuit still work? The data book says the "rise and fall time" of a

4013 clock input should be 5uS or less. Can the outputs of a 4069 achieve that?

Allen

It depends on the switch. There was a similar discussion a while back(hell, maybe a year ago). Some switches are very bad and have need several hundred miliseconds while others need just a few. Maybe you should do some tests or if you have an oscope you could look at how long it takes to settle down.

Obviously if you don't use need "rapid fire" then you can increase the time. So if, say, the button will not be pushed to rapidly you can increase the time and the only thing that will happen is if its pushed more than once in that time frame it will be ignored.

Alternatively you could get switches that are designed not to bounce or have the circuitry inside them the prevent bouncing. (don't remember the details of these switches but remember them being discussed a while back)

Jon

An _old_ favorite is to use an SPDT switch into an RS flip flop. Schadow switches and the like work very well for this. The nice thing about the RS is there is no time constants involved. The only possible fault would be if the switch bounced SO BAD it actually toggled both contacts.

Software debouncing works well if you try a microcontroller or you might be able to find some old stock switch debouncer chips. Hysteresis around an inverter can work well too.

GG

Not to forget the debounce circuit in Don Lancaster's CMOS Cookbook.... You'd get 6 per 4050 package as well.

Also, when entering 'debouncing' into google, the first hit is the semi-famous:

Which seems to have been updated quite recently (since I last looked at it, in fact.)

Jon

Okay. Now that I've looked, the "update" may be more a matter of adding some advertising for his new seminars at the end of it.

Jon

Well, I have thought about that too, but SPDT push buttons are hard to find here and the only one I can get is a micro-switch with a short lever. The whole will be covered under a thin membrance with word printed on it, so the switch has to somejow be small and able to lay flat on the PCB.

At the moment the IC count is 10 x 4013 and 5 x 4093 (or 4 x 4050) plus 20 pieces of 24V 10A relays. If I really can't squeeze all that many components onto the PCB, I'll have no choice but to turn to a PIC

16F877 or similar. Any other suggestion for the controller that might fit in? Software debouncing is so much easier!

Allen

Thanks Jon, that was great stuff, I will read it in my free time.

Allen

Since we are on this topic, I have been wondering for a long time if this modified form of RS flip flop has any debouncing effect ?

4011 __ +-------+--------------| \\ | | | )o---+------ Q | o | +--|__/ | |=|> | |+. _-+ | o | ``-.'' | | +'' ``-. | | +--| \\ `+ | | |\\ | )o---+------ /Q | +-| >O---------|__/ | |/ === 4069 GND

(created by AACircuit v1.28.6 beta 04/19/05

Regards,

Allen

--
None whatsoever.  Plus, when the switch is open you have floating
CMOS inputs, which is a definite no-no.

I'd use a 40106 to get six sections - it's got the schmitt trigger input, which is highly recommended for this sort of application, because of the hysteresis.

Cheers! Rich

No matter. The ad isn't until the end, and it's really a rather enjoyable read. :-)

Thanks! Rich

The problem with using either a 4069 or 4049 is that they do not have a snap action (Schmitt trigger), but just amplify the slow rise from the filter capacitor. You can expect a gain of at least 10, probably several times that much, but the rise time of 10 ms at the cap will only be shortened to .1 to 1ms, much slower than the 5us limit. You could cascade several inverters, to speed up that rise, but a Schmitt trigger is definitely the clean way to get a fast rise time out of a slow rise time. An example would be

74HC14.

Yes. I was well past his current discussion... about three decades ago. Some years ago, maybe two?, I had read it an earlier version to see if there was anything new or not already entirely crystal clear to me. There wasn't anything new. So this time around I was really just skimming to look for anything added of note. The first page talked about revision dates and I was curious, of course. I didn't see anything and quickly found myself at the advertisement at the end and that's when it instantly dawned on me that the ad was probably more of the reason why the revision dates were there than anything else. I'd like a note of that up front, so that returning readers could save a little time. In any case, I thought it was worth mentioning here so that any other past-reader of an earlier version had some idea what to expect.

I completely agree with you that it is worth reading, if you aren't already familiar with some of the details involved. I'd recommend a somewhat different discussion about the software, where software is appropriate, though.

Jon

74HC14.http://w3.id.tue.nl/fileadmin/id/objects/E-Atelier/doc/Datasheets/74h...- Hide quoted text -

I do have some 74HC14 but my application will be working at 12V but the max voltage of 74HCxx is only 6V, so it cannot be substituted as I want. But Rich has suggested using 40106. I think this should be my choice for this application.

Allen

I agree with your reasoning. I didn't see that your supply was 12 volts.

Another alternative for a 12 volt supply would be:

Oops, I am sorry about that as I didnt state clearly on the schematics. but I did mention 12V in the text.

Thanks.