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I'm trying to figure out the way of debouncing encoders and tact buttons with Schmidt triggers, but I'm confused with 2 existing schematics:

schematic

simulate this circuit – Schematic created using CircuitLab

Which of those two options are right? Values are shown for 5.0V inputs level, would the be the same for 3.3V? Or for 3.3V I should use another Schmidt trigger IC (not 7414)?

Thanks in advance!

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    \$\begingroup\$ Encoders should not need debouncing. Tact switches are often debounced in firmware. Are you sure you need to add a logic gate for debounce? \$\endgroup\$
    – user57037
    Commented Jul 10, 2017 at 5:05
  • \$\begingroup\$ @mkeith that "gate" is just for illustrating the presence of Schmidt trigger ( I didn't find the proper symbol in CircuitLab service). \$\endgroup\$
    – Araneus0390
    Commented Jul 10, 2017 at 5:14
  • \$\begingroup\$ @mkeith Yeah, some encoders need debouncing. This one, for example, when turned fast: sparkfun.com/datasheets/Components/TW-700198.pdf \$\endgroup\$
    – jonk
    Commented Jul 10, 2017 at 5:22
  • \$\begingroup\$ Option 1. (Don't use option 2.) You can always use the "very boutique" MC14490 part. Gives you 6 fancy debouncers in one IC package. PRICEY! \$\endgroup\$
    – jonk
    Commented Jul 10, 2017 at 6:01
  • \$\begingroup\$ I include the Schmidt trigger as a type of logic gate. I still suggest you debounce your button in firmware. That is the normal way to do it. I can't even remember the last time I saw external debounce hardware on a button connected to a micro-controller in a mass-produced product. \$\endgroup\$
    – user57037
    Commented Jul 10, 2017 at 8:09

1 Answer 1

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The first is the better option. Notice that the second circuit does not provide the output gate with the intended 0 V ground when the switch is closed. It is supplied a low level voltage through a voltage divider.

However, when the switch is closed in the first circuit, the gate's input is pulled down to ground, but slowly- as the cap discharges. This is what you want.

Edit also read all the comments. I agree with mkeith that, if this is for a microcontroller project, consider denouncing in firmware.

And jonk is right (as he often is) that there may be better, prepackaged, options depending on your needs.

Edit: 3.3 V will work fine with a 7414, provided you supply the IC with 5 V nominal on the VCC pin to power the chip. The resistors are up to you. The values of the resistors and capacitor affects the capacitor's charge/discharge time. Higher values provide more denouncing, but increase the wait time allowed between subsequent triggers. You might also want to add a voltage follower as an output stage.

For Example: Example circuit

and...

Regarding your 7414 question in the comments: 7414 Datasheet

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  • \$\begingroup\$ Thanks for answer. But what about resistor values? Would the be the same for 3.3V signal level input? Or for 3.3V the 7414 will not work? \$\endgroup\$
    – Araneus0390
    Commented Jul 11, 2017 at 18:03
  • \$\begingroup\$ See addition to the answer \$\endgroup\$
    – Blair Fonville
    Commented Jul 11, 2017 at 19:06
  • \$\begingroup\$ Well. Now it confuses me even more. If I understand the 7414 datasheet properly, it can give the output signal 0 or Vcc. If I'll do as you've described: "7414, provided you supply the IC with 5 V nominal on the VCC pin to power the chip", the 7414 output will be still 5.0V level (even if it will understand the 3.3V input as logic 1). What I wanted - attaching the output to STM32F103****, that has 3.3V signal level. Maybe, the proper question(s) is: "Will this circuit run with 3.3V input level? Could it use the 3.3V as supply? Could it produce the 3.3V level output?" \$\endgroup\$
    – Araneus0390
    Commented Jul 11, 2017 at 19:49
  • \$\begingroup\$ No, from the datasheet it looks like the supply voltage is 5 V nominal, and V_OH is 3.3 V. \$\endgroup\$
    – Blair Fonville
    Commented Jul 11, 2017 at 20:08
  • \$\begingroup\$ see edit. We're getting off topic though. I would advise you click Accept Answer, and post a new question if you need advice on using the chip. \$\endgroup\$
    – Blair Fonville
    Commented Jul 11, 2017 at 20:17

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