Protected Digital Input

This is a follow-up to the protected analog input post showing you how to implement protected digital input with noise filtering and debouncing. It can be used for interfacing remote switches and pushbuttons to your device with overvoltage protection up to 30V.

Everything that was said for the analog input is still valid here. So much that the actual circuits are almost identical with the difference being mainly the final stage. Schmitt-trigger NAND gate was used to shape-up the input signal into a clean square wave output and the RC low-pass filter (R1/C1) is here to filter out noise and debounce switch contacts. Second schmitt-trigger gate was used to invert the inverted signal (invertception?!) thus restoring the logic state present on the input. It can be removed but I had two unused gates and I just had to use them.

So, without further delays, here is the schematic:
Protected digital input schematic

Transient and overvoltage protection is identical to the analog input’s circuit. TVS diode for transient and a Zener diode for long-term overvoltage conditions. If you compare the two schematics you will probably notice that the value of pull-down resistor has been increased to 100k. This forms a voltage divider that scales the input voltage down to approximately 90% of its value. Assuming a minimum 5V input signal, this leaves 4.5V at the gate’s input while the positive-going threshold voltage is around 3.5V for the CD4093 IC when powered from a 5V supply. This translates to a typical value of 70% of the supply voltage and we have 90% available. Well above the threshold.


A few notes about this circuit. Read them, they might help you. Or don’t. Up to you.

  • Capacitor value should be left at 100nF or increased to 1uF if required. Do not replace with a lower value capacitor because the debouncing may not work with some switches.
  • Because of the low input current requirements any Zener diode with a 3.3-5.1V rating will do.


Schematic in PDF:
Arc’s Circuits – Protected Digital Input

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