enter image description hereI am connecting a digital output pin from an MCP23017 through a diode (1N4001, forward biased) to a CD74HC4067 input channel. VDD is 5V to all devices.

I am seeing 1.45V on the anode side of the zener and 0.7V on the cathode side. Since the CD74HC4067 is hooked up to the cathode side of the diode, I'm also seeing 0.7V there. If I hook up the CD74HC4067 to the anode side, I see 1.45V there.

I was expecting to see 5.0V-0.7V=4.3V at the anode side of the diode, why am I not seeing that?

If I just connect the diode from 5V to ground, I see 4.3V when it's forward biased.

Any help would be appreciated.


Edit: rectifier diode, not Zener.

Fix found! I had to disconnect GPA1 (accidentally still had it connected).

  • \$\begingroup\$ A 1N4001 is a regular rectifier diode, not a Zener. You should see about 0.7 volts across the diode when forward biased, and whatever the supply voltage is across it when reverse biased (it will be essentially open circuit when reverse biased). \$\endgroup\$ Commented Aug 18, 2018 at 4:09

1 Answer 1


I suspect you have the MCP23017 pin configured not as an output but as an input with a weak pullup and that you have an as-yet unreported 10K pulldown on the other side of the diode (on the input of the HC4067).

1N400x are just regular very slow rectifier diodes, not Zener diodes.

It's always better to supply an actual schematic, it prevents wasted time and guesses. If you use the schematic button you can edit your question to include a schematic.

  • \$\begingroup\$ Whoops, not Zener diode, just rectifier diode. When I disconnect the MCP23017 pin from the diode and measure it's output, I see 5V. When I connect the anode of the diode directly to VDD I see the 4.3V on the output of the HC4067. When I mate the two, I get the strange 1.45V out of the HC4067. I will create a circuit diagram. \$\endgroup\$
    – sizzle
    Commented Aug 18, 2018 at 4:23
  • \$\begingroup\$ Nothing you said there has changed my guess. Is there a 10K on the input or output of the HC4067? \$\endgroup\$ Commented Aug 18, 2018 at 4:32
  • \$\begingroup\$ Thanks for your help! Found the issue after drawing out the circuit. \$\endgroup\$
    – sizzle
    Commented Aug 18, 2018 at 5:19

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