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If I want to drive the Zener diode with the ESP32 GPIO, will that be appropriate? What factors to look in this case? I want this as a low voltage detector?

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    \$\begingroup\$ Your question is not clear. What do you mean by "driving" a zener diode? How do you intend to use this as a low voltage detector? Please use the built-in schematic editor to draw a schematic of your proposed circuit and fully explain how you want to use it. \$\endgroup\$ – Elliot Alderson Jul 16 '19 at 12:45
  • \$\begingroup\$ I am working on the 3.3v microcontroller which provides ~3.3v as logic 1 when gpio pin in output mode is driven High. If I configure one gpio pin as output and another gpio pin as input(pulled down) and connect the zener diode with 2.9v threshold to those pins, If I drive the microcontroller pin high that means the zener diode will conduct and the input pin will detect the high voltage. \$\endgroup\$ – Pro Jul 16 '19 at 13:18
  • \$\begingroup\$ If my supply voltage goes down, which will afect the gpio output voltage when driven high. so when the gpio output voltage reaches the 2.8v, which is not sufficient to drive the zener diode. the pin configured as the input pin wont detect any voltage. so I can assume this condition as a low voltage condition. \$\endgroup\$ – Pro Jul 16 '19 at 13:22
  • \$\begingroup\$ So your purpose is to detect a decrease of the supply voltage of your microcontroller? Maybe you have a look at: ti.com/lit/an/snoa981/snoa981.pdf or ti.com/lit/ds/symlink/lm8364.pdf \$\endgroup\$ – Daniel K Jul 16 '19 at 13:34
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    \$\begingroup\$ Schematic. We must have a schematic. \$\endgroup\$ – Elliot Alderson Jul 16 '19 at 14:07
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If I drive the microcontroller pin high that means the zener diode will conduct and the input pin will detect the high voltage.

With an output pin high at 3.3 volts, the voltage after the zener diode (2.9 volts) will be 0.4 volts at the input pin and not enough (possibly) to rise past the lower limit for detecting a logic 1. It sounds like a problem to me or, if it does work it'll be very flakey.

If I want to drive the Zener diode with the ESP32 GPIO, will that be appropriate?

Probably not - I'd consider using a BJT fed from the zener via a 220 ohm resistor. The BJT can be flakey in terms of knowing what the base-emitter voltage might be to get a mA or so through the collector but it's likely to be better than your proposed idea.

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