Consider an ESP-01 that is powered by a 3.3v (OR LESS) regulator hooked to a 18650 battery (3.7v ~ 4.2v) hooked to a TP4056 charger randomly obtaining power from 2 solar panels in series giving approx 5.5v when in full sun exposure.

Now ordering a KA75330 to also prevent an UNDER VOLTAGE condition, in the hope of preventing an ESP-01 brownout, which I just learned, almost always ends up completely stalling/halting the ESP-01. I was pulling my hair out today trying to figure out why the ESP-01 LED was lit, but the damn thing was doing nothing. This happens very often at random in the morning when the battery begins to charge up in the first sun rays and reaches around 2.5v and then the ESP-01 boots but it just immediately AND SILENTLY halts with its LED still on, giving the false impression that it is still running, but it's not! When this happens, I disconnect the 3.3v pin and wait an hour for the battery to charge a little more and reach the ESP-01 base voltage threshold (approx. 2.8v) and then I hook the 3.3v pin again and this resets the ESP-01 and it works for the rest of the day perfectly fine.

I saw this tutorial and it explains exactly that issue plus a solution (KA75330) but the video omits to explain what resistor value is needed if the Vcc is 3.3v or less.


QUESTION IS: What resistor value is needed in this case?


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1 Answer 1


Probably around 10k would be sufficient. It is only a pullup for the reset signal, so not super critical. The esp01 board might already have a resistor on reset, in which case you don’t need another.

P.S from what i googled, the esp-01 has no pull-up, so 10k should do it. Note: your battery is drawn the wrong way up.

  • \$\begingroup\$ "battery is drawn the wrong way up" >> That's what I was thinking too! But this is a screen capture from the mentioned YT video. I was thinking the same but then I was in doubt, since this youtuber seems to know his sh. \$\endgroup\$
    – that-ben
    Jun 2, 2021 at 11:55
  • \$\begingroup\$ Google will put you straight. Hint - only a very small fraction of youtubers know what they are talking about. The rest is just cat videos and people trying to be famous. \$\endgroup\$
    – Kartman
    Jun 2, 2021 at 12:58
  • \$\begingroup\$ Your battery wont appreciate being discharged to 2.5V, anything less than 3V is not good for a Li Ion cell. \$\endgroup\$
    – Kartman
    Jun 2, 2021 at 13:03
  • \$\begingroup\$ Yes but that is why I'm doing this, I install an under voltage supervisor in the hopes of only allowing the ESP-01 to power on when there is 3.3v therefore, with this setup, it would take weeks left completely in the dark to discharge below 3v since the ESP-01 would not power on below 3.3v. \$\endgroup\$
    – that-ben
    Jun 2, 2021 at 13:54

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