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I am trying to run an ESP8266-01S in short bursts from a supercapacitor/solar cell combination.

I have everything figured out (overvoltage protection, LDO regulator, etc) and it all works just like I want with one exception: when the sun comes up I have to manually reset the ESP device before it will run. Apparently, it goes into some kind of pseudo-alive state when awoken by insufficient voltage.

What I need is a simple discrete circuit that will hold the reset pin down until at least 3 V is available to properly power the processor. I figure that maybe a Zener/resistor/transistor combo or perhaps just a voltage divider on the reset pin may do it, but I'm sure this has been worked out before.

What's the easiest/best way to do this?

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The device you're after is called a supply voltage supervisor IC. These are designed to keep a system (i.e. microcontroller) in reset until sufficient supply voltage is available.

Digi-Key has lots of different types in stock in the PMIC - Supervisors category, with many different threshold voltages and output options.

You'd want to choose a part with low supply current, active-low reset, and a threshold voltage around 3V.

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