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I have hooked my HX711 to an ESP8266. I'm now faced with the problem that once I put my ESP into deep sleep mode the GPIO Pin which is hooked to the PD_SCK pin goes low.

This in turn will put the HX711 in "normal mode".

When PD_SCK Input is low, chip is in normal working mode. (Page 5)

How can I keep both devices in power down mode?

When PD_SCK pin changes from low to high and stays at high for longer than 60µs, HX711 enters power down mode.

I tried to pull up the PD_SCK pin and use the ESP8266 gpio pin as a drain but that did not work. Maybe I did something wrong.

I could introduce an inverting 74xx chip but that just seems wrong. Is there maybe something I overlooked?

https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf

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Use a FET to allow Open Drain operation.

schematic

simulate this circuit – Schematic created using CircuitLab

The RUM001L02 has a very low VGS(threshold) and is readily available. Remember your PD_SCK is now logically inverted.

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Set the PD_SCK pin to the output mode before readings, and to the HIGH-Z/INPUT mode before esp8266/hx711 powerdown, use pullup resistor.

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