YL-38 output to NodeMCU v1.0 analog input
To my understanding, the NodeMCU A0 pin operates from 0v-3.3v


Incorrect. The ESP8266's ADC can only measure between 0V and 1V.

You need to reduce your output voltage down to that range. How you do that very much depends on the output impedance of the YL-38.

The LM393 that the YL-38 is based around looks to have a maximum output current of 16mA. That should be enough to happily drive a simple voltage divider.  A 4.7kΩ and 1kΩ divider would give you 0 to 0.877v out from a 0-5V input.







simulate this circuit &ndash; Schematic created using CircuitLab

An alternative would be to attach an external ADC running at 5V to the NodeMCU. This would give you multiple channels of analogue input with a 0-5V range. I use the MCP3208 running over SPI (I actually bit-bang it on the ESP8266 rather than using SPI, since I haven't yet got SPI to work properly with it).  Ideally, you want a proper logic level translator for the SCK and MOSI signals to boost them to 5V. It all gets a little complex, though.



It has come to light that the NodeMCU includes a voltage divider already. For some reason they chose to use massive resistors, which give far too high an impedance for any reasonable ADC input to accurately work with.  I would suggest, if you can, removing those resistors from the board and replacing them with 4.7k and 1k resistors to form your own voltage divider. Otherwise you're going to have to use a pair of op-amps in inverting mode, one with a fractional gain, and one with unity gain (the second is to "undo" the inverting of the first one) to reduce the signal to 0-3.3V.

TBH I'd be inclined to use an external ADC anyway for this.