Using the following boards and sensors...
MakerFocus ESP8266 NodeMCU LUA CP2102 ESP-12E
SparkFun Multiplexer Breakout - 8 Channel (74HC4051)
HiLetgo Voltage Detection Module DC 0~25V Voltage Sensor
Gowoops DHT22 Temperature Humidity Sensor Module
I intend to use the ESP, the voltage sensor, and the temp/humidity sensors to log ongoing battery voltages and temp/humidity values to a RPI based database. This all seems straightforward, but I do have one concern. When I look at the spec sheets on ESP8266 ESP-12 ESP-12E modules it says that the max input voltage for the analog pin (A0) is 3.2v. But the overall voltage used to drive things is 3.3v. I assume that the input voltage to the sensors and multiplexer, at 3.3v, will drop a bit so the output they will send to the AO pin will be a bit lower, but can anyone shed light on my assumption? I really don’t want to over volt the analog pin on the ESP.
A NodeMCU already has a voltage divider built-in,
so you shouldn't use that voltage detection module (which also has a voltage divider).
Analogue input voltage of a NodeMCU, because of that built-in 220k:100k divider is about 0-3.2volt.
You should just use a single resistor between that voltage source and A0.
Value of that resistor should be 100k for every volt more than 3.2volt.
So for 15volt >> 15-3.2 = 11.8 * 100k = ~1200k (1.2Megohm).
Thx for the response. I already have the Hiletgo sensors laying around so I figured I’d use them. I do appreciate the recommendation, and I’d do as you explain, if that wasn’t so. I guess I’ll check the output voltage on the Hiletgo sensors and see what the output range is while using my LONGWEI variable DC power supply to roll thru the min and max voltages I expect.
But the basic question is still there...the spec sheets on ESP8266 ESP-12 ESP-12E modules say that the max input voltage for the analog pin (A0) is 3.2v (Max) tolerant. And I still would like to understand a bit better if that’s a hard voltage limit or not. I suppose it depends on how long you exceed it and by how much before the magic smoke come out...
I don't know which spec sheet you mean.
The A/D range of bare ESP8266 modules is actually about 0-1volt.
WeMos and NodeMCU boards with those modules use a 100k:220k voltage divider, to increase that range to about 3.2volt, to better match A0 to the 3.3volt logic of the rest of the board.
Never tried, educated guess, but this built-in divider should also protect A0 to 10volt and beyond.