A0 ADC converter inside

Caused by some own challenges here some guidelines how you connect to A0 to measure voltage.

Fist you have to acknowledge that the WEMOS MINI and NODEMCU are able to measure up to 3.3V and that the ESP8266 native boards are only measuring 1.0V.

The resistance of the ESP8266 at A0 is >10 MOhm. Also the WEMOS and NODEMCU uses this. To measure 3.3V there is a Voltage divider includes 220kOhm and 100kOhm. Therefore the resistance of these boards is 320kOhm.

Why is this interesting? If you want to measure MORE than 3.3V you must create your own voltage divider and therefore it is absolutely required that you know the resistance of the internal circuit.

For example, you want to measure up do 5V:

In this case on the WEMOS you need just a 200kOhm resistor before the A0. With the internal resistors: 200/320 * 5V = 3.12V this works perfectly. With the ESP8266 only you do 5MOhm and 1MOhm voltage divider and measure at the 1MOhm.

These are totally different resistors and also different current that goes through.