Voltage Divider

A GPIO can only tell 1 from 0. To read a voltage that changes continuously (0 V → 0.5 V → 1.0 V …), use the ADC (analog-to-digital converter).

A joystick or potentiometer would be ideal, but two resistors forming a voltage divider are enough to try it.

Parts

Two resistors of the same value (10 kΩ here)

Circuit

Breadboard diagram: two 10k resistors in series between 3V3 and GND, with the midpoint connected to GPIO13

From	To
3V3	Resistor 1
Resistor 1 / Resistor 2 midpoint	GPIO13
Resistor 2	GND

With equal resistors, the midpoint sits at half the supply voltage, about 1.65 V.

Program

require 'adc'

input = ADC.new(13)

loop do
  puts "Input: #{input.read_raw}"
  sleep 1
end

read_raw returns the raw ADC value.

Run

Save the program as adc.rb and put it in /home on the device (see File Transfer), then run it from the shell:

$> ./adc.rb

Input: 2012
Input: 2015
Input: 2011

(The exact numbers will differ on your board.)

The ESP32’s ADC roughly maps 0–3.3 V to 0–4095, but the recommended input range is somewhat narrower and the response is not perfectly linear. Treat the value as an approximate voltage rather than a precise measurement.