Measure Analog Sensors on Raspberry Pi Without Native ADC

The Raspberry Pi lacks built‑in analog inputs, yet that doesn’t preclude reading analog sensors. By combining a few resistors and a capacitor, you can employ a simple step‑response technique to determine resistance—ideal for potentiometers, photoresistors, thermistors, and more.

Background

This approach, popularized by Adafruit’s tutorials and adapted in the Raspberry Pi Cookbook, measures the time it takes a capacitor to charge past a threshold. The Raspberry Pi’s GPIO pins can detect the moment the voltage exceeds ~1.65 V, allowing you to infer the resistance of a connected variable resistor.

What You’ll Need

• Standard breadboard and jumper wires
• 10 kΩ trimpot (potentiometer)
• 2 × 1 kΩ resistors
• 220 nF capacitor

Python Code

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)

# Pin assignments
A_PIN = 18  # Capacitance discharge/charge control
B_PIN = 23  # Voltage threshold detection

def discharge():
    GPIO.setup(A_PIN, GPIO.IN)
    GPIO.setup(B_PIN, GPIO.OUT)
    GPIO.output(B_PIN, False)
    time.sleep(0.005)

def charge_time():
    GPIO.setup(B_PIN, GPIO.IN)
    GPIO.setup(A_PIN, GPIO.OUT)
    count = 0
    GPIO.output(A_PIN, True)
    while not GPIO.input(B_PIN):
        count += 1
    return count

def analog_read():
    discharge()
    return charge_time()

try:
    while True:
        print(analog_read())
        time.sleep(1)
except KeyboardInterrupt:
    GPIO.cleanup()

Running the script produces output similar to:

$ sudo python pot_step.py
101
210
101
623
435
367
728
610
512
314
317
0

The readings fluctuate between roughly 10 and 170 as you turn the potentiometer knob, reflecting the variable resistance.

How It Works

The technique relies on the RC charging curve. When the capacitor is connected to the variable resistor, its voltage rises exponentially. The GPIO pin registers a logical high once the capacitor voltage exceeds the input threshold (~1.65 V). The elapsed time—counted in loop iterations—directly correlates with the resistance: lower resistance means a faster charge, yielding a smaller count.

Because the Pi has no analog‑to‑digital converter, this timing method circumvents that limitation, offering a cost‑effective and reliable way to read a wide range of analog sensors.