Measuring Analog Sensors on the Raspberry Pi with a Single GPIO Pin

Unlike many microcontrollers, the Raspberry Pi offers only digital GPIO pins. They can source or sink high (3.3 V) and low (0 V) logic levels, but they cannot read analogue voltages directly. Fortunately, a simple RC‑charging technique lets you extract a range of values from variable‑resistance sensors such as LDRs and thermistors using just one GPIO pin.

Why It Works

In a classic RC circuit a resistor and a capacitor are connected in series. When a voltage is applied, the capacitor voltage rises exponentially. The time it takes to reach 63 % of the supply voltage is given by the time constant t = RC, where t is time (seconds), R is resistance (ohms), and C is capacitance (farads). By measuring how long the capacitor takes to charge to the GPIO’s high‑level threshold (≈2 V on a 3.3 V Pi), you obtain a value proportional to the sensor’s resistance—and therefore to the physical quantity it measures.

For example, with a 10 kΩ resistor and a 1 µF capacitor the time constant is 10 ms. An LDR that reads 1 MΩ in the dark will produce a time constant of roughly 1 s, making it trivial to distinguish between light and dark conditions.

Practical Implementation

To guarantee a defined resistance between the 3.3 V supply and the GPIO pin, insert a 2.2 kΩ resistor in series with the LDR. The complete measurement sequence is:

• Discharge the capacitor: Set the GPIO as an output and drive it low.
• Start charging: Reconfigure the GPIO as an input. Current flows through the resistor network into the capacitor, causing its voltage to rise.
• Measure the rise time: Increment a counter (or use a high‑resolution timer) until the GPIO reads high (≈2 V). The elapsed time equals the sensor’s RC time constant.
• Repeat: Set the GPIO back to output, drive low, and repeat as needed for continuous monitoring.

For more detailed guidance, see the official Raspberry Pi GPIO documentation and the RC circuit article on Wikipedia. An online RC time‑constant calculator can help you pick component values that match your sensor’s range.