23. Resistive Sensors, Dividers & the LDR

Why resistive sensors need a voltage divider

Arduino analogue pins measure voltage (0 to about 5 V on an Uno), not resistance. An LDR or an NTC thermistor only changes resistance. Place the sensor in series with a known fixed resistor and the mid-point voltage becomes a fraction of the supply that follows the sensor. That mid-point goes to an analogue pin.

This is the same divider idea as the potentiometer in Lesson 14, except one half is fixed and the other half is the sensor.

How an LDR works

An LDR (Light Dependent Resistor), usually a cadmium sulphide (CdS) cell, changes resistance with illumination. In the dark it can be megaohms; in bright light a few hundred ohms.

Light frees charge carriers in the photoconductive material, so resistance falls. LDRs are slow (tens to hundreds of milliseconds) and strongly nonlinear, so this course uses them for dark / bright decisions rather than calibrated lux.

Divider calculation (course wiring)

Course wiring puts the fixed resistor to 5 V and the sensor to GND:

5 V → 10 kΩ → junction to A1 → LDR → GND

Junction voltage: Vout = 5 V × Rsensor / (Rfixed + Rsensor). The ADC maps about 0-5 V to 0-1023, so the sensor resistance is:

Rsensor = Rfixed / (1023.0 / ADC - 1.0)

With this order, a darker room raises the LDR resistance, raises Vout and raises the ADC count. Swap the two parts and both the direction and the formula change - always match the code to the physical order.

Worked resistance example

Rfixed = 10000 Ω and analogRead returns 768.

Rsensor = 10000 / (1023.0 / 768 - 1.0) = 10000 / 0.332 ≈ 30 100 Ω - a fairly dim room for a typical LDR.

Vout = 768 / 1023 × 5 V ≈ 3.75 V. Measure the junction with your multimeter: if it reads 3.75 V but Serial shows a very different count, suspect the code or the pin, not the sensor.

If ADC is 0 or 1023, do not use the formula - it divides by zero or gives nonsense. Treat those readings as a wiring fault.

Choosing the fixed resistor

The divider is most sensitive when Rfixed is close to the sensor resistance at the light level you want to detect. Measure the LDR with the multimeter (power off) in the two conditions that matter, then pick a standard value between them.

A 10 kΩ resistor suits most indoor dark / bright decisions. Detecting dusk outdoors may need 100 kΩ; detecting a bright lamp may need 1 kΩ.

Thresholds and hysteresis

Set thresholds by experiment: cover the LDR and note the count, uncover it under room light and note the count, then choose values between them.

A single threshold chatters: noise near that value flips the output on and off many times a second. Hysteresis uses two thresholds - switch ON above one value, switch OFF only below a lower one. The gap between them must be larger than the noise you see in Serial.

const int darkOn = 700;     // count rises above this: dark
const int brightOff = 600;  // must fall below this to switch off
bool lampOn = false;

if (!lampOn && raw >= darkOn) {
  lampOn = true;
} else if (lampOn && raw <= brightOff) {
  lampOn = false;
}

Check it with the multimeter and scope

Measure the junction voltage with the multimeter while Serial prints the count. Predict the count as V / 5 × 1023 and compare - they should agree within a few counts. A disagreement points to a wrong pin in the sketch or a supply that is not really 5 V.

On the oscilloscope (DC coupling, 1 V/div) the junction is a steady level that moves as you shade the sensor. Under mains lighting you may see a small 100 Hz ripple: that is the lamp flicker, and it is one reason readings wobble near a threshold.

Wiring and safe build sequence

1. 5 V -> 10 kΩ -> A1 junction -> LDR -> GND
2. D9 -> 330 ohm -> LED anode; LED cathode -> GND (night-light output)
3. Multimeter COM to GND, V to the A1 junction for the calibration check

What this sketch is for: Run this first to choose your thresholds. It prints the raw ADC count and the calculated LDR resistance twice a second - cover and uncover the sensor and write the numbers in your logbook.

const float seriesR = 10000.0;  // fixed resistor to 5 V

void setup() {
  Serial.begin(9600);
  Serial.println("LDR on A1 (10k to 5 V, LDR to GND)");
}

void loop() {
  int raw = analogRead(A1);
  if (raw <= 0 || raw >= 1023) {
    Serial.println("Divider fault");
    delay(500);
    return;
  }

  float resistance = seriesR / (1023.0 / raw - 1.0);

  Serial.print("ADC ");
  Serial.print(raw);
  Serial.print("  R ");
  Serial.print(resistance, 0);
  Serial.println(" ohm");
  delay(500);
}