9. Input devices¶

Group assignment:¶

• Probe an input device(s)'s analog and digital signals
• Document your work on the group work page and reflect on your individual page what you learned

To see our group assignment click here

Individual assignment:¶

• add a sensor to a microcontroller board that you have designed and read it.

How the LDR Sensor Works¶

Sizing the Fixed Resistor & Voltage Divider Calculation¶

The voltage divider formula

The voltage divider formula

Vout = Vin × R_fixed / (R_LDR + R_fixed)

Where Vin = 3.3V (ESP32 supply rail).

Where Vin = 3.3V (ESP32 supply rail).

How to choose the fixed resistor

The goal is to place the fixed resistor close to the LDR's own resistance at the light level you want to detect most sensitively. At that point R_LDR ≈ R_fixed, which puts Vout near Vin / 2 — the region where the divider is most sensitive to small changes in light.

For a typical 5mm LDR (e.g. GL5528-type), resistance behaves roughly like this:

A 5 kΩ fixed resistor was chosen here, centering the divider's sensitivity around the lit indoor room condition.

Resulting output voltage and ADC reading

(ADC value ≈ Vout / 3.3 × 4095, assuming 11dB attenuation, required on the ESP32 to read the full 0–3.3V range.)

My Custom PCB¶

The sensor is connected to the custom PCB I designed during Week 06 — Electronics Design, based on an ESP32-WROOM microcontroller.

Custom ESP32-WROOM PCB with the LDR voltage divider circuit (J2 + fixed resistor) integrated on-board.

Wiring Diagram¶

Schematic of the LDR voltage divider: 3.3V - LDR analog pin - 5kΩ resistor - GND.

Physical wiring of the LDR sensor on the assembled board, matching the schematic above.

Code¶

The following code reads the LDR analog value and switches between the two modes:

#define LDR_PIN 34        // Analog pin connected to LDR (ADC1 — safe to use with Wi-Fi active)
#define THRESHOLD 2000    // Adjust based on your environment

void setup() {
  Serial.begin(115200);
  pinMode(LDR_PIN, INPUT);
}

void loop() {
  int ldrValue = analogRead(LDR_PIN);
  Serial.print("LDR Value: ");
  Serial.println(ldrValue);

  if (ldrValue < THRESHOLD) {
    // Room light is OFF → Night light mode
    Serial.println("Mode: Night Light");
    // Add your night light logic here
  } else {
    // Room light is ON → Visitor mode
    Serial.println("Mode: Visitor");
    // Add your visitor mode logic here
  }

  delay(500);
}

Serial Monitor¶

The video below shows the Serial Monitor output as the light condition changes in real time:

Live Serial Monitor output as the room light is switched on and off, showing the LDR value and resulting mode.

Final Project Development: Smart Mode Management via LDR Sensor¶

This section details the development and implementation of the LDR (Light Dependent Resistor) sensor within the final project architecture.

Operational Modes¶

Demo of the system transitioning between Visitor Mode and Sleep Mode as ambient light changes.

The LDR sensor integrated into the final enclosure, positioned to sense ambient room light.

Sensor Evolution: From LDR to Phototransistor¶

While the LDR-based system worked reliably for basic day/night detection, I wanted a more precise and repeatable sensor for the final version of this feature CdS-based LDRs age over time, respond slowly, and vary significantly from one unit to another.

TEMT6000 Phototransistor¶

Instead of adding correction filters to a photodiode, I switched to the Vishay TEMT6000, a phototransistor purpose-built as an ambient light sensor, with a spectral response close to the human eye and internal gain that removes the need for an op-amp entirely.

Circuit Design¶

Same simple voltage-divider topology as the original LDR :

Component sizing

With R = 10kΩ, the expected output swing is:

This gives a clean, well-separated range for the day/night threshold — with much better consistency across different light sources than either the LDR or the BPW34.

Comparison: LDR vs. TEMT6000¶

Files¶