Exercise 4: Photodetectors

Lesson Plan: Simple Light Intensity and Flicker Detection

Lesson Objectives:

1. Understand how a photodiode works and its applications in measuring light intensity.
2. Learn to read and process light intensity data using Arduino.
3. Use Red Pitaya to analyze high-speed changes in light intensity, including flicker detection.

Lesson Outline:

1. Introduction

• Topic Overview:
• Photodiodes generate current proportional to light intensity, making them suitable for light measurement.
• Flicker in artificial light sources occurs due to AC power cycles or poor design, which can affect human comfort and system stability.
• Why It Matters:
• Understanding light intensity and flicker helps improve lighting systems, environmental monitoring, and optical applications.

2. Hardware Setup

A. Components:

• Photodiode (BPW34): A photodiode responsive to visible light.
• 10 kΩ Resistor: Used to create a voltage divider with the photodiode.
• Arduino Uno
• Red Pitaya STEMlab 125-14

B. Circuit Assembly:

1. Connect the Photodiode:
• Anode to Arduino analog input (e.g., A0).
• Cathode to GND.
• Place a 10 kΩ pull-up resistor between the anode and 5V.
2. For Red Pitaya:
• Connect the anode of the photodiode to Red Pitaya IN1 for signal analysis.

Circuit Diagram:

3. Arduino Programming: Reading Light Intensity

A. Basic Light Measurement:

1. Write a sketch to read and display the photodiode voltage:
Copy

const int photoPin = A0;

void setup() {
    Serial.begin(9600);
}

void loop() {
    int sensorValue = analogRead(photoPin);
    float voltage = sensorValue * (5.0 / 1023.0);  // Convert to voltage
    Serial.print("Light Intensity Voltage: ");
    Serial.println(voltage);
    delay(100);
}

2. Observe changes in voltage when varying the light intensity.

4. Signal Analysis with Red Pitaya

A. Connecting Red Pitaya:

1. Connect the photodiode anode to Red Pitaya IN1 for high-speed signal sampling.
2. Ensure GND is common between Red Pitaya and Arduino.

B. Visualizing Light Signals:

1. Open Red Pitaya’s oscilloscope tool.
2. Configure the input channel to a range of 0–5V.
3. Observe the waveform as the light source changes:
• Ambient light changes (e.g., covering the photodiode)
• Flicker from artificial sources (e.g., fluorescent lights or LEDs powered by AC).

C. Flicker Analysis:

1. Use Red Pitaya’s FFT tool to analyze the frequency components of the light signal.
• Identify flicker frequencies (e.g., 50/60 Hz due to AC power).
• Detect higher harmonics or irregularities caused by poorly designed lighting systems.
2. Compare Red Pitaya’s flicker detection capabilities with Arduino’s averaged readings.

5. Analysis and Insights

A. Characteristics of Light Signals:

• Explain how photodiode voltage corresponds to light intensity.
• Discuss how the signal varies with changes in light source, distance, and angle.

B. Flicker and Noise:

• Highlight the effects of flicker on comfort and system performance.
• Discuss sources of noise in the light signal and their potential impact on readings.

C. Advantages of High-Speed Analysis:

• Compare Arduino’s averaged readings (suitable for slow changes) with Red Pitaya’s high-speed visualization, which captures transient events like flicker.

6. Wrap-Up and Discussion

• Key Takeaways:
1. Photodiodes are simple yet powerful sensors for measuring light intensity.
2. Arduino provides basic light measurement, while Red Pitaya enables advanced analysis of light flicker and noise.
3. Signal analysis helps identify and mitigate issues in lighting systems, ensuring better performance and user comfort.
• Real-World Applications:
• Light intensity and flicker detection in offices, homes, and industrial environments.
• Optical communication and environmental monitoring systems.

Homework/Extra Work

1. Add a capacitor across the photodiode and observe how it smooths the light signal.
2. Use Red Pitaya to analyze the flicker of different light sources (e.g., incandescent bulbs, LED lights, and sunlight).