NI myRIO: Photocell

Summary

This video delves into the characteristics of photocells, particularly a cadmium sulfide (CdS) version from Advanced Photonics Inc. It explains how the resistance of the photocell changes with light exposure (illuminance), following a predictable pattern where resistance decreases as illuminance increases. The video includes a practical demonstration of measuring photocell resistance using a voltage divider circuit, showcasing how to calculate the photocell's resistance based on voltage measurements. The presenter also explores the relationship between resistance, illuminance, and voltage, offering a hands-on approach to understanding photocell behavior in real-world applications.

Takeaways

• 😀 Photocells have a resistance that changes with illuminance, commonly using cadmium sulfide (CdS) material.
• 😀 The CdS photocell is sensitive to visible light in the 400-700 nanometer range, similar to human eyesight.
• 😀 The resistance of the photocell decreases as illuminance increases, following a logarithmic trend.
• 😀 At an illuminance of 10 lux, the photocell's resistance typically ranges between 10kΩ and 30kΩ.
• 😀 In complete darkness, the resistance of the photocell can exceed 40 megaohms.
• 😀 The symbol for a photocell consists of a resistor symbol inside a circle, with arrows indicating the light source.
• 😀 A measurement technique based on a voltage divider can be used to determine the photocell's resistance.
• 😀 In a voltage divider, the photocell serves as the upper branch, with a fixed resistor in the lower branch.
• 😀 The voltage divider equation can be used to calculate the resistance of the photocell by measuring the voltage (Vx), fixed resistance (R), and supply voltage (Vdd).
• 😀 As illuminance increases, the photocell's resistance decreases, causing the output voltage (Vx) in the circuit to increase.
• 😀 If the positions of the photocell and fixed resistor are swapped in the circuit, the output voltage (Vx) decreases as illuminance increases.

Q & A

What is the primary characteristic of a photocell?

A photocell has a resistance that changes with illuminance. Its resistance decreases as the amount of light increases.

What material is commonly used in photocells and why?

Cadmium sulfide (CdS) is commonly used because it responds similarly to human eyesight, especially in the visible spectrum from 400 nm (violet light) to 700 nm (red light).

What does the graph of photocell resistance as a function of illuminance show?

The graph shows that as illuminance increases (from 1 lux to 100 lux), the resistance of the photocell decreases, and the resistance is plotted on a log scale from 1 ohm to 1000 kilo-ohms.

How does the tolerance band affect the resistance reading of a photocell?

The tolerance band indicates a range for the photocell's resistance at a given illuminance. For example, at 10 lux, the resistance could be anywhere between 10k ohms and 30k ohms.

What happens to the photocell resistance in complete darkness?

In complete darkness, the resistance of the photocell increases significantly. In the case of the example given, the resistance reached over 40 megaohms.

How is the symbol for a photocell represented in a circuit diagram?

The symbol for a photocell is based on a resistor symbol, enclosed in a circle with arrows indicating that light can shine on it.

What is the function of the photocell in a voltage divider circuit?

In a voltage divider circuit, the photocell serves as the upper branch, and its resistance changes with light intensity, which in turn affects the voltage (Vx) measured across it.

How do you measure the resistance of the photocell in a voltage divider circuit?

To measure the resistance of the photocell, you measure the voltage (Vx) using a voltmeter or the analog input on a MyRIO, the value of a fixed resistor, and the supply voltage (Vdd), then calculate the resistance using the voltage divider equation.

What happens to the voltage (Vx) as illuminance increases in the original voltage divider setup?

As illuminance increases, the resistance of the photocell decreases, causing the voltage (Vx) to increase in the circuit.

How does the circuit behavior change if the photocell and fixed resistor are swapped in the voltage divider?

If the positions of the photocell and the fixed resistor are swapped, the voltage (Vx) will decrease as illuminance increases. The equation for Vx and the photocell resistance (Rx) will also be modified accordingly.