INST1015 - Photoelectric Sensors in Instrumentation

Study Notes

Photoelectric Sensors

Use light to detect the presence or absence of an object
Detection occurs if a light beam is interrupted or reflected by the object being sensed
Consist of five basic components: light source, light detector, lenses, logic circuit, and output circuit

Light Source (LED)

Light emitting diodes (LEDs) are the most commonly used light source
Can be visible or non-visible (infrared)
Operate due to electroluminescence – the generation of photons when electron-hole pairs are created by current flow through a PN junction
Characteristics:
- Very long lifespan (solid state devices)
- Light intensity does not deteriorate quickly
- Immune to shock and vibration
- Produce a single wavelength
- Popular for security applications (infrared)

Light Detectors

A light sensor that detects the absence of an object is called a light sensor, detector, or receiver
Most common light detectors:
- Photoresistors or photocells
- Photodiodes
- Phototransistors

Photocell

A passive resistive component that decreases resistance as exposure to light increases
Typical dark resistance: 500kΩ to 20MΩ range

Photodiode

A semiconductor device with a P-N junction that converts photons (light) into electrical current
Two main modes of operation:
- Photovoltaic or Unbiased Mode: illuminated photodiode generates a voltage which can be measured
- Photoconductive or Reversed Biased Mode: reverse voltage is applied to the diode where the diode will not conduct without incident light

Phototransistor

A semiconductor device that has a light-sensitive base region
Operation is identical to a normal transistor except the base voltage is replaced by light intensity

Photoelectric Applications

Nightlight
Smoke Detector
Coin Counter
Irradiance Sensor

Photoelectric Sensors in Industry

Can be categorized by:
- Sensing method
- Packaging style

Sensing Methods

Opposed sensing / through beam:
- Consists of two units, emitter and detector, positioned opposite each other
- Object is detected when it breaks the light beam
- Object must completely block the entire beam
- Detection mode is light-to-dark
- Most reliable sensing method
Retro-reflective sensing:
- Emitter and detector contained next to each other in the same housing
- Light beam reflected back to the receiver by a prismatic reflector mounted opposite the sensor unit
- Detection mode is light-to-dark
- Not reliable for highly reflective objects, may go undetected
Diffuse sensing:
- Emitter and detector contained next to each other in the same housing
- No need for reflector – relies on light being reflected by the surface of the object
- Reflected light indicates the object is present
- Detection mode is dark-to-light
- Reflected light is diffused and not very strong, range is limited
Convergent sensing:
- Similar to diffuse sensing – dark-to-light detection mode
- Emitter and detector contained next to each other in the same housing
- Emitter and detector are set at the same angle from the vertical axis to capture reflected light
- Sensor is designed to detect the object at one set distance – focal point
- Objects closer or further from the focal point will not be detected
- Typically used to detect parts in place or fill level
Specular sensing:
- Detects the difference between shiny and dull surfaces based on how the beam reflects (specular vs. diffuse)
- Emitter and receiver contained in the same housing
- The distance between the sensor and the object must remain constant

Description

This quiz covers the fundamentals of photoelectric sensors in instrumentation, including how they use light to detect objects and their basic components. Designed for students taking Introduction to Instrumentation at Conestoga College's Electrical Engineering Technology program.