A photo sensor, or photoelectric sensor, works by detecting changes in light intensity and converting them into an electrical signal.
Here's a breakdown of how it generally functions:
Core Components
A typical photoelectric sensor consists of two main parts:
- Emitter (Light Source): This component generates a beam of light, usually infrared, visible red, or laser light.
- Receiver (Photodetector): This component detects the light emitted and produces an electrical signal based on the amount of light it receives.
Operating Principles
The sensor's operation depends on how the emitted light interacts with objects in its path. There are typically three main operating modes:
-
Thru-Beam (Opposed):
- The emitter and receiver are positioned opposite each other, creating a light beam across a defined area.
- When an object interrupts the beam, the receiver detects a drop in light intensity and triggers an output signal.
- Example: Counting items on a conveyor belt.
-
Retro-Reflective:
- The emitter and receiver are housed in the same unit.
- A reflector is positioned opposite the sensor. The emitter sends light to the reflector, which bounces it back to the receiver.
- When an object interrupts the light beam between the sensor and the reflector, the receiver detects a drop in light and triggers an output signal.
- Example: Detecting the presence of a pallet on a forklift.
-
Diffuse (Proximity):
- The emitter and receiver are housed together.
- The sensor detects objects based on the light reflected directly off the object's surface.
- The emitter sends out light, and if an object is present, some of the light reflects back to the receiver. The intensity of the reflected light is used to determine the object's presence (and sometimes its distance).
- Example: Detecting the presence of boxes on a shelf.
Signal Conversion
Regardless of the operating mode, when the receiver detects a sufficient change in light intensity (either an increase or decrease, depending on the configuration), it converts this change into an electrical signal. This signal can then be used to activate other devices, such as:
- Relays
- Counters
- Programmable Logic Controllers (PLCs)
- Alarms
Summary Table: Operating Modes
| Operating Mode | Emitter/Receiver Position | Object Detection Method | Advantages | Disadvantages |
|---|---|---|---|---|
| Thru-Beam | Opposite | Beam Interruption | Most reliable, long sensing range | Requires wiring to both sides |
| Retro-Reflective | Same Unit | Beam Interruption with Reflector | Easier installation than thru-beam, good sensing range | Requires reflector, sensitive to reflector alignment |
| Diffuse | Same Unit | Direct Reflection from Object | Simplest installation, no reflector needed | Shortest sensing range, influenced by object color and surface |
In essence, a photo sensor acts as a light-sensitive switch that provides an electrical output when it detects a change in the amount of light it receives.
