Introduction: In its most basic form, a photoelectric sensor can be thought of as a switch where the mechanical actuator or lever arm function is replaced by a beam of light. By replacing the lever arm with a light beam the device can be used in applications requiring sensing distances from less than 25.4 mm (1 in.) to one hundred meters or more (several hundred feet).
All photoelectric sensors operate by sensing a change in the amount of light received by a photodetector. The change in light allows the sensor to detect the presence or absence of the object, its size, shape, reflectivity, opacity, translucence, or color.
Photoelectric sensors provide accurate detection of objects without physical contact. There is a vast number of photoelectric sensors from which to choose. Each offers a unique combination of sensing performance, output characteristics and mounting options. Many sensors also offer embedded logic or device networking capabilities that allow them to perform standalone in applications that would otherwise require external logic circuitry or a programmable controller.
Photoelectric Sensor Construction
A light source sends light toward the object. A light receiver, pointed toward the same object, detects the presence or absence of direct or reflected light originating from the source. Detection of the light generates an output signal for use by an actuator, controller, or computer. The output signal can be analog or digital. Some sensors modify the output with timing logic, scaling, or offset adjustments.
A photoelectric sensor consists of five basic components:
- Light source
- Light detector
- Lenses
- Logic circuit
- Output
Basic Components
Light Source: Most photoelectric sensors use a light emitting diode (LED) as the light source. An LED is a solid-state semiconductor that emits light when current is applied. LEDs are made to emit specific wavelengths, or colors, of light. Infrared, visible red, green, and blue LEDs are used as the light source in most photoelectric sensors. The LED and its associated circuitry are referred to as the emitter.
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Different LED colors offer different desirable characteristics. Infrared LEDs are the most efficient, generating the most light and the least heat of any LED color. Infrared LEDs are used in sensors where maximum light output is required for an extended sensing range.
In many applications, a visible beam of light is desirable to aid setup or confirm sensor operation. Visible red is most efficient for this requirement. Visible red, blue, and yellow LEDs are used in applications where specific colors or contrasts must be detected. These LEDs are also used as status indicators on photoelectric sensors.
More recently, laser diodes have also been used as photoelectric light sources. Laser light sources have unique characteristics including:
- Emitted light of a consistent wavelength (color)
- Small beam diameter
- Longer range
Laser sources tend to be more costly than LED light sources. In addition, the small beam size of emitted laser light, although extending the maximum sensing distance potential, may be more easily interrupted by airborne particles. Installers must guard against improper exposure to the laser beam, following typical safety procedures.
Rugged and reliable, LEDs are ideal for use in photoelectric sensors. They operate over a wide temperature range and are very resistant to damage from shock and vibration.
LED Modulation
One of the greatest advantages of an LED light source is its ability to be turned on and off rapidly. This allows for the pulsing or modulation of the source.
The amount of light generated by an LED is determined by the amount of current it is conducting. To increase the range of a photoelectric sensor, the amount of current must be increased. However, LEDs also generate heat. There is a maximum amount of heat that can be generated before an LED is damaged or destroyed.
Photoelectric sensors rapidly switch on and off or modulate the current conducted by the LED. A low duty cycle (typically less than 5%) allows the amount of current, and therefore the amount of emitted light, to far exceed what would be allowable under continuous operation.
The modulation rate or frequency is often in excess of 5 kHz, much faster than can be detected by the human eye.
Light Detector
The light detector is the component used to detect the light from the light source. The light detector is composed of a photodiode or photo-transistor. It is a solid-state component that provides a change in conducted current depending on the amount of light detected. Light detectors are more sensitive to certain wavelengths of light. The spectral response of a light detector determines its sensitivity to different wavelengths in the light spectrum. To improve sensing efficiency, the LED and light detector are often spectrally matched. The light detector and its associated circuitry are referred to as the receiver.
In a photoelectric sensor, the photodetector can receive light directly from the source or from reflections.
Logic Circuit
The sensor logic circuit provides the necessary electronics to modulate the LED, amplify the signal from the detector, and determine whether the output should be activated.
Output Device
Once a sufficient change of light level is detected, the photoelectric sensor switches an output device. Many types of discrete and analog outputs are available, each with particular strengths and weaknesses (discussed in Outputs & Wiring section).
Basic Circuit
Photoelectric sensors can be housed in separate source and receiver packages or as a single unit.
In the figure below the photodiode activates the output when light is detected. When an object breaks the beam of light between the source and receiver, the output turns off.
In below figure, receiver, and logic have been placed in the same housing. The output is activated when the light is reflected off an object back to the receiver. When the target object is present the output turns on.
Having the source, receiver, and logic in the same package makes it easier to design a control that limits interference (sensing other sources of modulated light).
Synchronous Detection
The receiver is designed to detect pulsed light from a modulated light source. To further enhance sensing reliability, the receiver and light source are synchronized. The receiver watches for light pulses that are identical to the pulses generated by the light source.
Synchronous detection helps a photoelectric sensor to ignore light pulses from other photoelectric sensors nearby or from other pulsed light sources, such as fluorescent lights. Fluorescent lights, using high frequency inverter type ballasts, require additional precautions.
Synchronous detection is most commonly found when the light source and receiver are in the same housing for all sensing modes except transmitted beam. Separate controls are also typically not capable of synchronous detection.
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