Optical proximity switch

An optical proximity switch consists of a light transmitter (often a light emitting diode or a laser diode ) and a light receiver (for example a light-sensitive resistor (LDR) or a photodiode ). The receiver (evaluation unit) evaluates the intensity, color, or the duration of the received light from the transmitter light from. The output signal of optical proximity switches is binary .

Light-emitting diodes with a wavelength of 660 nm (visible red light) and 940 nm ( infrared range ) are often used as the light source . Infrared light has the advantage of achieving a greater range on dark materials. The advantage of visible red light is that the sensor system is easier to set thanks to the visible light spot. For particularly precise applications (small part detection, high repeat accuracy ) i. d. Usually red laser light is used.

Modes of operation

With the light barriers, the light beam sent by the transmitter is either sent directly to the receiver (one-way light barrier) or reflected by a reflector (reflection light barrier) and then picked up by the receiver. In both cases, the signal is triggered when the light beam is interrupted.

A light button has the optical transmitter and receiver integrated in one unit. It reacts to the light reflected from the object. The switching distance therefore depends on the reflection properties of the object surface.

Proximity switches based on the principle of time-of-flight measurement use a photonic mixer as a sensor. The advantages are long range, reliable background suppression, and high functional reserve.

Proximity switches with laser triangulation have a spatially resolving photodiode or a CCD line as a sensor.

Further optical proximity switches react to the reflected color and contain a color sensor and threshold value switch .

Use in industry

The optical proximity switches are often operated with a nominal voltage of 24 V in the field of automation technology. At the output they always have a switching output (potential-bound to the supply voltage ), the switching position of which is controlled by the received light. In addition, many proximity switches offer a second, inverted switching output. Depending on the design, these switching outputs can also be electrically isolated by a relay .

Use in the smartphone

In smartphones, optical proximity sensors work with an LED that works in the wavelength range around 900 nm. The light has a longer wave than humans can see, but shorter wave than infrared sensors of, for example, remote controls (around 1000 nm). There are “binary” sensors that only register whether an object is closer than a threshold value (typically 2–4 cm) to the sensor, and sensors that measure the distance in centimeters. In order to be able to work independently of interfering light , the LED emits light in a pulsing mode. Using the principle of the lock-in amplifier , the intensity of the reflected light can then be measured independently of changes in the ambient light.

advantages and disadvantages

Optical sensors offer the advantage over magnetic, capacitive and inductive sensors that they can not be disturbed by ferromagnetic substances or by electrical fields or by magnetic fields . Photoelectric sensors generally have a long range . Optical sensors are particularly sensitive to fog or particles in the air , as this can severely dampen or interrupt the light beam . Soiling in the area of the optics usually leads to malfunction or failure of the optical sensors. Furthermore, even a small change in the reflection behavior of the measurement object with optical proximity switches often results in a change in the settings. An important source of interference for optical proximity switches is interference from the sun or from artificial light sources, such as fluorescent tubes, welding arcs, and lightning. Their level is often subject to strong fluctuations and is significantly higher than the transmitted light level. In many cases, this can be remedied by optical modulation of the transmitted light. Instead of a constant luminous flux, pulses of a fixed frequency are sent out. The receiver only evaluates the impulses of this frequency and suppresses all superimposed interfering components.

Web links

Optical sensors Basic ifm electronic gmbh (PDF 4.16 MByte)

Individual evidence

↑ Greg Milette, Adam Stroud: Professional Android Sensor Programming . John Wiley & Sons, May 17, 2012, ISBN 978-111818348-9 , p. 85.

[MiletteStroud2012-1] Greg Milette, Adam Stroud: Professional Android Sensor Programming . John Wiley & Sons, May 17, 2012, ISBN 978-111818348-9 , p. 85.