PCB Photoelectric Sensors; Operation mode:diffuse-reflective

Photoelectric sensors are used in numerous applications in state-of-the-art electronic engineering industry. They are installed both in simple consumer devices and complex industrial systems. As far as components “for printing” are concerned, these are mostly semi-finished products, i.e. these components can be used to manufacture precise, professional sensors, however, they can also be used in less precise circuits installed in household appliances, small electronic equipment or even toys.

Photo electric sensor design and types

A photoresistor belongs to the simplest photoelectric elements, but the majority of sensors “for printing” are based on semiconducting structures, i.e. photodiodes and phototransistors. Similarly to other photosensitive sensors, the general division of components is related to their method of operation.

Reflective sensors (diffuse-reflective)

Reflective sensors receive light reflected by an object located within their “field of view”.. The most frequently detected radiation is emitted by a LED. The operating distance of these elements depends on such factors as the light intensity and sensor sensitivity. In some cases, sensitivity can be adjusted by means of external controls.

Transmitter-receiver set-up (through-beam)

As for PCB-mounted components, a transmitter (LED) and receiver (e.g. phototransistor) are frequently enclosed in a single housing, usually a few millimetres away from each other. Such elements detect interruptions of a light beam in the sensor gap (e.g. initiated by a moving perforated tape).

Motion sensors

Motion sensors detect changes in the intensity of light reaching a photoelectric element (in most cases, they are characterised by sensitivity within a wide spectrum of colours , so the radiation source must be carefully selected). They often come with a semi-spherical, milky lens which collects light beams emitted from the widest possible angle. These products are very often used to manufacture automatic light switches or as anti-intrusion system components, etc.

Photoelectric sensor applications

The PCB-mounted sensors are used for two purposes: to create precise, factory-calibrated (industrial or automotive) sensors or in circuits with other functionalities, where light detection is a process-related method, but is not directly connected with the purpose of the target device (e.g. a motor RPM encoder described below).

Object detection

In industrial automation systems, detection of objects (e.g. on a production line to count their numbers) is a very important “sense” employed by machine controllers. One of such methods used to count objects is the application of photoelectric (mostly reflective) sensors.

Safety and protection systems

As it has been mentioned above, the motion detection functionality facilitates designing both reliable and sensitive safety systems. Here, at least two important applications must be mentioned. The first one involves anti-intrusion systems. As a photoelectric motion sensor can operate using infra-red light which is invisible to the human eye, it is perfectly suited to the needs of equipment responsible for protecting premises that typically operates in night-time conditions or darkened rooms. A similar application involves employing sensors to protect the health of employees and machine operators – they can be used to design light gates and curtains whose interruption is logged by a controller to emit an acoustic warning or even stop the machine, etc.

Industrial automation and robot-assisted systems

In extensively automated industrial plants, light sensors are used to determine the position of objects, but they also help determine the current position of machine parts (e.g. robotic arms or outriggers). Installing an encoder on the electric motor axis is a common method of controlling the operation of such motors. Such a set-up can be obtained in several ways, one of the simplest (but quite effective) being a combination of a transmitter-receiver type sensor and a disc made of opaque plastic in which radial slots are extruded. Each time a slot uncovers a sensor light channel, the encoder logs a single pulse. As the response time of such sensors is really short, they facilitate very precise control of shaft speeds, wheel positions or lever displacement. Photoelectric sensors are also used in consumer robot-assisted applications, e.g. in autonomous vacuum cleaners or Smart Home systems. Yet another typical industrial application of photosensitive sensors involves employing them to detect cuts or prints placed on packaging and materials to precisely position strips of paper, cardboard, textiles, etc.

Key photoelectric sensor parameters

While matching a photoelectric sensor to a given application, one should pay attention not only to basic parameters, e.g. the mounting method (THT, SMD), operating voltage (from 1 V to 24 V for components included in the TME product catalogue) or energy consumption, but also to other key characteristics. They include the range, i.e. from tenths of a millimetre to as much as several meters. Obviously, for practical and design-related reasons, motion sensors have a range that is several orders of magnitude wider than in the case of the transmitter-receiver pair, but this parameter can have completely different values even for a single sensor type. One should also note various output configurations of photoelectric sensors. Currently, products with digital or transistorleads are most commonly available, but the TME product catalogue also features analogue models. An operating mode is related to a sensor output, and it can assume the following values: DARK-ON (activation at light intensity below the preset threshold) or LIGHT-ON (reverse operation, i.e. radiation presence detection).