Apr 062013

Photoelectric sensors, also called photosensitive switches, serve to automatically turn on outdoor lighting in reduced ambient light. They can be integrated into the luminaire itself or they can be located in proximity to it. Their main component is a photoelectric sensor, which is responsive to light. Very often these photoelectric switches are used for managing and switching street lighting in cities or for illuminating parks, ports, railway stations, public areas of airports, barracks, storage areas, as well as facades of single-family homes and villas, automatic lighting of shop windows, etc. This device can operate without human intervention, and this is its main advantage. It is not affected by daylight saving time, it has very low consumption, the more complex models have options for setting the sensitivity by a potentiometer, it has model variations with an external sensor


uncontrolled photosensitive switch


(a photo switch with an external Photo-sensor), connected to the actual switch by a cable, which allows the installation of the switch in the electric panel, rather than outside, under adverse weather conditions. There are photoelectric switches produced for 12 volt DC voltage and switches for 220 volts AC voltage. Usually the lower threshold of switching-on of unadjusted photosensitive switches is 30lx, and the upper threshold of switching-off of the luminaires, connected to them – 150lx.


a photo switch with an external Photo-sensor



When installing photoelectric switches, people make a common error, which leads to incorrect operation of the device and causes its entry into on and off cycling mode. It is the following: if you place the photosensitive switch so that the photo-sensor falls within the scope of lighting of the lamp, switched-on by the light sensor, it of course exceeds the upper threshold, at which it switches off and respectively switches-on the lights, and then the photo-sensor falls again in the dark, passes the lower limit of activation, and again switches on the lights. Then the process is repeated and the photo-sensor switches off the lighting again. To avoid this, you need to consider very carefully the location of the installation of the photoelectric switch. It is better to choose where to install the photosensitive sensor during the nighttime, with switched on lighting. This will also take into account the scattered light coming from other light sources, unrelated to the lighting system commutated by the photo-switch, e.g. from windows of neighbouring buildings, etc.

If the appropriate place for the sensor is such that there is a parasitic light from one side, the photosensitive surface of this side can be painted black to insolate its sensitivity from this side particularly. If, ultimately, the sensor cannot be activated on the selected location, when the lighting is switched on in the usual way, when there are side parasitic lights, etc., then the location is selected properly. Of course, the installation itself may be carried out during daylight after the location has been already selected. The good thing about this type of equipment is that even the simplest and cheapest variation is enough to create an intelligent system for lighting control for your yard or around your store. Let’s look at a simple unregulated photoelectric switch. At a price of only 7.50lv, VAT added, you get a light-dependent switch that allows you to automate your outdoor lighting. If the switch has insufficient power, you can connect the device to the operational chain of a contactor and the load (the luminaires) – to its power circuit.



Photo-sensors are used in various devices with various applications. Тhey can be used in printers, scanners, copiers, in CD players, DVD players, camcorders, even in computer mice, as well as in various amateur constructions. In the majority of these cases we talk about photoelectric switches, responsive to the interruption of a light beam or an infrared beam when an object passes through it, or a change in the intensity of a beam.

Photo-resistor (Light dependent resistor). Constructively, photo-resistors consist of a metal housing with a glass window, through which you can see a gray tile with a zigzag path. More recent models have a plastic housing. The photo-resistor’s performance is not great, so they can be used only at low frequencies, which severely limits their use. If you want to check the condition of a photo-resistor, you can use a multi-meter. In the absence of light, the so called dark resistance must be high, of the order of 20-30 MOhm. When the LDR is illuminated, its resistance drops to several kOhm.


light dependent resistors




Photodiodes. They very much resemble a conventional rectifier diode, if not their ability to respond to light. If we test the condition a photodiode with a multi-meter, in the absence of light, the results will be the same as when measuring a simple diode: in forward direction the device will register low resistance, but in reverse direction – the resistance will be high. It is considered that the diode is connected in reverse direction when practically no current flows through it. But if at that moment we switch on light, the resistance will drop sharply. This mode of operation of the photodiode is called a photodiode mode. Photodiodes also have a photo-galvanic mode of operation, in which in case of direct light on the diode, it generates small voltage, like a solar battery, which, if reinforced, may be used as a useful signal. But still photodiodes operate mostly in a photodiode mode. Old models of photodiodes represented a metal cylinder with two outlets and a glass lens on top. Modern photodiodes have a body made of transparent plastic and look very much like LEDs. ()



Phototransistors. They look just like photodiodes – the same transparent plastic cylinder or a cylinder with a lens on top and two outlets at the bottom: an emitter and a collector. The base, as an outlet, is not used in phototransistors, because here, light acts as a base. Despite this, some phototransistors still have a base, which allows them to be controlled not just through a luminous flux, but through electrical flux as well. This can be seen in some transistor optocouplers. Photoresistors can be checked for reliability also through a multi-meter, even if they have no base. When you set the multi-meter to measure resistance, the resistance between the collector and the emitter must be high when the phototransistor is darkened. When the lens gets enough light in terms of intensity and spectral width – the ohmmeter should show low resistance, of course if the polarity is respected. This behavior of the phototransistor very much resembles the behavior of an ordinary transistor, with the difference that the phototransistor’s control signal is light, not electric. It is worth mentioning that in addition to the intensity of the light, its spectrum plays a considerable role as well.

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