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Automation systems and devices, both those found in industry and in intelligent installations for residential buildings, use limit switches as intermediaries between control systems and controlled objects. Also similar application, together with measuring systems, have strain gauges, by means of which we can measure the pressure force. Most of electronic scales are based on this technical solution. The contact force sensors and limit switches offered by DFRobot from the Gravity series allow the implementation of many interesting projects, such as automatic gates.
Analog vibration sensor with piezoelectric membrane. It is powered with the voltage from 3.3 V to 5 V, it works with Arduino modules.
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Limit switches - ideal for use in automation systems
From the point of view of wiring diagrams, limit switches are in fact monostable buttons, whose task is to switch electrical circuits on and off. In everyday practice, we can observe it during opening and closing of refrigerator door - in this case, the limit actuator is mechanically connected with the moving contact. When the fridge door is opened, such a switch switches on the power supply to the lighting circuit and switches it off when the door is closed. Other common examples of limit switches used in household appliances are microwave ovens and washing machines. In these devices, limit switches act as a safety device, which switches off the device in case of opening the working area, in which there are working moving elements. Limit switches are also used in automatic gates. When gate leaf during opening or closing process approaches extreme deadlock position, pusher of limit switch contact is pressed and disconnects power supply to the actuator driving the gate leaf.
Force Sensors - Build an Electronic Scale!
One of the most common design solutions for strain gauges are resistance sensors. The principle of operation of such a sensor is to change the output resistance in accordance with changes in the weight load acting on the surface of the sensor. Under the pressure surface of the sensor, there are two layers separated by air - a conductive layer and a dielectric layer. When the sensor surface is unloaded, the output resistance of the sensor is the highest. When the sensor surface is unloaded, the output resistance of the sensor is at its highest, while when it is loaded, the output resistance of the sensor decreases according to the load due to the deformation of its conductive layer. The most common applications using strain gauge sensors are electronic scales. In addition, such sensors are used in electric keyboard instruments (the so-called aftertouch), where according to the force of the keys we can control the output volume of the instrument or control parameters such as e.g. the depth of frequency modulation of sounds played on the keyboard.
Besides the resistive sensors, very common technical solution are also capacitive sensors. The principle of operation of such a sensor is to detect the presence of objects whose electrical permeability is significantly different from the measuring medium, i.e. usually air. This fact has been exploited due to differences in the value of electrical permeability of air and human skin. The contact surface of capacitive sensors is usually made of glass. When we touch the surface of such a sensor, then changes in the electric field strength cause a change in the logic state at the sensor output.