user Electronics Engineer

city Innovuze Solution Inc.

  • Contactless Inductive Proximity Switch

  • Created: Apr 11, 2016

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In rugged environment that has presence of dirt, oil, or moisture, switches like mechanical switches, reed switch, or contact switch will fail. The LDC0851 is a close range inductive switch for contactless and robust applications. Unlike other products, it does not require magnets and not affected by DC magnetic fields. The switch is contactless and can be triggered when a conductive object come within close proximity of the sensing coil. It has hysteresis to ensure a reliable switching threshold that is immune to mechanical vibration. The Environmental factors such as temperature variation or humidity effects can be prevented with its differential implementation to prevent false triggering.

The circuit has three parts; 5V micro-USB to 3V voltage supply, battery and LDC0851, and sensor. The circuit has a 3V supply from the 5V micro-USB and from the 3V battery coin that can be switch depending on the circumstances using the switch (U5). The LDC0851 is an inductance comparator with push/pull output. It uses the sensing coil (Lsense) and reference coil (Lref) to determine the relative inductance of the system. The output pin switches low when the sense inductance drops below the reference and returns high when the reference inductance is higher than the sense inductance. By matching the sense and reference coils it maintains a consistent switching distance over temperature and to compensate for other environmental factors. The switching point occurs when the inductances of both coil Lsense and Lref are equal. The ADJ pin has a 50k trim potentiometer for easy adjusting of the target distance. The EN pin is used for setting a new threshold, To set a new threshold, toggle the EN reset switch (U4) to set a new threshold. The Led (L2) will turn on if there is a presence of a conductive material in its switching point. It is recommended to have a target distance of less than 40% of the coil diameter from the reference coil. The characteristics of the sensor are 20mm outer diameter, 7 turns/layer, 6 mil trace, 6 mil space 2 layer, 6.2 uH. The sensor has 4 layers stacked, top 2 layers used for the sensing coil, and the bottom 2 layers used for the reference coil.

The circuit is contactless and robust that is suitable in applications such as Event counting like fan speed (RPM Detection) and flow meter, simple buttons used in consumer and industrial applications, and position detection.



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