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  • Capacitive Sensing with 4 Touch Pads

  • Created: Sep 17, 2014

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Summary

Capacitive sensing is a technology that operates by detecting the change of capacitance due to the influence of external objects like human body. This project circuit was developed to demonstrate the capability of the PCF8883 capacitive proximity switch.

The sensors used in this project are four different copper plates with a jumper pins for selection (additional external sensors can be connected). The shape, size, distance, and material of the sensor can be varied to adjust its switching behavior, this defines the sensitivity and reaction speed of the sensor for typical applications. The sensor is connected to the pin “IN” of the PCF8883 IC. Switching behavior modes can be also be selected by the jumper pin configured at the pin “TYPE” of the PCF8883 IC. There are three modes, toggle mode, momentary mode, and pulse mode. The toggle mode (jumper at “TOG” and “TIM”) operates in toggle touch-ON and touch-OFF. In momentary mode (jumper at “SW” and “TIM”), the output switches as long as the capacitive sensor is active. In pulse mode (no jumper placed), the output is switched for a defined time adjusted by the capacitor between “TIM” and “SW”.

Power supply for this circuit can be selected using a mini USB cable or a nine volts battery. The op-amp is included on the circuit to measure the voltage at the “CPC” pin of the PCF8883 IC. A capacitor is connected between pin “CPC” and Vss used to adjust the sensitivity of the switch. The circuit has three parameters that influence the switching behavior, first is the switch sensitivity configured by the capacitor at “CPC”, second is the calibration of the total capacitance on the sensor input using the parallel resistor and capacitor, and the third is the Switching speed configured by the capacitor at “CLIN”.

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