SENSORS

PROXIMITYKey Benefits of IntroducingCapacitive Sensing toElectronic Equipmentby Emmanuel T. Nana, NXP SemiconductorsNXP’s capacitive sensors use a patented auto-calibrationcapacitance-change detection circuitry to monitor thecapacitance at the input of the device. The device digitallyfilters out very slow and very quick changes in capacitance atthe input and continuously adjusts itself to the environment.As a result, the performance of the NXP devices is affectedless by conditions that can impair or prevent correctfunctions in other devices. The auto-calibration functionensures that long-term environmental changes such as dirt,humidity, freezing temperatures, or damages to the electrodedo not affect the device function.PROXIMITY90There is an increasing use of capacitive sensing technologyin many different applications that require touch switches,proximity detection, fluid level measurement, slider andwheel controls, position determination, human interface andmany other features. These capacitive sensors are changingthe face of today’s electronic equipment as they require noactual pressing on the sensor area and theywork when the user is wearing gloves.NXP Semiconductors’ capacitive sensordevices offer the added benefits of autocalibration,configurability, low powerconsumption, and increased reliability indifferent environments.NXP’s PCF8883 single-channel capacitivesensor has a wide operating supply voltagerange from 3V to 9V. A sensing plate connectsto the IN pin of the device and any change ofcapacitance on the IN pin will cause the open-drain OUT pinto toggle.Sensitivity and response time are the two primary items totake into account when fine-tuning the device performance.The sensitivity is adjusted by altering the value of thecapacitor on the CPC pin, while the response time iscontrolled by varying the value of the capacitor on the CLINpin.Fig.1 Typical connections of PCF8883The ability to vary the sensitivity of the devices makes itsuitable for use as either a touch sensor or a proximitysensor. For proximity sensing applications, the sensitivity isincreased by increasing the CPC capacitance; and for touchsensing applications, the sensitivity is lowered by reducingthe CPC capacitance. Proximity sensing is widely used inequipment to detect the closeness of an object,in touch-free operations such as hazardousenvironments or medical applications, and inhermetically sealed applications where thesensing plate is separated from the user witha dielectric material.Power consumption is always a mainconsideration for battery-operatedequipment using capacitive sensingtechnologies. The PCF8883 deviceconsumes less than 5μA, even under worst-caseoperating conditions (typically 2.2μA at 3.0V supply). Thismakes the device very suitable for applications wherebyall the electronics may be powered down or put in deepsleep mode and the PCF8883 is configured in the proximitysensing mode to scan the environment for any user’spresence.By enabling human interfaces that can be controlled byproximity or touch, capacitive sensors have the ability totransform the way we interact with electronic systems. Untilnow, a limiting factor has been reliability, since environmentalfactors such as humidity, dirt, freezing temperatures, andchanges in the moving object can have a negative impact onsensitivity and responsiveness.C SENS= Sensing plate capacitanceThe coaxial cable is optionalNXP Semiconductors, with its low-power PCF8883capacitive sensors, takes advantage of a patented autocalibrationfeature that improves reliability. Designing withauto-calibration requires a certain amount of fine-tuning,since the switch has to be optimized for typical applicationrequirements, but once the initial configuration is set, thesensor offers enhanced performance and longevity. As aresult, these auto-calibrating devices are expanding thereach of capacitive sensors, and bringing a new level of userfriendliness to a wider range of applications.www.FutureElectronics.com/Sensors