Basics and implementation of capacitive proximity sensing

Posted on Feb 6, 2014

Capacitive proximity sensing has been a very popular topic with our audience over the past year. For your convenience, at the end of this article, there is a hot-linked list of related articles published at Planet Analog. To read Part 1 of this article, click here. ) Now that we have discussed the theory about the sigma-delta modul

Basics and implementation of capacitive proximity sensing

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ator, let us design a simple proximity sensor that will detect an approaching hand and switch on an LED. A 0. 5 mm2 wire, 10 cm long, is chosen as a sensor. Figure 7 shows the schematic of a proximity sensor circuit built using the CY8C21434 mixed-signal array from Cypress Semiconductor. The resolution of the sigma-delta modulator is set to 14. CINT and Rdis are the only external components required for the sigma-delta modulator. The count value from the sigma-delta modulator is passed through an IIR filter to form a reference count called Baseline. By selecting the filter response of the IIR filter, high-frequency noise on the instantaneous counts are rejected, but slow changes caused by temperature, humidity, and other factors are tracked. If the difference between the instantaneous count and the baseline exceeds a threshold, the firmware reports proximity. Figure 9 is the C code for the above flowchart. This is the most important stage of the development. Using the debug port and a PC GUI, display the Instantaneous counts, Baseline and Difference counts from the sensor. Perform the following steps: 5. The HYSTERISIS value in the firmware should be set in such a way that noise in the system does not create oscillations in the output. A few counts more than the noise level should work fine. We now have a proximity sensor. Watch the LED being turned ON and OFF as you bring your hand near and away from the sensor. Proximity...

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