We are using CY3280_MBR EVM to determining the frost formation between evaporator coil. The evaporator coil (acting as one electrode) that we have connected to CY3280_MBR GND pin J2 (pin 12). And 2nd electrode is copper plate of area 255 Sqcm which is connected to CS7 (J2 pin 6) by small wire length approx 1.5 inch the evaporator coil and copper electrode are separated by 5mm distance. CY3280 board is interface via I2C to Microcontroller which display raw ADC count .
When we are testing this board with 2nd electrode (copper plate connected to CS7) the touch application works fine i.e. when we are touching the copper plate or touching ground pin (pin 12 to CS7(copper plate) the raw count goes up to max value of 65030 and button press is detected.
But when same fixture we are fitting inside the refrigerator and connecting GND pin to evaporator coil the CAPsense is not working even if we touch the 2nd copper plate . It is just showing the raw base count around 35000, and no button press detected. For better understanding connection diagram is attached.
Please let us know the cause for the problem. If you have any other suggestion on the same kindly let us know.
First thing we would like to say is the physical dimensions of the electrodes you are using are very high. Ideally the dimensions of the buttons we recommend are 1.5cm * 1.5cm (in diameter max). Due to the the large size of buttons the parasitic capacitance of the sensors will be very high. For the operation of MBR3 sensors, the parasitic capacitance of the sensors should be less than 45pF.
Also the parasitic capacitance of the sensors will depend majorly up on the environment factors like temperature and humidity.
The smart sense algorithm inside the MBR3 will calibrate the sensors on every power up. Due to this you may have to calibrate the sensors again when there is power off.
In your case please check the parasitic capacitance of the sensors in various conditions like outside and inside refrigerator with worst case frost conditions. The parasitic capacitance in all the cases should be less than 45 pF. Please ensure that the electrodes are not touching the ground. Also please do not touch the copper plate directly.
However, MBR3 is primarily designed for detecting human touch or proximity. For this kind of applications, we recommend you to go with programmable device called PSoC especially PSoC 4000S/4100S/4100S plus families. These devices have latest CapSense IP which can allow the parasitic capacitance of the sensors upto 200pF. You can buy a very cheap stamp board of these devices and start testing your application.
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