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Proximity Detection in the Presence of Metal Objects | Cypress Semiconductor

Proximity Detection in the Presence of Metal Objects

Last Updated: 
Jan 19, 2010

This application note describes implementing proximity detection in the presence of large metal objects. Recommendations about sensor mechanical construction and proximity sensing best practices are provided. An example of proximity sensing implementation for microwave ovens is also provided.

The ability to use proximity detection in white goods and automotive applications is often essential. For example, proximity detector is used to turn on the backlight in a kitchen stove or the internal lamp in a microwave oven when the palm is close to the door. In various home appliances a proximity sensor turns on the display when the user tries to adjust some parameters. Cypress provides a CY3235 kit that demonstrates proximity sensing. The CY3235 kit has a detection range of 30 cm when the sensor is located far away from conductive objects such as metals. When a wire sensor is placed on a metal surface, detection range dramatically decreases from 30 cm to 2 cm. Most white good and automotive applications have a metal frame or case that is a challenge for proximity sensing devices.

This CY3235 kit contains a wired sensor and small PCB with CY8C21434 chip on board, as shown in Figure 1. The reasons why the proximity detection range reduces dramatically when conductive objects are placed close to the sensor are:

  • The sensor stray capacitance increases. Stray capacitance reduces the proximity response value by providing a higher full scale range. Larger stray capacitance often requires operation frequency reduction, causing the additional detection distance to decrease.
  • A grounded metal plane catches a part of the sensor electric field and reduces the added by palm capacitance.

Figure 1. CY3235 Proximity Detector Demonstration Kit