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Metal Tactile Switches Capacitive Transducer
Jul 17, 2017

Metal Tactile Switches Capacitive transducer

How to replace the mechanical switch with capacitive sensor?

The capacitive touch sensor interface is usually composed of a capacitive sensor, a capacitive digital converter (CDC) and a main processor. The sensor USES a standard two-layer or four-layer PCB line (trace) or flexible circuit manufacturing, so no external components or materials are required.

Reliable sensors must not be affected by external environmental changes and can maintain accurate sensitivity at any working conditions. Changes in temperature or humidity can lead to changes in the properties of PCB materials, so the output level of the printed circuit capacitor sensor will drift. For example, this can happen when a user starts from an air-conditioned car to a hot and humid environment. To avoid intermittent contact errors, the CDC must include real-time drift compensation.

As environmental conditions change (e.g. temperature or humidity), the sensor's environmental parameters (measured by the CDC during the user's absence of contact with the sensor) will drift. In order to compensate, it is necessary to dynamically change the high and low threshold power to determine effective sensor contact. The reference level of position 2, 3, 5 and 6 is adjusted to keep the optimal threshold reference level, thus tracking and compensating the drift error automatically.

PCB may also be affected by the parasitic capacitance, which can reach up to 20pF. It will change the sensitivity of the threshold when the capacitive touch sensor is considered to be pressed. In order to compensate for parasitic capacitors, the method of DAC programming can be used to offset the CDC input. For the PCB, the parasitic capacitance is consistent, so it can be when manufacturing PCB, simple adjustment, so there is no need for external RC tuning elements, thus make the lowest cost of materials, assembly and testing. Independently adjust the offset of each sensor so that the designer can make full use of the converter's resolution.

Traditional mechanical switches have user's familiar sensitivity and tactile feedback, and these parameters must be considered and optimized for capacitive sensors. Different sensors may require unique sensitivity depending on the switching function or the physical location of the switch in the product. Also, a set of sensitivity Settings cannot be suitable for all users, so users should be allowed to set different levels of sensitivity, if can through the sensitivity control menu choice will be the most ideal. For example, AD7142 supports these sensitivity requirements and allows a single 16-bit sensitivity control register to be programmed for each sensor. These registers can also be embedded in the host firmware and are provided in the menu display, allowing users to select different levels of sensitivity to meet their specific needs. When the user is not in contact with the sensor, the battery charge will be wasted if the input of each sensor is taken. To maximize battery efficiency, the CDC should be able to detect a user's stop touching the sensor and automatically switch to a low-power mode. When the sensor is touched again, IC will automatically re-enter the normal working mode.

In order to save more power, it should include a complete cut-off mode. In this case, you can disable the entire IC as long as you disable the sensor. In portable products, disable sensor switches are usually done by setting a mechanical switch or selecting blocking mode from the control menu.

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