AN2869 datasheet(33/43 Pages) STMICROELECTRONICS

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AN2869

Projected sensor design

Doc ID 15298 Rev 6

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Projected sensor design

Projected sensors are generally used to implement keys, rotary sensors or linear sensors

for use with microcontrollers such as STM8TL5x which embed a ProxSense interface.

4.1

Touchkey sensor

A projected sensor is composed of 2 electrodes, one connected to a transmitter and the

other to a receiver.

The touchkey sensor can be of any shape, however it is recommended to use a square as

this shape is the simplest. The touch sensing library and ProxSenseTM IP automatically

compensate for capacitance differences, but the acquisition time and processing

parameters can be optimized if the sensors have a similar capacitance. For this reason, it is

recommended to use the same shape for all sensors.

An electric field surrounds the Tx and Rx electrodes (see Figure 27). This field is dependent

on the permittivity

R of both the front panel and the PCB. It should not be dependent on air

bubbles or moisture which can be trapped between them, because they must be sufficiently

well bonded by the adhesive.

Figure 27.

Electric field between 2 surface electrodes

The above figure only shows a simplified representation of the sensor; for specification details, refer to

Figure 28 to Figure 30.

Hence, the sensitivity is dependent on known materials and is optimized. This will ensure

that the disturbance caused by the user’s finger is detected and measured with accuracy.

It is recommended to define the sensor size in relation to the panel thickness (d) and vice-

versa. The following formula helps the designer to define the external size of key sensors

(L), and also for linear touch sensor and rotary touch sensor parcels:

L (mm) = 3 + (1.75 * d)

For example, a typical 10*10 mm touchkey should be used with a 4 mm Plexiglas panel.

MS19683V1

Dielectric panel

Electric field

Electrodes

Electric field

Electrodes

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