DocID024816 Rev 1

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AN4313

Projected sensor design

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Projected sensor design

2.1

Touchkey sensor

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

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.

The physical principlefor a touchkey, linear or rotary sensor is that an electric field surrounds

the Tx and Rx electrodes (see Figure 1). This field is dependent on the permittivity



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 1. Electric field between 2 surface electrodes

1.

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

Figure 2 to Figure 4.

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 touchkey

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.

2.1.1

Diamond type sensor

In this “diamond type” implementation, the Tx and Rx electrodes form a diamond shape

where the two electrodes are on the same layer, using a bridge (see Figure 2).

Note:

This symmetrical implementation allows swapping Rx & Tx.

MS18983V1

Dielectric panel

Electric field

Electrodes

Electric field

Electrodes