AN1250 datasheet(13/22 Pages) MICROCHIP

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DS01250A-page 13

AN1250

CAPACITIVE TOUCH SLIDER

The capacitive touch slider senses the position of a

finger along a strip of the circuit board. The area desig-

nated as the strip has two capacitive sensor channels,

each formed in the shape of a triangle (see Figure 7 and

Figure A-2). The schematic of the PICDEM Touch

Sense 2 Demo Board with the cover plate removed

shows the slider toward the right hand side of the board.

FIGURE 7:

SLIDER IMPLEMENTATION

COPPER PAD SHAPE

Notice that the trace between the two triangular pads is

grounded. This trace provides additional isolation

between the two triangular capacitive touch sensors by

reducing the effect of capacitive coupling.

The slider works by reading the amount of shift in

capacitance from a normal untouched sensor. The slow

moving average described earlier is used to track the

untouched “base” capacitance of each of the two

sensors. When the value is shifted from this base value

to a value where the trip point is reached for either of

the two slider sensors, the base value is then locked

and the slow moving average is no longer used for

slider position calculations until the value of both slider

sensors is again above the trip threshold. The base

values for each of the two sensors is captured and

used for the slider position calculations. The software

algorithm then looks at the change in the slider value

from the captured base value. The position of a finger

on the slider is then determined by the following

equations.

EQUATION 1:

Since the left triangular sensor has its point at the

bottom of the slider, it is expected that the shift in

capacitance from a finger on the slider at the bottom will

be minimal, and maximum when a finger is positioned

at the top. The position of the left sensor (scaled for a

value of 0 to 100) is shown in Equation 2:

EQUATION 2:

Similarly, the right triangular sensor is large at the bottom

of the slider and a finger positioned at the bottom would

cause the largest shift in capacitance, while a finger at

the top of the slider would produce a minimum amount

of capacitance shift. The position of the right sensor

(again scaled for a value 0 to 100) is shown in

Equation 3:

EQUATION 3:

Finally, the two values may be averaged for the final

position calculation:

EQUATION 4:

It should also be noted that when neither the left nor

right slider has crossed the tripped value (base or

unpressed), the position is not calculated. This

prevents jitter between values that are displayed on the

slider LEDs.

To achieve maximum linearity and accuracy of the slider,

it is important to keep the capacitance for each of the

triangular sensors matched as closely as possible since

the software algorithm is using and comparing the

values for two triangular sensors. The trace lengths to

each triangular sensor and adjacent sensors, LEDs and

other board traces become important. The CTMU cur-

rent trimming capability, mentioned in the “Multiple

Keys Press”

section, is applicable to the slider sensors

as well.

To Input Pin B

To Input Pin A

Note: Slider is not to scale.

Delta Left = (Captured Left Base Capacitance – Current Left Value)

Delta Right = (Captured Right Base Capacitance – Current Right Value

Left Position = (Delta Left * 100)/(Delta Left + Delta Right)

Right Position = ((1 – Delta Right) * 100)/(Delta Left + Delta Right))

Position = (Left Position + Right Position)/2

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