Battery & Power Switch
The power switch is located next to the power connector. Due to the current
drain of the board, it is advised to leave this switch off when not in use.
Always use a 9V alkaline battery.
Although power is regulated onboard via a 78L05 device, raw power should be
free from switching noise and short-term fluctuations for best performance.
Circuitry
The sensing circuitry is located entirely in the center area. This includes the
voltage regulator, the QT160, the six Cs sampling capacitors, and series-
resistors in-line with each key to attenuate RFI.
The LED driver transistors are located under each key.
The LED’s are bypassed to circuit ground via bypass capacitors to prevent
them from reacting with the capacitance of the keys when the LED’s switch on.
This is essential to prevent signal ‘stiction’ from occurring. The junction capaci-
tance across an LED drops as the LED is forward biased; any coupling from the
LED to the electrode ring will be influenced by this change. The bypass caps
act to swamp this capacitance change with a much larger one, thus stabilizing
the detection.
Keys
The six keys on the board are simply wide circular traces surrounding open
holes in a PCB layer under the plastic panel. The plastic panel couples the
sense field up from the ring and into the centers of the keys.
Key sensitivity is governed by the value of Cs for each key; Cs values can differ
on a key-by-key basis. Increasing Cx increases sensitivity in direct proportion.
Recalibration Timeout (Max On-Duration)
The keys have an automatic recalibration timeout function that allows recovery
from 'stuck key' conditions that may be caused by foreign objects. The timeout
period is set via the option switches. This can be ten seconds, sixty seconds or
infinite (no recalibration).
If a key has been in continuous detection for the recalibration timeout period, it
will automatically recalibrate itself to the signal present at the moment of
Board
Details
The gains of the keys are set high enough by using appropriate values of Cs
sample caps so that the sense fields will penetrate up and through the plastic panel
into the centers of the circles. The keys do not have to be isolated from each other
via ground traces; since the QT160 channels acquire in time-sequence, they do not
cross-interfere with each other. Furthermore, each sense electrode is connected to
ground while waiting its turn to sense, so that the touch action from one key does
not bleed across into adjacent keys.
This configuration lets the sense fields permeate the center of each key but not
outwards from each key. Touching near the edge of one key will not cause an
adjacent key to detect. The AKS feature also helps to suppress cross coupling
between adjacent keys.
A schematic of the E160 can be found at the end of this guide.
How it
Works
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