Electronics Design and Manufacture
ABN: 57 995 688 558
QKEYPAD-K16 is a 16 key proximity touch sensing keypad intended
to detect human touch through any dielectric panel up to a thickness of
10 mm. This product is designed specifically for OEM users needing
to provide data entry into a sealed or protected environment.
The Qkeypad features:
as glass, wood, or plastic the contacting surface becomes a touch
sensitive keypad with 16 discrete keys. During operation minute
amounts of charge is coupled by a set of 16 electrodes through the
overlying panel. Any contact with the panel over the area of the
electrodes causes charge to leak away from the electrodes and a
corresponding drop in voltage is detected as a key activation.
Equivalent circuit of 4X4 analog crosspoint output.
Two interfaces are available on the Qkeypad-K16. A 4x4 analog
crosspoint matrix, and serial uart output. The serial output can be in
RS232 or TTL levels depending on which pin is connected.
Shown here is the equivalent circuit output when using the analog
crosspoint matrix. The row to column connections for inactive keys are
effectively open circuit. When a key is selected the row to column
connection remains active until the key is released.
The serial output will transmit key codes whenever the corresponding
key is activated. Codes are sent once at the point of contact detection.
Principle of operation:The Qkeypad proximity keypad unit has 16 proximity key sensing areas. The sensing areas consist of simple
PCB tracks of conducting material that when pulsed with an electric charge creates an electrostatic field
surrounding each proximity key area. When a non-conductive panel is placed over the Qkeypad the
electrostatic field permeates through the panel above the sensing area. When contact is made by the human
finger some of the charge flowing through the overlying panel is "leaked" away as shown in this figure below.
Qkeypad under 10mm glass
This leaking away of charge is detected by the associated circuitry as a change in voltage levels. When the
change in voltage beyond a certain reference level reaches a defined trigger point the Qkeypad detects this
as a key press.
In practice any object near the sensing electrodes will cause some leaking of charge to occur. Also water
droplets and other surface contaminates will cause coupling of the electrostatic field between the
individual proximity keys. To avoid a false trigger an individual reference voltage is generated for each
proximity key area. Additionally the reference voltage is adjusted periodically to allow for any fluctuating
influences such as water moisture that would change with time at a rate much slower than would be the case
for a genuine key press.