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The
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. |
Interface Selection 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 simplePCB 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. |
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