HEXWAX - - Capacitive Touch KeyPad Demo Board |
AccessTouch is an integrated combination key access controller featuring tough-sensitive keys for vandal-proof operation. An accompanying design blueprint is available for a complete low cost, access control product.
AccessTouch uses capacitive touch sensing technology. The PCB keyboard shown can be overlaid by a non-metallic covering. It automatically calculates signal averages to compensate for track capacitance, water splashes, etc. A buzzer output provides audio feedback as buttons are pressed.
AccessTouch is firmware for the PIC16F631 microcontroller and is available as a preprogrammed chip from electronics distributors or as a firmware download from www.hexwax.com.
When the correct code is entered, the door release output is activated for a fixed time delay. The sequence of keys pressed prior to entering the code is unimportant.
To change the access code, an electrical switch is closed and the new code is entered twice.
- Tiny volume BOM cost
- Vandal-proof design, no parts need to be physically exposed
- Codes may be up to 8 digits
- Audio feedback on touch press
- Safe Controller mode available
- 20-pin SSOP package
- Supply voltage Vdd 2.0 – 5.5 VDC
- Current, sleep scan mode, 3V Vdd ~50ìA
- Operating Temperature –40°C to 85°C
Bill of Materials
The Table 2 shows the recommended components for the bill of materials. The table does not include components for providing power or driving the lock release, which will be application specific. Package sizes apply to the gerber files in the product blueprint.
How It Works
The touch pads measure the capacitance of the interleaved touch contacts using a relaxation oscillation oscillator which scans the contacts in turn. When a finger is placed close to the contacts, the capacitance will increase, reducing the frequency of the oscillator. A sudden drop in capacitance is interpreted as a button press.
The touch pads measure changes in capacitance relative to a recent moving average. This provides some resilience to changes in humidity and water splashes. In order to achieve reliable operation, keys do not need to be pressed hard, but they must be held for approximately 250ms.
When power is applied, all three LEDs light for approximately 15 seconds while the average capacitance is initially measured. After a period of inactivity, an optional sleep state reduces the key scan rate to once every 500ms. If this mode of operation is used, the first key must be held for this time in order to wake the device.
Vss is the power supply ground reference. Vdd should be connected to a DC supply of 2.0V – 5.5V.
TouchA – Touch D
The TouchA to TouchD pins configure the relaxation
Touch1 – Touch4
The Touch1 to Touch4 pins are capacitance sense inputs. They should be connected to the touch pads.
Vpp, PGC, PCD
TEAclipper programming pins. Refer to the Delivery and Programming section for details. Note that the Vpp pin may be subject to voltages as high as 12V during programming.
The pin is an active low reset input. It is important that this is connected to a 22k pull-up to Vdd, the TEAclipper connector, and nothing else.
Outputs high when the lock release is triggered. This pin can drive up to 25mA.
Outputs high when the lock release is not triggered and the device is not in a sleep state. This pin can drive up to 25mA.
Set Code LED
Outputs high when the SetSw input is high. This pin can drive up to 25mA.
Set Code input control. Set to Vdd to enter a new code. The code must be entered twice and is acknowledged with a beep. This minimizes the chance of entering in an incorrect code while keeping user operation simple. It does mean, however, that codes made up of repetitive sequences longer than the minimum code length cannot be specified, e.g. 12341234.
Typically the set switch will only be accessible from inside the secured-access area, or will be implemented as a key switch.
The code set by default is 123.
The lock outputs control the access lock. These pins can drive up to 25mA, sufficient for a small relay.
Connect to a buzzer to provide audio feedback:
High note: Button press registered
Medium note: Code set
Low note: Lock open
These pins can drive up to 25mA, so a current limiting series resistor may be required. This is the function of R12 in figure 1, which also serves as a volume control.
Touch Pad Design
The touch pads need to be designed for maximum capacitance when a finger is present, and minimum capacitance when not present. The design shown in figure 1 has been shown to be effective, with a track width of 1.1mm and gap of 0.33mm. The overall pad size should be at least 13mm square, with a minimum gap between pads of 5mm.
No physical contact is required and the touch pads may be overlaid with, for example, a label or thin acrylic panel. Alternatively, the pad may be placed in an inner PCB layer immediately below the exposed PCB surface.
Button presses are detected as a rise in capacitance in one or two sense lines. Too high a trigger sensitivity can be just as troublesome as low a trigger sensitivity.
Therefore the sensitivity setting may require adjustment to suit a particular physical setup. (See non-volatile settings section below.)
To avoid ambiguity, it is important that fingers do not induce capacitance where not wanted. Avoid
unnecessary routing tracks on the exposed surface of the PCB. Apart from the touch pads, the circuit should avoid being placed where fingers might get close. Avoid metal enclosures.
Various settings can be specified in the code memory at programming time (see the Programming AccessTouch section). Alternatively, they may be set in EEPROM memory at a later date using a PIC programmer.
|Man Part No.||Description|
|KIT, EVAL BOARD, TOUCH-ACCESS|