We had a customer come to us recently with a design proposal for a small portable device. The product concept included a small LCD display, a couple LED’s, and a mylar keypad with 8 buttons. I had been looking for a project which I could use as a vehicle to check out the Quantum QT1081 8 channel touch controller. (Quantum is now owned by Atmel, see here.) Because the product is lightweight, and may be either hand held or placed on a table when used, the force required to activate a typical mylar keypad seemed like it would interfere with the use of the product. Plus there’s the cost of fabricating the keypad. The button array of a capacitive touch keypad can simply be created on a standard single sided printed circuit board, and requires no force to activate. Because a finger touch can be reliably sensed from several mm away, in this product the copper pads are on the back of the switch array, which is overlaid with a layer of plastic on which the key legends are printed. Because of the dielectric characteristics of FR4 material used to make printed circuit boards, the board helps to “focus” the electric field coming off the pad. As a result, the sensitivity to touch on the back side of the board is increased.
I found with the QT1081 chip that key presses were very reliable, with no issues with double hits, etc., that one might encounter with a mylar keypad. The sensitivity of each key pad is easily adjusted to compensate for the differing amounts of stray capacitance associated with each key pad. Best of all, the power consumption is very low. A system using a PIC16F883 processor running at 4MHz with the QT1081 detecting a single key consumes only 2.4mA from a 3.0V supply. An overview of the operation of digital capacitive touch technology can be found here.
Adjusting QT1081 key touch sensitivity
A datasheet for the QT1081 may be found here. Please download this and turn to page 6, as figure 1.1 contains a reference schematic which will be used for the remainder of this discussion. The values shown for the resistors and capacitors connected to the SNS** pins are good starting values. The 1M resistors that are used to set the AT1081 operating modes will affect the sensitivity of their respective key inputs. I discovered that SNS pins that were pulled to ground were less affected than SNS pins that were pulled off to VDD. The values for CS0 through CS7 are MINIMUM values it seems. I found that with the CS* capacitors set to 1000pF (1nF), keys with option resistors pulled off to VDD would not reliably detect a touch. However, increasing CS slightly for those inputs to 2000pF resulted in working keys. The datasheet claims that capacitors as large as 50nF (0.05μF) can be used for sensing capacitors. Power consumption increases as larger values are used for CS. Use no more capacitance for CS than necessary to achieve reliable operation. Capacitor selection is discussed in section 3.3, page 11, of the datasheet.
Capacitive touch opens up many new design possibilites. Other touch controllers in Quantum’s product line include slider bar and wheel sensors with up to 256 discrete positions that can be sensed. Elimination of mechanical switches can be a huge win in terms of product styling and reliability, as well as cost reduction. Because the Quantum components use a digital algorithm they are able to adapt to long term changes in the capacitance of the button array, such as water droplets on the sensing surface. This reduces issues with “stuck” keys.