Capacitive touch circuit implementation process

Seven steps to design a capacitive touch circuit

Figure 1 is a schematic diagram of the principle of capacitive sensing technology.

Figure 1 Schematic diagram of technical principle

Capacitive sensing technology is becoming the preferred technology for touch control due to its durability and relatively easy to implement at low cost. In addition, due to its scalability, this technology can also provide user functions that other technologies cannot achieve. Providing a user interface in the form of soft keys on a display is usually called a touch screen.

Touch input scrolling/pointing devices, such as the click wheel on the iPod music player, have gained widespread acceptance in the consumer market and are gradually appearing in more consumer devices. There are two basic types of scrolling devices: the first is the absolute reporting type, which provides a direct position output report; the other is the relative type, which provides a direct report used to increase or decrease a value.

The development process of designing a sensor switch circuit board using a capacitive sensing IC is slightly different from that of other circuits. Because the design of a capacitive switch is affected by the structure and other circuit designs, there will be more adjustment procedures, so a more complicated development process is required. Now I will introduce the development process and key points of the earlier and more representative "Quantum" product to you, hoping to help friends in need. The following figure is a development flow chart:

1. Institutional design

a. The panel must be made of plastic, glass, or other non-conductive materials.

b. During the mechanism design stage, the operation process must also be designed to select the appropriate product. If it is a button product, it is necessary to consider whether there is a composite button design, or a combination of sliding operation and button operation. If it is a sliding operation product, it is necessary to consider whether the button needs to be cut out.

c. Since there cannot be any gap between the sensing electrode and the panel contact point, the mechanism design must consider directly sticking the sensing path test plate to the inner side of the shell panel and considering the assembly method of the panel.

d. Similarly, there cannot be a metal layer between the sensing electrode and the finger, so the panel cannot have any metal plating or paint containing more than 15% metal that will form a conductive layer.

e. If electroplating or high metal content paint is required, please leave a circle around the edge of the key area without electroplating or painting to isolate other sensor switches.

g. The shell thickness of the mechanism design will affect the size of the sensing electrode, so the mechanism design must be completed before continuing the development process.

h. If there is a large piece of metal or circuit board behind the sensing circuit board, some space must be reserved to avoid reduced sensitivity or interference with the sensing electrode. If it is a metal plate, the metal plate must be grounded and the gap must be at least 0.3mm. If it is a circuit board, minimize the passage of high-frequency circuits and retain at least 1.0mm of space.

i. For the sensing circuit board with the above condition, although sufficient spacing is retained, it is better to enlarge the sensing electrode to facilitate the subsequent sensitivity adjustment steps.

j. The sensing electrode can be made of copper foil of circuit board, or conductive materials such as FPC flexible circuit board, ITO etching, ORGACON (CARBON) printing, etc.

2. Determine the size of the sensing electrode

a. The minimum size of the sensing electrode is determined by the panel thickness of the mechanism design. When the panel thickness is 1mm, the minimum diameter of the sensing electrode is a circle of 3mm. When the panel thickness is 7mm, the minimum diameter of the sensing electrode is a circle of 10mm. The sensing electrode should be as large as possible within the space allowed by the mechanism and circuit board.

b. The minimum area of ​​the sensing electrode cannot be less than 1/3 of the finger area.

c. Pay attention to whether there are large metal objects such as metal screws or iron plates near the sensing electrode. If so, the metal object must be grounded and the area of ​​the sensing electrode must be increased to avoid reduced sensitivity.

d. If a hole is opened in the sensing electrode to install an LED, the sensing electrode must be enlarged to make up for the area lost by the hole. Therefore, the increased sensing electrode area must be at least equal to the area of ​​the hole.

e. The sensing electrode can be in any shape, but it is best to use a circular or square shape. If all the space must be used to increase the area of ​​the sensing electrode, try to avoid designing the sensing electrode into a narrow and long shape.

a. The circuit design can be based on the sample circuit in the IC specification sheet.

b. A voltage regulator IC must be used to ensure that the power supply of the QUANTUM IC is clean and free of noise.

c. The resistors and capacitors attached to the sensing electrodes should be as close to the IC as possible. If it is a double-sided or multi-layer board, try to avoid passing high-frequency lines under the resistors and capacitors, and lay ground wires or wider lines.

d. If it is a single-layer board, there should be no high-frequency circuits near the sensing electrodes, and other circuits should be kept as far away from the sensing electrodes and their connections as possible. If the selected IC has an AKS function, please try to use this function to reduce the mutual interference between adjacent sensing electrodes.

e. If the AKS function is not enabled, adding a ground wire between the sensing electrode and its connection can also reduce the mutual interference between adjacent sensing electrodes. The ground wire must be placed in the center of two adjacent sensing electrodes, and the line width should not exceed 1/5 of the distance between the two sensing electrodes. Alternatively, the sensing electrode and its connection can be surrounded and separated by a ground wire. In principle, the farther the ground wire is, the better.

f. The line from the accessory of the sensing electrode to the sensing electrode can be laid with the minimum line width. The distance between the connection line of the sensing electrode and other lines is at least 5 times the line width. The distance between the connection line of the sensing electrode and the connection line of another sensing electrode is as far as possible, and the closest distance is more than 2 times the line width.

g. If the spacing between the connecting lines cannot be achieved, it is best to use a ground wire to isolate the lines. Use the smallest line width to lay the ground wire, and use the general safety spacing for line spacing.

h. The length of the line from the accessory parts of the sensing electrode to the sensing electrode should not exceed 30cm. The line of the sensing electrode can pass under the sensing electrode and avoid surrounding other sensing electrodes.

i. Avoid high-frequency lines under the sensing electrode lines, lay ground wires, or wider lines. If it is difficult to avoid, try to stagger them, and try not to let other lines run parallel to the sensing electrode lines.

j. If you really need to reduce the interference from under the sensing electrode and need to lay a ground wire, do not lay a whole piece of solid ground wire. Use a grid-like copper surface with a grid size of 1.27mm or more and a minimum grid line size.

k. The natural capacitance of the human body is about 5pF to 30pF. The ultimate principle of wiring is not to exceed the minimum value of 5pF of the natural capacitance of the human body.

4. Test the circuit board

a. This stage mainly tests whether the wiring of the circuit board is correct and whether the sensing action is normal.

b. At this stage, only the sensitivity needs to be roughly adjusted. No precise adjustment is required because it will change after the circuit board is installed in the casing.

c. Pay special attention to the placement of the circuit board during testing. The test circuit board cannot be placed directly on the desktop, nor can it shake when pressed during testing. The ideal test environment is to stick a panel or a replacement panel of the same thickness and material as the panel, and then use rubber feet to raise the circuit board and stabilize it.

d. The circuit board to be tested must not have jumpers. If there are jumpers, they must be unrelated to the sensing switch circuit, and must not pass through the sensing IC and its associated circuits, nor be near the sensing electrodes.

e. If the test is unsatisfactory, go to Step 4-1 to determine the cause of the failure. If the sensing electrode can work normally when you touch it directly, it can be determined that the sensitivity is poor.

f. If the induction switch is automatically triggered, or is released after a long time after being triggered, first check whether the power supply is stable. If the power supply is stable, it may be oversensitive. Reduce the Cs capacitor value to reduce the sensitivity and test again.

g. If the wiring is poor or wrong, return to step 3 and re-wire.

h. Once the sensitivity is adjusted to a point where there is no instability or inaction, you can proceed to the next stage and adjust the sensitivity.

5. Install into the casing

a. It is a necessary step to install the sensor circuit board into the casing for testing. It can be a handmade casing, and the trial casing is preferably a mass production casing. The printing and painting of the casing's exterior must be the same as the paint used in mass production.

b. The components in the case must be complete. It is best if other circuit boards have been properly installed and can be connected to the power supply. It does not matter whether other circuit boards work properly or not, as long as the button action can be tested.

c. During the testing phase, general non-substrate double-sided tape can be used for bonding, but for formal mass production, it is recommended to use 3M 468MP or NITTO 818 non-substrate double-sided tape.

6. Test sensitivity

a. Appropriate sensitivity means that the sensor switch will move when the finger lightly touches the panel. If the sensor switch needs to be pressed with great force to move, or the sensor switch moves before the finger touches the panel, it is a case of poor sensitivity.

b. Increasing the Cs capacitance value can improve the sensitivity, while decreasing the value can reduce the sensitivity. It must be noted that different ICs have different value range limits. Please refer to the IC specification document.

c. Improving sensitivity does not mean increasing the sensing distance. The thickness of the panel must be determined at the beginning of the design, and the area of ​​the sensing electrode must be sufficient.

d. Individual sensor switches will be affected differently by mechanisms or other components due to their different locations, so the sensitivity adjustment is performed individually for each sensor switch.

e. Ideal sensitivity can eliminate the false operation of the inductive switch and increase the tolerance of ESD test. (Already tested and passed 25KV)

7. Determine BOM

a. Only at this stage can the BOM be fully determined and trial production and mass production can begin.

b. SMD components are recommended for induction-related resistors and capacitors. There are no special requirements for resistors, and generally ±10% error is sufficient. X7R components with an error within ±10% are recommended for capacitors.