Capacitive Touch Key principle

At this stage, with the capacitive touch button in appearance and service life and other aspects are superior to the traditional mechanical keys, capacitive touch keys are increasingly widely used in the field, including home appliances, consumer electronics, industrial control and mobile equipment. In this paper, a specific capacitive touch switch chip SJT5104 introduces the basic working principle and material selection of capacitive touch button.

a working principle

There is an inductive capacitance between any two electrically conductive objects, a key is a pad and the earth can also constitute an inductive capacitance, in the surrounding environment unchanged, the capacitance value is a fixed constant small value. When a person's finger is close to the touch button, the inductive capacitance between the human finger and the Earth's inductive capacitance parallel pad and the earth will increase the total inductance capacitance. The capacitive touch key IC outputs a definite signal that a key is pressed when it detects a change in the sensing capacitance value of a key. Capacitive touch Buttons Because there is no mechanical structure, all the detection is a small change in power, so the various disturbances will be more sensitive, so touch key design, touch panel design and Touch IC selection is very critical.

two touch pad design

1. Touch pad Material

Touch pad can be used PCB copper foil, metal sheet, flat-topped cylindrical spring, conductive cotton, conductive ink, conductive rubber, conductive glass ito layer. Regardless of the material used, the key sensor disk must be tightly attached to the panel, the middle can not have air gap. When using a flat-topped cylindrical spring, the touch line and the spring connection of the PCB, the diameter of the hollow floor should be slightly larger than the diameter of the spring, to ensure that the spring even if it is compressed onto the PCB, will not touch the floor.

2. Touch pad shape

In principle can be made into any shape, the middle can be left holes or hollow. The authors recommend the shape of the edges to be rounded to avoid point discharge effects. Generally the use of circles and squares is more common.

3. Touch Pad Area Size

Key sensor disk Area size: Min 4mmx4mm, Max 30mmx30mm. The actual area size depends on the sensitivity, and the area size is proportional to the sensitivity. In general, the diameter of the key sensor disk is greater than 4 times times the thickness of the panel, and increase the size of the electrode, can improve the signal-to-noise ratio. The shape and area of each sensor disk should be the same, to ensure consistent sensitivity. Typically in most applications, 12mmx12mm is a typical value.

4. Touch the distance between the pad

The distance between each touch pad should be as large as possible (greater than 5mm), thus reducing the interaction between the electric fields they form. When using the PCB to touch the pad, if touch pad between the distance closer (5mm~10mm), touch pad must be covered with ground isolation. If each touch pad is far away, it should be as isolated as possible. It is helpful to increase the touch sensitivity by pulling the distance between each touch pad properly.

three, touch panel selection

1. Touch Panel Material

The panel must choose the insulating material, can be glass, polystyrene, poly * * Vinyl (PVC), nylon, resin glass, etc., the key is above 1mm can not have metal, touch button 50mm within the metal must be grounded, otherwise the metal will affect the sensitivity of the case. In the production process, to maintain the material and thickness of the panel, the surface coating of the panel must use insulating paint.

2. Touch Panel Thickness

The panel thickness is usually set between 0~10mm. Different materials correspond to different typical thicknesses, such as acrylic materials are generally set between 2mm~4mm, ordinary glass materials are generally set between 3mm~6mm.

3. Double-sided Adhesive

Touch button PCB and touch panel through double-sided adhesive bonding, double-sided adhesive thickness of 0.1~0.15mm more appropriate, recommended the use of 3M468MP, its thickness of 0.13mm. Requires no air between the PCB and the panel, because the dielectric coefficient of air is 1, and the dielectric coefficient of the panel is significantly different. The air will have a great influence on the sensitivity of the touch keys. So double-sided adhesive and panel, double-sided adhesive and PCB bonding, are touch key production assembly process, must ensure quality.

PCB and double-panel adhesion, PCB with double-sided adhesive and panel assembly are to use positioning fixture to complete the assembly, after the completion of the assembly, to manual or clamping clamp. In order to ensure that there is no air between the PCB board and the panel, it is necessary to open holes and exhaust slots on the double-sided board, and to cooperate with the PCB opening. When the clamping fixture is designed, the key presses the touch key to ensure that there is no air in the sensing area.

four, touch IC selection

At present, the world's well-known electronic components suppliers have increased the capacitive touch key IC application research, and launched a number of professional chips (this is referred to as Touch chip), there are many MCU-based integration class IC, designers choose a larger space, According to the requirements of the function and the cost-effective chip to choose the IC suitable for the design needs, you can also design MCU-based A/D port to realize the touch IC.

Five, SJT5104 touch IC Introduction

1. Basic Introduction

The SJT5104 is a low-cost, high-reliability capacitive touch sensing IC with 4 touch-sensitive keys and 4 direct output ports, built-in Ldo regulator circuitry, high power supply noise tolerance, low peripheral components, and simple design with minimal components to complete the hardware design. Available in 2 output modes, selectable output high/low, 2 modes of operation, multi-key weight-dissipation, 2 output types. The sensitivity of each touch-sensitive button can be adjusted freely as needed, increasing the operability of the product and making the design more flexible and changeable.

The SJT5104 is a low-power , low-cost, 4-channel capacitive touch sensing IC designed for four-way touch switches, four-button touch keys, and power-saving electronics, with SSOP-20 small package, four-button sensitivity independently set, and a built-in regulator system Anti-power fluctuation drop interference ability, SJT5104 with ambient temperature, humidity adaptive ability, no weather or environmental changes affecting sensitivity and work stability, stable operation in the 2.5v~5v voltage range, standby current is only 3uA, The direct output mode of the analog mechanical button and the trigger output mode of the analog mechanical switch are provided, and the high/low level of the corresponding output can be set directly by the function setting port by the touch detection. Multiple keys can be set to eliminate the function, the detection of a few keys while touching the effect is only allowed one to take effect.

2. Basic parameters and benefits

4 Capacitive touch-sensitive keys;

Operating voltage: 2.5v~5.5v;
Power consumption: vdd=3v no load, low power mode 2.5uA, fast mode 9uA;
The sensitivity of each key can be freely adjusted by the external capacitance.
Direct mode and trigger mode, high impedance output and CMOS output are provided;
Embedded Ldo regulator circuit, strong resistance to power fluctuation drop interference;
Environmental temperature and humidity change automatic adaptation function;
Multi-button cancellation function: Allow single-key mode and multi-key mode;
Ultra-strong anti-EMC interference capability.

3. Basic Application schematic diagram

Among them, S0, S1, S2, S3 are 4 touch keys;

The OUT0, OUT1, OUT2, and OUT3 are the corresponding 4 outputs connected to the ADC device or MCU with ADC peripherals.

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