Design skills of capacitive touch sensor

Welcome to the IT technology community forum and interact with 2 million technicians. touch sensors have been widely used in the industry for many years. However, with the recent development of hybrid signal programmable devices, capacitive touch sensors have become a substitute for traditional mechanical switches in a wide range of consumer electronic products. Capacitive induction switch is very attractive

Welcome to the IT technology community forum and interact with 2 million technicians> touch sensors have been widely used in the industry for many years. However, with the recent development of hybrid signal programmable devices, capacitive touch sensors have become a substitute for traditional mechanical switches in a wide range of consumer electronic products. Capacitive induction switch is very attractive

Welcome to the IT Community Forum and interact with 2 million technicians>

Touch sensors have been widely used in the industry for many years, but until recently, with the development of hybrid signal programmable devices, capacitive touch sensors have become a substitute for traditional mechanical switches in a wide range of consumer electronic products.

The capacitive induction switch is a very attractive switch, but it requires an appropriate physical size as well as a 3mm or thinner thin film stack used in a typical capacitive sensor design. As the thickness of the laminated film increases. It becomes increasingly difficult to sense fingers through film laminated. In other words, as the thickness of the laminated film increases, the processing process of the tuning System is changed from "Science and Technology" to "design skills. To demonstrate how to make a capacitive sensor break through the limits of today's technology, the glass covered thickness used in this example system is set to 10mm. Glass is easy to use, easy to obtain, and transparent. You can see the circuit pad of the sensor under the glass. Glass coating is also applied directly in the so-called "white appliances" (household appliances.

1. Finger Capacitor

The core of any capacitive sensing system is a group of conductive bodies interacting with the electric field. The tissue of the human body is filled with conductive electrolyte, which is covered by the human skin, which is a loss of insulator. This finger conductivity makes capacitive touch sensing possible.

A simple parallel board capacitor has two conductors separated by a media layer. Most of the energy in this system is concentrated between two parallel plates, but some energy may flow out of the two plates of the capacitor. The field line related to this effect is called the edge field of the electric field. One of the challenges facing the production of practical capacitive sensors is to design a set of cabling for printed circuits. This cabling method allows users to obtain an effective sensing area. For this sensor mode. Parallel board capacitor is not a good choice.

Placing a finger near the edge electric field increases the area of the conductive surface of the capacitor system. The additional charge storage capacity caused by the finger is generally referred to as the finger capacitor (Cf ). When no finger is placed on the surface, the capacitance of the sensor is expressed in Cp in this article. It represents a parasitic capacitor.

A common misunderstanding about capacitive sensors is that fingers need to be connected to the ground to make the system work. The reason why the fingers can be felt by the system is that the fingers are charged when the fingers are floating or grounded. The system can feel it.

2. PCB layout of the Sensor

Displays the top view of a printed circuit board (PCB). In this design example, this PCB implements one of the capacitive sensor buttons.

The button is 10mm in diameter, equivalent to the average size of an adult fingertip. The PCB for this demo circuit consists of four buttons for the above design method with a center spacing of 20mm ., The top layer is connected to the ground. There is an even gap between the sensor pad and the grounding surface. The gap size is an important design parameter. If the gap is too small, too much field energy will be directly transferred to the formation. If the configuration is too large, the field will pass through the stack directly, and the control will be lost. The 0.5mm-size gap is the optimal spacing for guiding edge fields through the 10mm glass covering layer.

Displays a cross section of the same sample sensing mode.

Connect a sensor pad to a bottom-layer track through a pass through the PCB, as shown in figure 2. When the electric field tries to find the shortest path of the Back-to-earth, the dielectric constant ε r will affect the closeness of the electric field energy in the material. The dielectric constant of the standard window glass is about 8, and the dielectric constant of the FR4 material of the PCB is about 4. The dielectric constant of high hardness Glass Used in white household e-commerce products is usually about 5. In this design example, standard window glass is used. It should be noted that the glass plate is installed on the circuit board with the non-conductive adhesive tape 468-MP of 3 m.

3. capacitive sensing: 101

A basic component of a capacitive sensing system is a programmable current source, a precise analog comparison circuit, and a analog multiplexing bus. The bus can be sorted by a capacitive sensor array. The relaxation oscillator in the system described in this article serves as a capacitive sensor. The Simplified Circuit of this oscillator is shown.

The output of the comparator is used as the clock input signal of a pulse width modulation (PWM) circuit. It is responsible for selecting a 16-bit counter with a frequency of 24 MHz. When the finger contacts the sensor, it will increase the capacitance, thus increasing the total value of the counter. This is how a finger is perceived. The typical waveform of this system is shown in Figure 4.

Shows the circuit schematic to achieve this project.

In order to realize capacitive sensing and serial communication, the circuit design adopts the PSoC chip of CY8C21x34 series of saipath. It contains a set of analog and digital functional modules, they are set by the firmware stored in the onboard flash memory. The second chip handles RS232 level shifting to provide a communication link with a host so that capacitive sensing data records are transmitted to the host at a 115200 baud rate through a serial port. The pin distribution of the four capacitive sensing buttons is shown in the table in. Program the PSoC through The ISSP interface, the programming pins such as check and SDA. The ISSP Interface contains power supplies and locations, while a PC host connects to a capacitive sensing Board through a standard DB9 connector.

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