Design of touch button based on electric field sensor MC33794

introduction

With the continuous development of touch button technology, touch buttons have been widely used in electrical appliances, mobile phones, kitchen equipment, home appliances, industrial control switches, etc. Compared with traditional mechanical buttons and plastic membrane switches, touch buttons have many advantages, such as no mechanical wear, good durability, high reliability, and a more intuitive touch button interface and easier to operate.

The electric field sensor MC33794 is a new sensor product launched by Freeseale, which is suitable for applications that require non-touch sensing of objects. A new touch key control system is designed using MC33794. The touch key system can sense the key press of fingers non-contactly through the electrodes of the electric field sensor. The system design only requires a microcontroller and MC33794 to realize touch detection and control of keys. The circuit is relatively simple and suitable for many household appliances.

2 Introduction to the Electric Field Sensor MC33794

MC33794 can generate low-level electric fields internally, and all functions are integrated into a single chip, which can measure the electric field load caused by the movement of objects in the electric field. The frequency of the low-frequency sine wave generated by this device can be adjusted by external resistors, and the optimal frequency is 120 kHz. This sine wave contains very few harmonic components to avoid harmonic interference. It is the only integrated circuit in the industry that can generate and detect low-level electric fields and support microcontrollers. MC33794 is applicable to:

(1) Household appliances can be easily implanted with automatic switching functions. If a few electrodes are embedded in the control end of certain appliances, these appliances will automatically start when they are moved, and will automatically shut down when they are put back. In addition, more electrodes can be embedded and their functions can be defined, so that users can control household appliances by lightly touching these electrodes.

(2) It can easily realize applications including liquid level detection, overflow detection and humidity detection. For example, the stove fire can be programmed to automatically reduce or turn off when the liquid on the stove is heated to boiling and overflows.

(3) Applied in touch screen input, it can sense input without membrane switches or expensive multi-layer resistive touch panels, and without contact mechanical devices. It can eliminate the problems of wear, dust and corrosion commonly encountered by contact mechanical devices, and is particularly suitable for working in harsh environments.

(4) Application in automobile safety. Elesys uses Freescale's MC33794 electric field imaging device in its SeatSentry passenger protection system to detect the body shape and position of passengers to determine whether the airbag needs to be deployed to prevent passengers from being injured by the deployment of the airbag.

It can be seen that the electric field sensor integrated circuit launched by Freescale has expanded the application scope of this sensing technology. It can be used in consumer products, electrical appliances, and industrial fields, improving the reliability and ease of use of equipment in these fields.

3 MC33794 Working Principle

The working principle of MC33794 is shown in Figure 1. The low-frequency sine wave generated by it is used to drive the electrode. When the electric field of the electrode changes, the corresponding capacitance value changes, and the measured voltage is proportional to 1/C, that is, the detected voltage decreases when the capacitance increases. The detector and low-pass filter in Figure 1 are both inside the MC33794 chip, and only need to connect a 10 nF capacitor when in use.

4 Touch Button System Design

4.1 Design Principle of Touch Button

The electric field sensor can sense the movement of objects in the electric field in three-dimensional space, as shown in Figure 2. When the palm enters the electric field area generated by the electrode, the current on the electrode will change accordingly, thereby sensing the distance between the palm and the electrode.

The dielectric constant εr of glass is 7.8, the thickness of glass is 4 mm, the electrode area is 2 cm2, and a single electrode is used as a key, so there can be 9 keys. If more keys are needed, 2 or more electrodes can be combined to form a key. For example, 2 electrodes can form a combination key, and the maximum number of keys can reach 45. Generally, the number of electrodes that constitute a key should not exceed 4, because the more electrodes there are, the weaker the electric field is, and it is difficult to detect.

When a finger touches the glass surface, the capacitance value between the electrodes changes, and the output voltage of the MC33794 pin LEVEL changes, so we can know whether the finger touches the glass.

4.2 Touch Button Hardware Design

The main control MCU is Freescale's MC9S12DG128B microcontroller, which integrates a 10-bit A/D converter with a sampling frequency of 140 KS/s, a timer, up to 91 I/O ports, SCI and other interfaces.

The hardware circuit of the system is shown in Figure 3. Connect the A, B, C, and D of MC33794 to the PA port of 9S12DG128, and select the electrode to be sampled by controlling the I/O port output through software. MC33794 has a total of 9 electrodes that can be used, and the required buttons can be formed according to the actual situation.

When a finger touches the glass, the output signal of MC33794 drops by about 0.2 V. The signal change is amplified by about 1 V and sent to the A/D converter of S12 for detection and processing.

4.3 Software Design

The software of the touch key system mainly consists of a main program, an initialization module, a touch sensitivity module, a reading electrode voltage value module, a digital filter module, and a key function submodule. The main program flow is shown in Figure 4.

The initialization module mainly initializes the timer, A/D converter, etc., and sets the microcontroller bus speed. The A/D converter clock frequency is required to be about 1 MHz.

The sensitivity of all buttons can be adjusted by changing the sensitivity parameters to adapt to different glass panels.
When the touch controller is not "working", the microcontroller is set to "low power wake-up" mode in software, while maintaining a fast response when the button is pressed. This mode can be used to wake up the product when a person touches it.

5 Conclusion

A touch key system composed of electric field sensor MC33794 and microcontroller MC9S12DGl28B is proposed. A series of experiments are carried out on it. It has been applied to the key part of the human-machine interface of automation instruments. The circuit is simple and reliable, and good results have been achieved.