There are three main styles of mobile phones on the market now: European Nokia-style bar style, Japanese and Korean trend shell folding machine, and Ericsson classic style lower flip design in the past. For every friend who often uses the folding machine, he will often be very curious to answer the flip of the folding or flip model or hang the lid. So how do they work? Why do you need to press the key to answer when the lid is open? Although this article is very professional, after reading it, you will know more about the principle of answering calls with flip phones. Please sit patiently and look down.

In fact, there are three main types of devices used to control the on-off of lines in mobile phones: switches, reed switch and Hall elements. The difference is that the switch is generally controlled manually, while the reed switch and Hall elements control the on-off of the circuit through magnetic signals.

Reed switch is a passive electronic component, which is widely used in various communication equipment. It is a circuit switching device controlled by magnetic field signal, also called "magnetron". The shell of reed switch is generally a sealed glass tube, which is filled with two iron elastic reeds and an inert gas called rhodium. Usually, the two reeds in the glass tube are separated. When a magnetic substance is close to the glass tube, under the action of magnetic field lines, the two reeds in the tube are magnetized and attract each other, so that the circuit connected by the two pins is connected. After the external magnetic force disappears, the two reeds are separated due to their own elasticity, and the line is disconnected. In practical application, a permanent magnet is usually used to control whether two metal sheets are connected, so it is also called "magnetron".

Magnetron is often used in flip circuit of mobile phone, especially Motorola mobile phone. Through the action of the flip cover, the magnet on the flip cover controls the opening or closing of the magnetron, so as to answer or hang up the phone. For example, some Motorola mobile phones will have a status indicator flashing after the flip is closed. This is because the flip cover is equipped with a magnet. When the flip cover of the mobile phone is closed, the magnetic disk will attract the reed switch on the keyboard/display panel, so that the mobile phone recognizes that the flip cover is closed and the status indicator light starts flashing. If this indicator doesn't light up, there may be something wrong with this reed switch. At present, the magnetron basically adopts the technology of Japanese OKI company.

Compared with reed switch, Hall sensor is more advanced. But its function in mobile phone is the same as reed switch's, and its working principle is similar, which directly produces the function of on-off under the action of magnetic field. Hall sensor is an electronic component, which is packaged like a triode. It consists of Hall element, amplifier, Schmidt circuit and open collector output transistor. When the magnetic field acts on the Hall element, a tiny Hall voltage is generated, which is amplified by the amplifier and Schmidt circuit to turn on the transistor and output a low level. When there is no magnetic field (that is, when the flip cover is opened), the transistor is turned off and outputs a high level.

Compared with reed switch, Hall sensor has longer service life, is not easy to be damaged, is insensitive to vibration and acceleration, and has faster switching time, usually 0. 1 ~ 2 ms, which is much faster than 1 ~ 3 ms in reed switch.

Let's take T39MC, a classic Ericsson mobile phone, as an example to specifically talk about how the flip of the mobile phone works. At the same time, this principle is also applicable to other folding machines.

The Hall sensor in T39MC consists of a switch-type Hall module and two power switch control tubes. Whether it is turned on or not is completely controlled by the Holgard level signal output by the mobile phone CPU, and the power supply comes from the battery. When the flip is closed, the magnetic field of the magnet in the flip acts on the Hall sensor (generally, the flip/folding mobile phone is equipped with a magnet on the flip), and the triode in the Hall circuit is turned on, which outputs a low level from the sensor pin and sends it to the CPU as an "on-hook" signal after a call. This is why the phone hangs up after closing the flip.

When the user opens the flip cover, the Hall is not induced by the magnetic field, the triode in the Hall circuit is turned off, and the output is at a high level. If the signal is generated during an incoming call, when it is sent to the CPU, the CPU will answer the call as a power-on signal. However, if only the user does other operations, such as inputting short messages, the phone number simply opens the flip cover, and the circuit signal is used by the CPU as the background light control signal to light the background light. (T39MC, turn on the background light every time you open the lid, and record the number of flips). Someone has done a special test, and the tool will not light up if it is carefully lifted a little, indicating that there is still a magnetic field on the Hall element at this time. When it is turned on to a certain angle, the transistor of the Hall circuit that loses the magnetic field will stop inputting high level, and the CPU will drive the backlight circuit to light the backlight after receiving the signal.

When the user cancels the "flip answer" option, the Hall signal sent by the CPU is low, so that the control tubes of the two power switches are turned off, and there is no power supply. Even with or without the magnetic field signal, the output voltage will not change, thus losing the switch function. So in this case, after the incoming call, your flip CPU does not connect the phone according to the setting, so you need to press the answer key to answer it at this time.