Practical information | Xiaomi water purifier perfectly transformed into "invisible version"

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The so-called "invisible version" is the under-counter version. This modification involves a problem: how to change the touch button to a water pressure switch control?

The solution is as follows: a wire is led out of the touch pad of the touch panel to connect a capacitor, then connected to a water pressure switch and a key switch, and then to the ground.

Here, the principle of touch buttons is fully utilized: when a human finger presses, a capacitor is equivalently connected. After modification, the equivalent logic function is achieved through external capacitors and switches. There are two rewiring schemes: 1: Adding capacitors and water pressure switches to simulate the logic of human buttons as mentioned above. 2: Direct and fierce, use the switch to force the output voltage of the touch chip to be lowered (low level is a valid signal) Write down the analysis process first, and jump directly to the second scheme if you are impatient. First look at the disassembly:

Then there is the tap touchpad

After opening it, there is a layer of sealant covering the circuit board for waterproofing, but it is found that it is not fully covered, so if water enters, there is still a risk of damage. Here we can only blame the workmanship. At least in the design, the engineer took this into consideration and added this process, but the glue was not enough ...

Let's talk about the chip. Is there a main control chip with ADC2012 printed on it? What the hell is this? I have never seen it, but there is a familiar Jtag interface next to it, so I thought I could do something with it.

There is also a solenoid valve control chip, but it was accidentally burned and smoked during the modification process, but it did not affect other parts, so I took it out and put it aside. However, later users must be careful when modifying it, because this chip is still quite useful (you can remove the original solenoid valve and replace it with an external one connected to the tap), but it is not a big deal if it burns, at worst, you can connect a water pressure switch to control the solenoid valve.

The touch pad here uses a spring pancake, which is connected to increase the contact area and distributed capacitance. It's clever. After opening it, you can see 4 lines, red, black, white, and yellow. The red and black are 12V and ground, and the other one is tested by a multimeter for 3.3V, which is probably the serial port! Because the two logic lines of this touch panel are definitely not enough.

I used an oscilloscope to verify my guess. Sure enough, both the yellow and white wires had serial port signals, with a baud rate of 2400. Then I disconnected one wire, and used the two probes of the oscilloscope to measure the disconnected wire at the same time, and measured the communication direction.

Here we analyze that the faucet and the host are constantly communicating to detect the state of the faucet or other functions. The dumbest idea came to my mind at the first time: use a logic analyzer to see what the host and the faucet are talking about, and then use the stm32 in my hand to write a small program to replace the touch circuit board. Unfortunately, my Taobao logic analyzer worth more than 20 yuan is in the office, not at home, so I gave up and changed my mind. So there is plan 1

This is the logic of adding capacitors and water pressure switches to simulate human button pressing.


The schematic diagram of solution 1 is a bit ugly, just bear with it, I'm too lazy to draw.
S is a switch. AB is connected to the spring pancake on the touch panel. Forgive me for giving it this name: spring pancake. The
104 capacitor here can be a little bigger.

When there is no pressure on the water purifier tap, that is, when the machine is just turned on, closing S is equivalent to touching the pure water button. The machine starts working.

When the faucet is closed, the pressure increases and the water pressure switch switches to state B, which is equivalent to turning on the tap water.

This solution has a small problem:
because the touch signal is calculated by the TS02N chip, and this chip has a calibration function, so when the machine is powered off or turned off, you need to disconnect S again, wait for the calibration to be completed, and then close S before using it.

Adding a switch, the wiring and welding are not simple and beautiful. Moreover, when the power is turned on at the breakpoint, if S is closed, the calibration data will be weird, and the water pressure switch will not work. Can you guess what the logic is? Haha, because S is closed during calibration, the IC thinks that the circuit distributed capacitance is a 104 capacitor. At this time, closing S will turn on the water purification function, but the control logic keeps changing. It is estimated that this IC can detect two gestures: approach and leave. (I glanced at the PDF, but didn't look closely)

Let's do something drastic. Use a switch to force the output voltage of the touch chip to be lowered (low level is a valid signal).
I glanced at the PDF above, and now I have a new idea. According to the PDF, the output of this touch chip is a low-level valid signal, so I thought of using a switch and
a pull-down resistor to control this signal. Isn't it rough?

First look at the pin diagram & typical application diagram of TS02N

According to the diagram, pins 7 and 8 are the output signal lines, and then I checked and found that these resistors are all connected to the circuit board.

Schematic diagram of solution 2

Use a thin wire to lead out the 78th pin, and connect a 1K resistor to GND. Be careful not to connect it wrong! The logo and the pad are not aligned. The safe way is to connect it directly to the black wire. But it is not simple enough.
Finally, the effect picture is posted (it has been tested on the machine, the faucet is turned on and off, the logic is normal, and the machine is normal after power off and restart)


The modified touch panel and pressure switch are pictured together.
This high-pressure switch is to be connected to the clean water, that is, between the water outlet and the faucet.

Next, connect the two wires of the original solenoid valve to a solenoid valve like the one in the picture below. I don't think it will be a big problem if you don't connect them, but I'm afraid that the water pressure will fluctuate too much and affect the water purifier. As for advertising, I don't do business anyway. I searched Taobao for a long time and only found this one: 3 points 12V, quick connector, solenoid valve for water purifier.

Finally, let's talk about the modification of this high-pressure switch (the Taobao keyword is water purifier high-pressure switch)

Generally, high-voltage switches or low-voltage switches are either normally open or normally closed and need to be modified.

Take out the micro switch inside. You can discard the spring. If you do, it can improve the response sensitivity and prevent the machine from over-pressure.

Replace it with a single-pole double-position micro switch, Omron's is only one or two dollars. As long as it looks the same and is of standard size, if it is a little difficult to install, it needs to be polished a little bit. Once it is installed, it will be OK.