Waterproof touch

[brain13]

Hi Espressif,

I want to make a waterproof touch button. I read this
https://docs.espressif.com/projects/esp ... h_pad.html
article where touch_pad_waterproof API mentioned.
Also I read this article
https://github.com/espressif/esp-iot-so ... sign_en.md
but there is no description how the waterproof technique works.

From my understanding the waterproof sensor consists from 3 signals: Touch, Shield, Guard. Am I correct?
The Touch signal is pulled between thresholds. The Shield signal just repeats the Touch signal.
But what is the purpose of the Guard signal?
How to route the Guard signal?

We made PCB like this:

photo5323405470410584364.jpg (214 KiB) Viewed 5433 times

The hatched polygon on both top and bottom layers is the shield. I use the shield under the Touch sensor to protect the sensor from objects on opposite side.
Outer blue ring is the Guard signal.
The inner circle electrode is the Touch signal.
The Touch circle is about 10mm diameter. The Guard ring is about 20mm diameter and 1mm thick.
Is the design is correct?

[Zhang Zhiwu]

Hi Brain13,

Firstly, it is correct to adopt touch sensor, shield sensor and guard sensor in a waterproof touch system. And GPIO 14 must be assigned for shield sensor only.

Secondly, we recommend to reserve a 470 ohm serial resistor from touch sensor to GPIO and from guard sensor to GPIO. A 0 ohm serial resistor is recommended to placed between shield sensor and GPIO. Sometimes, the resistors can affect the sensitivity of the sensor and improve system ESD and EMI robustness. Value of the serial resistors may be adjusted accordingly.

Thirdly, guard sensor and shied sensor act as different roles in touch waterproof function.
If the size of water droplet is similar to touch sensor pad size (see Fig.1): Shield sensor and touch sensor will be charged at the same time, thus there will be seldom charge transfer on the droplet. Under this condition the water droplet will not affect the function of touch sensor.

If the size of water droplet is far more bigger than the touch sensor pad size or there is a water stream (see Fig.2): The water may cover several sensor pads or system ground, thus it introduces a high parasitic capacitance. Under this condition, once guard sensor is triggered all the touch sensor functions are disabled intentionally by software to avoid any mistake. We usually set the thickness of guard sensor to 2-3mm, and the distance from guard sensor to touch sensor is more than 10mm. If the guard sensor is too close to touch sensor, the guard sensor will be mis-triggered by finger and affect the function of touch sensor.

Finally, please evaluate your application environment and then make design.
In your current design, if there is no water stream or water droplets more than 20mm ( the water droplet can cover touch sensor pad, shield sensor and system ground simultaneously), you can just remove guard sensor. 20mm is not an exact value, and I estimated this value according to your figure attached.
In your current design, 1) the distance of guard to touch is too small, thus guard will be mis-triggered by finger. And 2) any water deoplets with a diameter of 20mm or bigger can trigger guard sensor then disable touch fucntions. Thus, please adjust the size of shield and guard sensor to enlarge the distance of guard sensor to touch pad according to your application circumstance and product ID design.

Hope that it may help you.

Thanks & best regards,
Zhiwu

[franzhoepfinger]

Hello,

is there any recommendation about ESD Diodes ?