Locate hall effect sensor for disabling

Question

We are wanting to use windows tablets in a high magnetic field. The problem we have encountered is the sensor used to detect a "smart cover" and shut off the screen is being triggered upon entered the field. I am wondering how best to go about locating the sensor in the circuit board and disabling.

For the sake of simplicity, I am comfortable enough to open the tablet (yes I understand this may void the warranty), dissemble any attached cables/components to access circuit board, and desolder the chip and jumper.

My question is how to detect where the sensor is located. I can pass a magnet around the tablet to identify the general area that triggers the screen shutoff/lock but need recommendations for how to locate the actual sensor.

Some of the tablets we are pursuing:

• HP 608 Pro G1
• Xiaomi MiPad 2

Answer 1

One cannot assume that the sensor is an integrated circuit; a small reed switch is often used. If you listen carefully, while probing with a magnet, you might hear it 'click' when your probe initiates a shutdown. Such reed switches look like glass tubes with a wire coming out each end.reed switches

Neither assume it is on a circuit board; for best sensitivity it is likely to be taped to the case, as close as can be managed to the landing zone of the magnet in the other half of the clamshell. And, in my MacBook Pro, that means it's behind the glass of the display.

I'm unfamiliar with smart covers on the specific models mentioned, but if you can localize the sensitive area, a soft steel plate might be cemented over that zone, to lessen the field gradient and desensitize the switch.

Answer 2

If you are lucky, it well be a hall device -- tiny part is 3 or 4 pins. Example (see step 4): https://www.ifixit.com/Teardown/iPad+2+Smart+Cover+Teardown/5089

You will have to use scope to find the output pin. The unamplified sensors have microvolt output; you can disconnect the power pins and you would be good. Unfortunately, they are very rare in high-end consumer devices. The analog amplified sensors will usually output a fixed voltage when there is no field (1 volt, or Vcc/2). The digital sensors output logic value, so you may have to force the output to either 0 or 1.

If you are unlucky, the device will have a single sensor for accelerometer / gyro / magnetic field with I2C or SPI interface. In which case the best you can is to try to disconnect it and hope software can handle this. You will lose all other functions of this chip, including screen orientation detection from accelerometer.

An alternative solution, if your company has people with the right skills, is to go to Android -- if your tablet can be rooted, there is usually a way to disable smart cover.