Unboxing¶

I’m working with 2 used pHin sensors, obtained recently from a former user on 17/12/2021, following the announcement that the pHin service would shut down on 20/12/2021.

After unboxing both units, neither of them were sending out BLE Advertisements. Reading through the User Manual didn’t help much. A bit of googling brought me to pHin’s Setting up your pHin Monitor, which mentions that the bottom cap must be removed and swiped over the water drop on the casing in order to activate the device.

At this point I was assuming there must be a reed switch under the water drop, connected to other parts of the circuit in any of the following ways.

  1. a GPIO of the MCU, acting like a user-button

  2. the reset pin of the MCU

  3. the enable a power regulator

  4. to the gate of some transistor, gating the power supply of the MCU

The last three options are the most likely: when you’re designing a battery-powered device that’s completely sealed, having a magnet to power-cycle your chip can be a big help. The reed switch could be useful for activating the device after delivery to the user. The magnet could also be a smart way of saving power while the product is shelved: the monitor could be set in a very low-power mode after factory tests by some special command on a serial port. The magnet could then make the device exit the standby mode and start measuring and advertising over BLE, either through a GPIO interrupt or a power cycle depending on how the reed switch is wired.

Swiping both devices with the bottom caps and other types of magnets did not produce the expected result: the LED of neither pHin monitor blinks when a magnet is brought close. Seeing that a magnet had no effect, I concluded that the batteries were dead (which they were, as I observed later when opening the monitors).

This leads us sooner than expected to the most thrilling part: the Teardown.