Magnetic Switch

The FunKey S features no ON/OFF switch. Instead, a small magnet placed near the LCD screen in the lid is detected using a magnetic sensor that is placed in front of it when the lid is closed.

The magnetic sensor controls the PMIC N_OE (Negative logic Output Enable) input, which turns off all voltage rails except the +3.3V Always On when the signal is at the high level.

We thus need a sensor that will close when a magnetic field is detected, and in order to reduce power consumption when the FunKey S is turned off and preserve its battery when stored on a shelf, it should be a fully passive device. This rules out magnetic Hall effect sensors, wich require some power for their operation.

The only practical solution is to use a "Reed switch", which is a relatively thin flexible piece of metal inside the switch envelope that is sensitive to a magnetic field, which opens or closes a contact accordingly.

Most of the Reed switch available are made of a tiny glass tube, which is rather fragile and may break if the device falls on the floor, something quite usual for a keychain...

We found only a unique plastic-molded model which almost meets our low-profile requirements of 1.5 mm (1.6 mm height!): the Standex Meder MK24.

Here is the corresponding schematics, already covered in the PMIC discussion:

The global PMIC chip enable signal N_OE is activated by default through a 47kΩ resistor R17 to GND, but the magnetic Reed switch S14 can disable it by forcing its level to +VOUT, with a filter capacitor C83.