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43 lines
1.7 KiB
Markdown
43 lines
1.7 KiB
Markdown
The **FunKey S** features no ON/OFF switch. Instead, a small magnet
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placed near the LCD screen in the lid is detected using a magnetic
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sensor that is placed in front of it when the lid is closed.
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The magnetic sensor controls the PMIC N_OE (Negative logic Output
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Enable) input, which turns off all voltage rails except the +3.3V
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Always On when the signal is at the high level.
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We thus need a sensor that will close when a magnetic field is
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detected, and in order to reduce power consumption when the **FunKey
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S** is turned off and preserve its battery when stored on a shelf, it
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should be a fully passive device. This rules out magnetic Hall effect
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sensors, wich require some power for their operation.
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The only practical solution is to use a "Reed switch", which is a
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relatively thin flexible piece of metal inside the switch envelope
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that is sensitive to a magnetic field, which opens or closes a contact
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accordingly.
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Most of the Reed switch available are made of a tiny glass tube, which
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is rather fragile and may break if the device falls on the floor,
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something quite usual for a keychain...
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We found only a unique plastic-molded model which almost meets our
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low-profile requirements of 1.5 mm (1.6 mm height!): the [Standex
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Meder MK24][1].
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Here is the corresponding schematics, already covered in the PMIC
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discussion:
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{.lightbox}
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The global PMIC chip enable signal N_OE is activated by default
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through a 47kΩ resistor **R17** to GND, but the magnetic Reed switch
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**S14** can disable it by forcing its level to +VOUT, with a filter
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capacitor **C83**.
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[1]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/MK24_V02.pdf
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--8<--
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includes/glossary.md
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--8<--
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