This article details the assembly of the momobox. Before you start, ensure that you have:

1. Gathered all required tools
2. Acquired all electronic components.
3. 3D printed all mechanical components

Lid Assembly

Determining Magnet Polarity

The first step is to determine and remember the correct polarity of the magnet that gets embedded in the momobox lid. The simplest way to do this is to check your magnet with a magnetic compass. You can also do this with a phone app like Magnetometer. Note that your standard compass app may or may not work for this.

Bring your magnet near the compass such that the the compass points towards the magnet. The side of the magnet near the compass is the south pole of the magnet:

Now turn the magnet around. The compass should point away from the magnet. The side of the magnet near the compass is the north pole of the magnet:

Mark the magnet that will go in the momobox lid accordingly.

• The south pole of the magnet will point up out of the lid.
• The north pole of the magnet will point down into the box.

Failure to correctly determine magnet polarity will cause the hall sensor to not work properly. There are a couple of opportunities along the way check that this was done properly.

Building the hall sensor board

A hall effect sensor can be thought of a switch that is activated when placed in the vicinity of a magnet. The hall sensor used in the momobox is unipolar and low-sensitivity. Getting the placement of the hall sensor relative to the magnet correct is critical for momobox to function.

Using prototype PCB board, construct the following circuit with the hall sensor mounted as shown in the photos below.

Hall Sensor	RPi Pico	10 kOhm Resistor
1	3V3	X
2	GND
3	GP8	X

Make two 90° bends in the hall sensor leads such that the sensor will mount with the front of the sensor (South pole in the image above) faces the edge of the magnet. The (very crude) example below shows this board as constructed.