Quantity   Component name
1 × Arduino Pro Mini (3.3V) 3.3V 8MHz type. Mine was an "eBay Special" straight from China
1 × 18650 LiFePO4 Battery The ever-popular Li-ion cell. The kind that go into those DIY battery packs.
1 × 18650 battery holder I intend to take out discharged cells and replace them with freshly charged cells, so the battery holder was a must for me.
1 × DC-DC Boost Converter Another "eBay Special" item. These are cheap, and I got an adjustable one with a potentiometer to select the output voltage. Unless your project is okay with a voltage that varies from about 3.6V down to 2.4V, you need a DC-DC converter to get the output you want.
4 × 1N4148 Diode Really, any diode will work in this circuit, but these are a jellybean part that I keep on-hand.
2 × 1 megaohm resistor The mighty resistor! The value is large to reduce vampire currents from draining the battery before I can replace the old one with a new one.
1 × 330 kilohm resistor This combines with the 1M resistor to create a voltage divider to measure battery voltage.
4 × 100nF capacitor This is the kind of capacitor I would recommend using. I used random electrolytics, but I think the values are way bigger than the circuit actually needs
1 × N-channel MOSFET Almost any "logic-level" MOSFET will work in this circuit, as long as it can handle the current you expect to draw from the battery. The charge pump can get the gate-source voltage up to about 4 or 5 volts, even on a low battery