Quantity   Component name
1 × ATmega 328P Any AVR from the Atmegaxx8 family with over 5kb flash should work fine. Make sure it is a P version as the consume less power
2 × 1 -Bit Seven Segment Display A 2-bit version would be helpful but I did not have any on hand so I wired two into a multiplexing configuration. I used common anode version but a common cathode would work just fine. You would need only to replace logic low with logic high
1 × Arduino Board Any board will do as long as the micro controller is not an SMD. We would need to remove the microcontroller to program the standalone chip. If you have an FTDI cable then you would not face this problem
1 × CR 2032 3V Lithium coin cell Any 3V coin cell would do
1 × Holder for the coin cell You can buy one or improvise one from wire or paper clips, etc
1 × Regular epoxy Needed to make the housing as I did not use a 3D printed enclosure
1 × 32.768 Hz Clock crystal These go by the name 32 kHz clock crystals and can be bought for very cheap or salvaged from old clocks and watches
2 × 0.1 uF ceramic capacitor
1 × 10 k resistor