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Countdown timer - 2014

Small gadget purposefully trying to look like a countdown timer for a bomb. I've made it for a friend, who had some undisclosed plans with it. I do not know what he planned, but I suspect, not all people involved were as amused as him.

Countdown timer

Turning it on starts a 10-minute countdown (form 9:59.99) displayed on five 7-segment displays.

Pressing the red button subtracts 1 minute, cutting one wire significantly speeds up the countdown, and cutting the second one stops it.

For the full effect, I suspect that taping it to a cardboard box or a couple of water bottles can give at least somewhat believable results.

Countdown timer

Inside, there is an Atmega8, 4 AAA batteries, 5 7-segment displays, some transistors for driving the segment displays, and a power switch.

Schematic, board, and firmware here