“A general purpose countdown timer. Easily settable for up to one hour. Great for the kitchen. Perfect 5 minute eggs!
In todays world of Apps for your smart phone, a countdown timer might seem like a bit of a redundant build considering the prolific amount of free timer Apps. As a project, this was a interesting build.
Microprocessor
I chose to use a cheap ATtiny1614 processor to drive a 4-Digit 7-Segment clock display. As the ATtiny1614 is a 14 pin device, it became a challenge to connect the 11 pins of the display, 4 switches and a buzzer to the 11 available IO pins (ignoring reprogramming the RESET/UPDI pin as another IO pin).
The switches form a voltage divider and is read as an analog input allowing all switches to use a single IO pin. The buzzer also uses a IO pin. This left 9 pins to connect to the 11 pins of the display. I achieved this using a technique called charlieplexing.
Charlieplexing
This technique uses the fact that an IO pin can have 3 states (HIGH, LOW and INPUT). When a IO pin is configured as an INPUT it is in effect disconnected from the circuit.
Consider the following circuit where 6 LEDs are driven by 3 IO pins.”