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Big digit 7-segment Clock using ATtiny3216

This is a giant digital clock, featuring four large LED displays that provide the time in a format you can read from the other side of the room

You could also use it as the basis for a stopclock, for timing events such as sports and speedcubing.

Introduction
This clock was inspired by some 2.3” one-digit 7-segment displays I saw on AliExpress [1]. I decided it would be fun to have a clock that you can see clearly from a distance. The displays come in a variety of colours and voltages; I chose the red 4V versions so I could drive them directly from the microcontroller’s I/O lines without needing driver transistors. My displays are common-cathode, but a minor change to the program would make it work with common-anode displays.

The circuit is based on one of the new ATtiny chips which have a Real-Time Clock peripheral you can run from an external 32.768kHz crystal; see Getting Started with the New ATtiny Chips. I chose the ATtiny3216, a 20-pin member of the family with 32Kbytes of flash which has enough I/O lines to drive the displays in a straightforward way, without having to resort to charlieplexing. The program is actually small enough to fit on the 4Kbyte ATtiny416, which is otherwise equivalent.

The current-limiting resistors are in series with each segment to give consistent display brightness. A push button is provided to allow you to set the time, and I’ve provided access to the UPDI pin for programming the ATtiny3216, and the serial input pin, RX, in case you want to connect the clock to a GPS module; see Further suggestions below. “

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