6 Digit LED Clock

I recently found six 2.3″ red 7 segment displays in my collection which I purchased from Rapid Electronics when they were clearing out non-RoHS stock. Since I have no clock at my work bench I decided to construct one from some prototyping pad board, a Microchip PIC18F26K20 microcontroller and a Dallas DS32KHZ temperature compensated 32.768kHz crystal.

The displays I used are SA23-12SRWA, which don’t appear to be available from Rapid any more, the closest is SA23-12EWA, however, Farnell and the other usual distributors sell them.

I wanted this to be a fairly quick project to complete, so the schematic is fairly simplistic. The schematic diagram of the clock is shown below:

6 Digit LED Clock

The 7 segment displays are common anode displays, with segments consisting of four red LEDs in series and the decimal places consisting of two red LEDs in series. To reduce the number of GPIO used to drive the display, various methods can be used – such as multiplexing, shift registers or dedicated display drivers. I’ve chosen to use plain display multiplexing, requiring one GPIO for each digit common anode, and one GPIO for each segment – with the colon separators, that gives 7 common anodes, and 8 cathodes with the decimal places and colons commoned together. There are many methods to reduce the number of GPIOs needed further than this, but the chosen microcontroller does not have a shortage of GPIO for this application.

The forward voltage of the segments is slightly high – the datasheet specifies between 7.4V and 10V. For this reason, the anode drivers for the displays have to perform both voltage translation in addition to sourcing enough current for the displays.

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