Powering the LED of the clock

Obviously the hardest part of the clock was all the LED. For a display of 21x15 you already have 315 LED that need to be powered, and even the large 16F877A has only 40 pins. The first step to simplify things is to group the LED in groups of 7, with a shared cathode. At that point you can select a group of LED by setting the cathode to 0V, and light a specific LED by setting its anode to 5V. LED burn very brightly at 5V, but can handle this only for a short duration of time (the so-called duty interval). For the LED I used the duty interval was 1/16th, but because of the amount of LED I stayed well below that. To select a group of LED I used the 74LS154, which demultiplexes a 4-bit input into 16 separate lines that can be individually pulled low. Attaching these to each of the cathodes would allow the control of 16 groups of LED. As they are grouped in groups of 7, this meant approximately 1/3rd of the LED could be controlled this way. Using 3 74LS154 the entire board could be controlled (in the schematic not all LED are drawn, one has to imagine the missing LED in between). To make things easy the 74LS154 allows you to individually select which 74LS154 is active at a given time, so with only 7 lines you can control all cathodes, and with 7 additional lines all anodes. With 45 cathodes, this means the duty cycle is, at best, 1/45th. However, it turned out that the drain on the 74LS154 is not high enough to power 7 LED at the same time, and it has to be split in 2 groups (one of 3 LED, one of 4 LED). This means the duty cycle became 1/90th. It turns out that even at 1/90th the LED burn quite brightly and are well visible.

The anodes could obviously be powered directly from the PIC16F877A, as it can output 25mA per line. However, it can only output 100mA in total, and it is therefore recommend to add the 74LS125 buffer chip, which can output 35mA per line. Do not forget to add the 0.1uF decoupling capacitors at every IC used: in this case there are 3 74LS154, 2 74LS125 and 1 16F877A that need decoupling. The result is a display that can be used for many purposes, but was used, in this case, for a clock. Alternatively the LED could be all put in a long line, making a scroll text display.