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The Binary Clock in BCD Mode

Instructions to read a binary clock in the classic "BCD mode", along with a visual JavaScript simulation with hints.

JavaScript Simulation

1 7 4 1 2 3
Hints: LED Values

If your browser supports JavaScript and you have it enabled, the example above should be "live", showing the current time from your computer's clock and updated once a second just like the real-life binary clock, but with extra hints to help you understand the display. Click on the checkboxes to disable/reenable the various levels of hits as you gain experience.

If your browser does not support JavaScript or you don't have it enabled, the picture above will be static, sorry.

How to read the time in BCD Mode

Here you see the clock in BCD mode. This the standard mode in most binary clocks, including my own design, because it is arguably the easiest to read.

In this mode, each decimal digit corresponds to exactly one column of LEDs, as shown in the simulation above. The first two columns are the two digits of the hour; the middle two columns are the two digits of the minutes; and the last couple of columns are the two digits of the seconds.

The value of the six LEDs in the bottom line are 1. The values of the LEDs in each line above is twice the value of the line below. So the LEDs in the second line have value = 2; the LEDs in the third line are worth 4; and the 3 LEDs in the topmost line are worth 8.

To read the time, just add the values of the LEDs that are lit on each column and read them left-to-right as usual. With practice, you can do that nearly instantaneously. It is rather easy to memorize because for each column there are only 10 possible LED patterns (since they represent the 10 possible decimal digits):

0 1 2 3 4 5 6 7 8 9

If you want to impress your friends (or, depending on the audience, sound a bit pedantic), you may point out that although many people call this a "binary clock", the correct term for this mode is actually "BCD mode", for "Binary-Coded-Decimal": each number is first converted to decimal, then each decimal digit is converted to binary. A real true binary clock would be too hard to read to be practical.

Going further on, you may point out that there are three number bases here: the time is first converted to sexagesimal, then each sexagesimal "digit" is converted to decimal and, finally, each decimal digit is converted to binary.

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