Binary Clock
The classic BCD clock โ read the time from lights, learn binary free.
About this clock
Six columns of lights โ HH:MM:SS โ each column one decimal digit in binary: bottom light is 1, then 2, 4, 8; add the lit ones. Digits show under each column until you hide the answers and read the lights alone.
BCD, and the ten-minute path to fluency
Strictly this is binary-coded decimal โ each DIGIT is encoded separately rather than the whole number โ which is how the famous desk clocks work and why the columns need at most four lights. Reading practice that works: learn the four bottom-row values (1, 2, 4, 8), read the seconds column aloud as it climbs 0โ9 on repeat, then hide the answers โ most people are fluent inside ten minutes, which makes this the gentlest possible introduction to binary proper. Party fact: at 10:48:36 and its friends, the board makes pleasing symmetric patterns, and 11:11:11 unusually lights a neat pair of columns. The ordinary clocks โ digital, analog, world โ live next door for when you just need the time.
Frequently asked questions
How do I read a binary clock?
Each column is one digit of HH:MM:SS. From the bottom, lights are worth 1, 2, 4, 8 โ add the lit ones to get the digit. Left pair hours, middle minutes, right seconds.
Why do columns have different heights?
Each column only needs enough bits for its maximum digit: the first hour digit tops out at 2 (two lights), tens of minutes/seconds at 5 (three lights), ones digits at 9 (four lights).
What's the difference between binary and BCD?
True binary would encode 59 as one number (111011); BCD encodes each decimal digit separately (0101 1001). Clocks use BCD because humans read digits โ it's also how early calculators worked.
How long does it take to learn?
About ten minutes of watching the seconds column with answers on, then off. The 1-2-4-8 bottom-to-top pattern does all the work.