74LS90 Binary Coded Decimal Clock

Description

The 74LS90 Binary Coded Decimal Clock is a binary clock in it’s purest form. You won’t find any micro controllers in this clock. The clock is built using 4000 and 7400 Series digital logic chips, and it keeps the time by counting the ripples from a 32.768Khz quartz crystal oscillator. The time is displayed as a 6 digit binary coded decimal using 5mm LED’s. At the heart of this clock is the 74LS90 Binary Coded Decimal counter IC. These neat little chips count to ten in binary, and start over at zero. The ten’s digits on a clock stop at 6, so three AND gate chips reset the count at 12, or 60 and advance the next digit. 12 and 24 hour formats can be selected with a flip of a switch.

How do you read the time?

Reading numbers in binary is surprisingly easy. Each digit on the clock has 2-4 LED’s that represent 8, 4, 2, and 1. Simply add the values of the LED’s that are lit in each column.

How it works

A 32.768Khz Quartz crystal oscillator generates a very small 32.768Khz wave form that’s fed into the inputs of the CD4060. 32.768Kzh is the ideal frequency, because if you divide 32768Hz by 2 15 times, you get an very accurate 1Hz square wave. The CD4060 is a 14 stage ripple counter. It’s sensitive inputs are ideal for counting the tiny ripples from the crystal. But the 4060 only has 14 stages, so it outputs a 2Hz square wave. This 2Hz square wave is fed into a CD4027 flip flop that divides by 2 one last time to get 1Hz. The 1Hz wave is fed into the Seconds ones binary counter. This digit counts to ten and resets to zero. When it resets to zero, it advances the next stage. When the seconds tens digit reaches 60, a 74HC08 quad, 2 input AND gate advances the hours ones digit and resets the seconds tens. The minutes counters work exactly the same. They count to 60, reset to zero and advance the hours. When the hours reach 13 or 25, a CD 4082BE resets the hours tens to zero. The clock starts at 1, not zero so  a CD4002 NOR gate and a 4082 AND gate send a single pulse to the hours ones digit to change it from 0 to 1.

Easy to solder

This soldering kit is made entirely with through hole parts so the soldering is easy. It is an excellent choice for soldering beginners. You’ll want to make sure to get the correct orientation for the LED’s, digital logic IC’s, electrolytic capacitor, and the diode. Thorough assembly instructions can be found on our soldering kit assembly page.