WorldSkills Electronics Tips: Level 2 week 2

The second week project is a simple digital clock based on CD4026 - counter and BCD decoder.

Again, no programming involved, only hardware.

The circuit is composed by the following blocks:

- Crystal oscilator;

- Frequency divider;

- Up counter/BCD decoder;

- Monostable using schmitt triggered NAND (CD4093);

- Logic gates converted to NAND logic;

- Astable multivibrator based on 555 timer;

The full schematics of the circuit is below:

Digital clock full schematic

Overview

The circuit is a digital clock that exibits 00:00 to 23:59. Is powered by a 9V battery and has bright control to prolong battery life.

All circuit is based in a BCD counter IC CD4026 and uses CD4060 within a crystal to precisely oscillate and avoid variation on clock ticking.

Blocks diagram

To simplify the circuit, it can be functionally represented by the following blocks diagram:

Digital Clock block diagram

Clock generator:

Circuit based on IC CD4060, uses a 32762Hz crystal to precisely oscillate. This frequency is the best option to clocks because it can be successively divided by 2 until reach 1Hz. As the 4060 has only 14 outputs, the final frequency will be 2Hz instead of 1.

Frequency divider

It's a divisor by 120. Regular counter CD4040 that resets when reach 120. In the input cames the 2Hz and it divides by 120 to provide 1 pulse every minute to the minutes counter stage.

BCD counter and reset circuit

Aplication circuit of CD4026. Every clock pulse counts 0 to 9. As the clock needt to count minutes until 59 and hours until 23 it has some logic gates to provide a reset circuit. In this specific application all logic circuit uses only NAND gates.