Introduction: 24 Hr Digital Clock Only With Basic CMOS Chips
PCB art is included in this instructable so you can easily make it!
A 24 hour based clock based on dual decade counters, AND gates, OR gates, BCD to Decimal Converters, and a crystal oscillator. No micro controllers here. This instructable contains PCB art so you can easily make your own.
Quantity is in parentheses. Item number at Digikey.com is in brackets.
(6) 74HC390 Dual Decade Counters [568-1442-5-ND]
(6) 4543 BCD to Decimal Converters [568-3138-5-ND]
(2) 4020 Divide by 16384 Counter [296-2039-5-ND]
(1) 74HC32 Quad OR Gate [568-1434-5-ND]
(1) 74HC08 Quad AND Gate [TC74HC08APF-ND]
(39) 500Ω or 1000Ω drop down resistors for 7 segment displays
(3) 10kΩ Resistors
(6) Common Anode 7-Segment Displays [160-1575-5-ND]
(4) 3 mm Green LEDs
(1) 100 uF Capacitor
(1) 0.1 uF Capacitor across Crystal Oscillator
(1) CMOS 32.768 KHz Crystal Oscillator [SER3618CT-ND]
(3) Momentary Switches [P8071SCT-ND]
(4) 3 mm Green LEDs
(1) Power Plug [CP-102A-ND]
(1) 5V Power Supply
Step 1: The Gist of How It Works
A 32,768 Hz oscillator provides the clock pulse and this is divided by two CMOS 4020 Divider chips down to 1 Hz. The 1 Hz signal is fed into the first dual decade counter [IC3] that runs from 0-9. The binary coded decimal from IC3 is fed into a 4543 [IC4] that is converted for a seven segment display. The block diagram omits showing the drop down resistors, but they're really there. Use either 500Ω for a bright display or 1000Ω for a dimmer display.
IC3 triggers IC5 by feeding its IC3's q3 output into IC5's clock input. IC5 runs from 0-5 and resets via an AND gate when it hits 6. The AND gate also feeds into IC7 which is the minutes section. IC9 resets at 6 as well. This completes the minutes section.
The AND gate from the minute section increments the hour section. Another AND gate in the hours section resets IC11 and IC13 simultaneously when they collectively reach 24.
This probably sounded all convoluted, dense, and confusing. Refer to the diagrams below for more clarity.
Notice part numbers from DIgikey.com are included in the block diagram
Step 2: PCB Layout & Explanation
The PCB is a 5x8 copper clad. Refer to the images below to understand the layout of the board. The red lines represent jumper wires. The next step will include the PCB art for the actual etching.
Caution: The 4543 BCD to decimal decoders are placed upside compared to all the other chips
Step 3: PCB Art
The actual PCB art for etching. It should come out as 5x8. I always like to place actual ICs on the paper to make sure everything lines up incase the page setup size needs to be increased or decreased slightly.
Etching the board:
1) Lightly sand the copper clad board. This seems to make the pattern stick better.
2) Set your laser printer on max toner density
3) Print out the PCB art on magazine paper. It may jam a few times, but it gives great results.
4) Iron the PCB art onto the board at your iron's maximum temperature. Apply alot of pressure for 15 minutes.
5) Soak in water to dissolve paper. Remove any excess paper.
6) Fix any broken traces with a sharpee.
7) Etch in ferric chloride which you can get at Radio Shack.
Step 4: Drilling Holes & Example Images
I drilled my holes with a testors hobby hand drill. It worked out pretty well, except for a few poorly drilled places where I wasnt paying much attention.
Step 5: Populate the Board
Populate the board with components. Be careful with the 4513s as they are upside down in orientation compared to the other chips on the board. Also, the crystal oscillator is a SOIC chip. I got mine on by pre-soldering the pads and having the chip melt into the pads. I am not sure if this is the best approach, but Iv never dealt with SOIC chips before.