Digital Clock
Description
This is a 12 hour digital clock using Digital IC's, Seven Segment Digital Displays,
a crystal oscillator for the clock signal and a 12V power source. Below is a demo of it.
The Design
The Hours
I started with the hours. They have the smallest amount of numbers (1-12), so seemed like a good place to start. For this, three components are needed.
- BCD(Binary Coded Decimal) to Seven-Segment Drive IC
- Seven-Segment Display
- BCD Counters
BCD To Seven-Segment IC: The purpose of this is to convert the binary values for 01,02...12 to decimal values for the seven-segment display. The highest digit on each segment is 9, NOT 12! Even though each seven-segment display package has two digits, each digit operates on their own. For the hours, the left digit will be between 0 and 1, and the right will be between 1 and 9. With the highest number being 9, we will have a total of 4 bits (Since 2^4 = 16, values 0-15).
The CD4543 IC will do the conversions for us. The good thing about this IC is no resistors are necessary.
BCD Counters: For the BCD counter, we have two available, the CD4510 and the CD4518. For the hours, I will use the CD4510 since it's presetable. Why is that important? Since it's a 12 hour clock, there is no 00 hour, we need to make sure it starts at 01. So these ICs have carry in and carry out signals. You can use these to cascade the IC's, and be able to go from 12 back to 01. To cascade, the least significant digit's carry out must be connected to the carry in of the next significant digit. Then, the preset pins, P0-P3 are the pins used to tell what value to reset to. It's just a binary value, so 01 in binary is 0001.
Now how do we get the chips to use this preset value? Very simple. We need to know when there is a 1 on the left and a 2 on a right. The keyword is AND. I have some chips that have AND gates, so I can connect the digit values to the AND gate and when it outputs true it can be connected to the preset enable pin of the CD4510. Now before we do that, there is something to think about. Do I reset the clock at 12? Or when it goes to hit 13? Looking at the data sheet, since this clock trigger starts on the positive edge (right at a new pulse), we want to preset when it actually hits 13. If it was 12, it would never reach 12 and go back to 01.
Go to the next page for the hours and minutes.