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Digital Clock

I've been playing around with electronics for a long time. When I was a kid I used to take apart old radios and TV's and try to figure out why they had stopped working. Sometimes I would even get them running again. When I was in the Computer Networking Program in college ('97-'99) ... almost 20 years after I had graduated from H.S., I had several electronics classes. After finishing a Digital Concepts class, I decided to apply some of the stuff I had learned.

This is the digital clock that I designed and built. It was developed at the component level using TTL circuitry. The crystal oscillator IC runs at 1.8432 MHz, with the frequency division being performed primarily through the use of a 14 bit CMOS counter. The output was taken from Q13 (225 Hz), and further divided down to 1 Hz using 2 additional binary counters. The 1 Hz signal is used to clock all the other IC's through the chain. Decade counters, logic gates, and a single J-K flip-flop are used to decode the clock logic. 74LS47's were used to drive the four 7-segment displays used to show the hours and minutes. The seconds are represented in BCD (Binary Coded Decimal) with the use of 7 LED's.

Since the picture was taken indoors with a flash, the time on the clock did not show up. It was 4:28 when the picture was snapped. You can see the LED on the left in the group of three, and the LED on the left in the group of four lit up, representing 48 seconds. The two toggle switches on the left of the clock are for set fast, and set slow - just as you would have on many alarm clocks. The single toggle switch on the right turns the 7 LED's on and off. Having those LED's flashing and changing every second can get quite annoying after a while. A simple block diagram of the clock function is shown on the right.

The Circuit Maker Program is a very cool simulator I found years after I had designed and built the clock. It's kind of like the poor man's version of SPICE. Anyway, it allows you to run simulated circuits in real time. Another words, the clock actually works when you run the program! The red and blue lines represent the binary states that each device is in. Red represents a high state, and blue is low. So as each of the seconds tick off, and the states of some of the device are changing, the lines for those devices change color. As all this is happing, the time on the clock is advancing. So, you can actually see how the clock works. Kind of like the model car engines we used to build as kids. The engine block was clear plastic, so you could actually see the pistons and rods going up and down while the LED's that represented the spark plugs would light up in the proper timing sequence.

If you would like to download the full size block diagram for printing, click the link below. The file size is 594 KB