5 Minute Digital Clock

Posted on Feb 5, 2014

The clock shows time in 5 minute intervals. The hour and minute can be set. There is no alarm or any AM/PM indicator. For the clock face I used a 2 pieces of lenolium glued together to increase thickness. (A couple of floor tiles would work as well. ) The numerals are just inexpensive adhesive characters I purchased at a hardware store meant for us

5 Minute Digital Clock
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e on mail boxes. The LEDs are glued to the clock face from behind. If you wanted to get fancy, a nice brass or wooden clock face can be made along with a wooden cabinet like a "grandfather" style clock. NOTE : In the following schematics, TTL devices are shown as 7400 series. These are used as generic 7400 series part numbers. During construction, 74LS, 74ALS or 74HCT should be used because of the low power requirements and availability. (I know most of you know this already. ) The power supply is not shown in the schematics. A simple 5 volt regulated supply of 500mA to 1A will work well. I used a 7. 5V, 600 mA power transformer, a full wave rectifier and a 220 uf capacitor as a filter. Regulation is done with a LM340-5 - a 7805 would work just as well. For the full wave rectifier, I used a rectifier set in a TO-220 package that I salvaged from a switching power supply. The oscillator is a LM311 comparator along with a 32768Hz tuning fork type clock resonator. The 10pf trim cap is adjusted using a 6 digit frequency counter with the resolution of 1Hz. If you don`t have a 6 digit frequency counter, a fixed capacitor of 5 to 7 pf should get you close. I chose the LM311 circuit because the 311 is commonly available and inexpensive. There probably are better time base ICs available. The 32768Hz signal is divided by 16384 to get a 2Hz signal. This is done with a 74HCT4020 counter. The 2Hz signal is then divided by 12 with a 7492,...

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