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|Written by Hans Summers|
|Thursday, 19 March 2009 18:52|
My first major digital electronics project, aged 14 I embarked on this digital clock project, sponsored by my Dad. It took several years to complete, during which time it evolved into a sidereal clock. The inspiration for this project was an article in the July/August edition of Elektor Electronics, 1984. Every summer Elektor did this bumber edition containing over 100 small projects. I didn't buy Elektor regularly but every summer this bumper edition was a must. 1984 was the first one I bought, I don't know if they did it before then. Anyway, number 17 was an alarm clock for cars. (See right. 08-May-2001: Regrettably Elektor Electronics have requested that I remove this article due to copyright infringement, and have declined me permission to reproduce it here. Read more...). It used a MM5387 chip which is almost a complete clock on a chip, plus 3 CMOS support chips. It looked like a nice project and captured my imagination. I have an MM5387 datasheet on my datasheets page. Alternatively I prepared a pinout and some brief notes. (Note: The LM8360 and LM8361 chips are equivalent to the MM5387). Somewhere along the line, thinking about it, I thought it would be more fun to design my own using individual logic chips. The MM5387 seemed like cheating somehow. And thus was this project born!
In my circuit diagram below you'll notice the logic for adding the 4 extra pulses is pretty similar to the method given in the article. One thing I did differently was that I count 1461 seconds, then add 4. In the article they count 1461 fiftieths of a second (or sixtieths in the US) then add 4 fiftieths. My extra pulse generator comes after the divide-by-50 counter which turns the 50Hz mains frequency into 1 Hz for the clock's seconds counter. I think I did this while trying to get the thing to work so that I could see what was going on better. It would be better though to add the extra pulses before the divide-by-50 because as it is, every 20 minutes or so my clock jumps forwards by 4 extra seconds which means that an extra error of 0 to 4 seconds is present as the 20 minute cycle progresses. A 4 second jump is also noticeable (if you're watching for it!) where as a 4-fiftieths of a second jump is invisible to most of us!
Top left on the back, there are some holes drilled in an attempt to keep the case cool. The three red buttons set the time. The top one resets the seconds count to zero, the middle one increments the minutes at a rate of 1 minute per second, the lower one increments the hours at a rate of 1 hour per second. The display on/off switch and brightness knob are to the right of that.
The back of the 7-Segment displays are at the right of the internal picture. The heat sink of the voltage regulator is at the top, with the transformer top left. In the middle of the picture you can see the second small circuit board which contains the extra counters for the Sidereal Time modification. The wires at the left go to the controls in the back of the case. In the foreground, just to the right of that prominent electrolytic capacitor, you can see the wire can be switched to ordinary time by connecting it to the one on the left instead of the right!
Errors, Omissions and Modifications
Leading zero blanking circuit
The sidereal clock can also be used in countries where the power line frequency is 60Hz. In my country we have 50Hz mains frequency. IC3 is a dual decade counter, whose two halves devide first by 10 then 5, producing a divide-by-50 in total. To decode the "5" we use check for a "1" on the Q1 and Q3 outputs (representing 1 and 4). These are pins 11 and 13 respectively. To instead decode "6", just use pin 12 instead of pin 11, it is this simple. This then decodes Q2 (2) and Q3 (4) outputs, which is a decode of "6". This is illustrated in the revised diagram fragment below:
Note that some datasheets label the four output pins of each decade counter as Q0 to Q3. Others use Q1 to Q4. I am using the latter convention in my circuit diagram.
Sidereal Clock PCB layout by Jose Izaguirre
Jose Izaguirre and I corresponded about the sidereal clock, starting with the modification described above, for operation from 60Hz mains supplies (e.g. in the US). Later in 2010, Jose sent me the PCB layouts shown below, and the following comments:
"I am almost finishing assemble Yours sidereal clock on a double sided PCB 180x159 mm.I made few changes,They are:1) Eliminate the diodes and resistor in IC15, pins 1,4-5 and 7 and connect 4-5 to pin 16; in order to show the decimal units for the hours indication.I also added 3 diodes to IC17, to pins 15 and 7, and 4 diodes to IC18 in the same pin 15 and 7, in order to reset the clock to 00hr 00min 00sec.And a switch to convert the clock in a 12 and 24 hrs clock.I am attaching the artworks for the printed circuit board,and the placement of the different components."
Update 12-Jan-11: "another modification from the original circuit. In order to set sidereal time, to different moments in an observation or for different places, I synchronize the counters 193,from 5b4 to 0 with the seconds reset.To do that I used the pin 14 of the 193 with a 470 ohms resistor, and use the same button and signal from the seconds reset.This way the counters is going along the seconds for the solar time,and You can check that every 1461 secs time interval solar, four pulses are added to maintain the sidereal time accurate.The clock is built and working."
Jose sent the following Adobe PDF files for his PCB layout (Component side, Solder side and Composite updated 12-Jan-11):
Update 21-Mar-11: "I am sending the final PCB with the indications for the different connection to the push buttons and switch."
If you build the clock, use the PCB layouts, or make any other modifications, please let me know.
|Last Updated on Wednesday, 19 November 2014 07:58|