Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!swbatl!ken From: ken@swbatl.sbc.com (Ken Gianino 5-9081) Newsgroups: sci.electronics Subject: Re: Sidereal Time Clock Message-ID: <1990Dec12.214854.19076@swbatl.sbc.com> Date: 12 Dec 90 21:48:54 GMT References: <1833@umriscc.isc.umr.edu> Distribution: sci.electronics Organization: Southwestern Bell Advanced Technology Laboratory Lines: 42 In article <1833@umriscc.isc.umr.edu> robf@mcs213j.cs.umr.edu (Rob Fugina) writes: >My roommate just asked me to design a clock for him. It's for astronomical >purposes. Relatively simple, really, but I need a little advice and some >chip numbers. > >The clock has to count from 00:00:00 to 23:56:03, then reset back to zero. >I figured it would be easy enough to design an oscillator with a 555 to >100Hz, calibrated with a 15-turn pot and an oscilloscope (I'll take it >to campus for one of my EE labs). From there, I can use 7400-series chips >to do the logic. BCD counters for both digits of the hour, and for the >ones digits of the minutes and seconds. What do they make as far as >divide-by-6 counters? I would need these for the tens digits of the minutes >and seconds. The outputs of the counters would go to 7-segment decoders. > >Next is how to set it. I figured I could, for each digit, put a momentary >switch directly to the digit's clock line, which would advance that digit. >Special attention would have to be taken to make sure they flip to 0 >correctly. A spst switch could enable/disable these switches. >B >Power supply...how easy is it to make it AC line powered, with a battery >backup? This I'm not so worried about, but I'd like advice on whether >it would be better powered by AC line or by battery...i.e., how long do >you think the battery would last? > >The whole thing would be made up of a 555, 6 counters, 6 BCD-to-7 seg >decoders, and 6 LED digits. > >Oh, and I nearly forgot...the most complicated part will be using >logic gates to reset 0 at the right time...I'm looking at two 24-input >NAND gates or something similar. Advice, please! > >Rob Fugina >robf@cs.umr.edu Why 23:56:03? Doesn't sidereal time use 24 hours and just lose 4 min/day? Boy, I've kinda forgotten. At any rate, Astronomy magazine had an article on how to convert a Radio Shack 24 Hr alarm clock (digital) to sidereal time. It just added a couple run-of-the-mill counter chips to change the 60 Hz count input the right amount. I don't have the date but try 1988 +1/-2 years. Ken