A simple (and useless) digital circuit for the fans of the HP-15C (and two other HP calculators)


I'm back to school for lack of something better to do (I'd rather be vacationing in southern France or Tuscany :). Lots of things have changed: one of my classmates in 1986 and 1987 is now my professor of Digital Electronics, for instance, and back then we used DOS PSpice :-)

After finishing my homework exercises on digital counters I decided to design a counter to cycle through the names of my first, second and third HP calculator. My first idea was doing it with three synchronous counters, for the two digits and characters. When I was done with the first one I realized a Johnson counter would do the job more nicely. Also, the second digits and characters could be easily derived from the first digit.

My first calculator was an HP-15C, as displayed. An easy question for those in the field: what were my second and third HP calculators? Comments and improvement suggestions welcome.


1) The 47 nF capacitor should be replaced with a 4.7 uF in the real circuit so that the clock frequency is around 1 Hz;

2) The Agilent multimeter is optional :-)


15C, 22S, 48S ?


Almost: 15C, 2?S, 4?S.




When the first digit is 4 the 4017 output O2 is high, low after the inverter, which means the segments "c" and "f" in the second display are not lit. Therefore it cannot display an "8".

Edited: 17 Oct 2011, 5:34 p.m.


15C 28S 42S


Yes. It's great it doesn't cycle through 20 or so calculators. But then again I would not pose the problem :-)


oops. swapped the 2nd and 3rd middle digits.


Yes, I noticed the swapping later, only after I had already edited my reply to you.


Haha and that produced actual models, too, by accident!

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