Switches to CPUs: Minutes counter

We make the minutes counter for the clock.
Part 1: Binary numbers - • Digital Logic Explained
Part 2: 2-input gates - • Switches to CPUs: 2 In...
Part 3: Relay based logic - • Switches to CPUs: Rela...
Part 4: Latches - • Switches to CPUs: Rela...
Part 5: SR Latch - • Switches to CPUs: Set ...
Part 6: Wired OR gates - • Switches to CPUs: Wire...
Part 7: Flip Flops - • Switches to CPUs: Flip...
Part 8: Ripple Counter - • Switches to CPUs: Ripp...

[@chrisg6597]

In the mid 1980's I was working for an electronics company in the test department with the ambition of moving into R&D. One day the manager of both test and production came to me and said that they had run out of 74C74's (Dual D type flip flop) and could 74HC74's be substituted. There was a mixture of 74C and 74HC devices all over the PCB which the substitution would be made, so I said OK. What I hadn't realised is that in this particular design both /Set and /Reset were sometimes brought low at the same time and that the truth tables for this condition were different. i.e. with both /S and /R low on the 74C74 both Q and /Q are Low and on the 74HC74 both Q and /Q are high. This is the only anomaly I've found but even now I still check the truth tables between 74 series families.

[@DrMattRegan]

Yes, it may have something to with NAND vs NOR implementations of the latched. I personally prefer the term undefined, as it indicates use at your own risk. I believe the table I used was for a 74HC74, which is to tie it in later. Will divide the stream soon, one I’ll continue to use relays, the other I’ll move to 7400 series logic.

[@mheermance]

The use of BCD is a good idea. Much easier to read, and the logic to detect the overflow and reset to zero wasn't that complicated.

[@DrMattRegan]

You tempted to make one?

[@mheermance]

@@DrMattRegan Yes, but maybe only part of one, as wiring relay D flip flops will get tiresome. I already have a relay ring oscillator, so a zero to fifty nine counter could be neat

[@DrMattRegan]

@@mheermance 1-12 might also be useable as wall art!

[@Raiment57]

I've just added like number 74. Nice.

[@DrMattRegan]

Thanks. Enjoy!

[@JaenEngineering]

Nice. Wonder how fast those little relays can switch? If it's less that 20ms you could half wave rectify the output of a small mains transformer and use that as a pretty stable timebase

[@DrMattRegan]

Yeah, i'm still deciding what to do for timing. I'm tempted to use a pendulum. It's cheating a bit, because they are quartz crystal controlled. Interestingly i once saw the timing mechanism in Big Ben, a big pendulum. They used pennies on top to adjust the rate.

[@frankowalker4662]

Neat

[@DrMattRegan]

Thanks

[@mndodd]

Now do dekatrons.

[@DrMattRegan]

Hmmm, i think i'll skip them. Was planning to go over Karnaugh maps and 7 segment displays. I'm trying to introduce the ideas as i need them to get to the finish. Any thoughts on this series?