Resetting a clock back to 1:00 after 12:00

[FastEddie]

I am trying to build a retro digital clock using LS TTL logic, My decade counters are 74LS90 for ones of seconds, tens of minutes and tens of hours. The remaining tens of seconds, Tens of minutes, and tens of hours, use a 74LS92. I have the 60Hz reference working and use a 74LS90 (divide by 10) and a 74LS92 (divide by 6) to obtain the one pulse per second to drive the clock circuit. The display decoders are the 74LS247's and the displays are Numitrons.

What I cannot figure out is how to reset back to 1:00 after 12:00 has passed? I have tried various gates to obtain a reset pulse. After about 30 or so hours of trying I am out of ideas and require some assistance.

I am not an EE or Tech person but was a QA Engineer for 35 years. I had two strokes and now spend my spare time tinkering with electronics.

Ed

 

[philba]

You need to preset to 1 after seeing a 13. The 90/92 isn't a perfect solution. maybe you could generate an extra count - see 13, reset (0), extra count (1). seems like a lot of extra effort.

I'd look at the '192 if I was starting from scratch. Use the preset for the hour 1s digit. a bit more effort and you can make a switchable 12/24 hr clock.

Of course, this can be done with a single uC...

 

[eblc1388]

I'd look at the '192 if I was starting from scratch. Use the preset for the hour 1s digit. a bit more effort and you can make a switchable 12/24 hr clock.

What the OP needs is a 12/24 hr lookup table sitting between the hour digits counter output and 74LS247 decoders. That can be done very simply by a single EPROM. The physical hour counters will count from 00 to 23 and reset to 00.

The address lines of EPROM accept output of the hour decade counters and the eight data output line of the EPROM will go to the 74LS247s as data. One additional address line of the EPROM will act as 12/24hr selection via a switch. Changeover can be done at any time with no concern of any logic problem.

Question is whether he can find a way to program the EPROM with the correct data. With so little data(48 bytes) to enter, it is doable via some switches on a breadboard.

 

[philba]

that is certainly one way to do it though I think using a presettable counter will work fine too. the reason I suggested the 192 was so he didn't have to chuck his design out. Programming is definitely an issue.

I do agree that eproms are really great as a "poor man's" programmable logic device. lots of gnarly logic can be reduced to a table look up scheme. Prior to the advent of cheap micros, it was pretty common place. Now it seems to have fallen by the wayside.

 

[eblc1388]

Have you considered the necessary logic to enable an user to switch back and fro from 12Hr display to 24Hr display, anytime of the day?

A presettable counter works if a fixed value is to be loaded into the counter so it will be good at 12:59:59 -> 01:00:00. What happens when the user wants to change the 12/24hr selection at 2:38pm? Resetting the time again?

There is little issue of programming the EPROM. A dip switch and a monostable is all one needs to do it.

 

[FastEddie]

Although the thought of using an EPROM is interesting I was trying to stay with TTL logic. In 1970 in a issue of Popular Electronics there was a digital clock project using 7490 counters with 7447 drivers. I remember the article because I made several clocks from this article. I remember the reset logic using 7474 flip-flops, an 7430, and a few 7400 gates. It would run in either 12 or 24 hour mode (although you would have to reset the clock if you tried to change time formats). Unfortunately, I no longer have the magazine as a reference as it was lost many years ago. Since my board is pretty much already built I was hoping for a simple TTL solution to go with this project.

Thanks,
Ed

 

[philba]

you probably aren't delaying the extra count pulse enough. I'd be tempted to use a 555 to do it. Kind of kludgy and not TTL but I know it would work. maybe you could post a schematic for us to pick apart. lol

switching between the 12 hr reset and 24 hr reset should be simple, have 2 sets of logic controlled by a dpdt switch. one pole for the reset logic (13 vs 24), one for the preset value (0 or 1).

so, eddie, when are you going to get to microcontrollers?

 

[TekNoir]

Not that it is much help, but one of these might be the article that you are thinking of -- if anyone happens to have any Popular Electronics magazines still laying around.

Build a digital clock. Uses modern TTL chips to display either 12- or 24-hour time in hours, minutes and seconds. Est. cost: $40.
POPULAR ELECTRONICS, Mar 1973 (v.3#3) pg. 51

Schematic diagram for an electronic clock which uses inexpensive 7400 series TTL ICs along with type 555 timers. Est. cost: $5.
POPULAR ELECTRONICS, Dec 1976 (v.10#6) pg. 93

 

[kchriste]

I know it uses 4000 series logic, but have you seen this thread?:
https://www.electro-tech-online.com...lock-with-7-seg-led-displays-hourly-ch.18901/

 

[FastEddie]

Does anyone have the March 73 issue of popular electronics? If this clock uses minitron incandesent seven segment displays, this is probably the one I was thinking about.

Thx,
Ed

 

[FastEddie]

so, eddie, when are you going to get to microcontrollers?

so, eddie, when are you going to get to microcontrollers?

.

I have been working on learning PICs for some time now. I have plenty of PICs, a PIC Start Plus, and a load of reference materials. I even designed and built my own demo board. It has buttons, LEDs, and a VFD (LCD dropin replacement).

The exercise with the TTL clock was a project I was going to put in a transparent plastic case because the chips and color wire look neat.