EEVblog #801 - How To Design A Digital Clock

[wraper]

All well and good, Dave.  Righteous clock design.  However, I must call bullshit on the Ahmed bandwagon.  Here are the updated facts and context:

1.  Ahmed did not design that "clock" from scratch.  It was an existing Radio Shack clock removed from its enclosure and remounted into a briefcase.
2.  The clock was altered into a countdown timer, rather than an RTC.
3.  Said clock/timer was executing an active countdown when the incident occurred.
4.  The entirety of the situation resembled nothing more than an improvised explosive detonator being taken into a public school.
5.  Ahmed Mohamed's father, instead of consulting with the school board (which is the legal and responsive authority on this matter), has contacted CAIR, which is in the process of manufacturing a racial incident out of this event.
6.  The same father, a known jihadist, has twice ran campaigns for the presidency of Sudan, which last I checked, is not known as a wellspring of democracy but is known for impressing children into suicide bombers, as part of President Bashir's policy of global jihad and arms supply to Hamas and Hizbullah.

Good Job, all factual.
I love it when someone posts the truth in such a concise form. 

Crap job, reposting bullshit (consisting of lies mixed with some real information) found in sewers of internet.

[AF6LJ]

All well and good, Dave.  Righteous clock design.  However, I must call bullshit on the Ahmed bandwagon.  Here are the updated facts and context:

1.  Ahmed did not design that "clock" from scratch.  It was an existing Radio Shack clock removed from its enclosure and remounted into a briefcase.
2.  The clock was altered into a countdown timer, rather than an RTC.
3.  Said clock/timer was executing an active countdown when the incident occurred.
4.  The entirety of the situation resembled nothing more than an improvised explosive detonator being taken into a public school.
5.  Ahmed Mohamed's father, instead of consulting with the school board (which is the legal and responsive authority on this matter), has contacted CAIR, which is in the process of manufacturing a racial incident out of this event.
6.  The same father, a known jihadist, has twice ran campaigns for the presidency of Sudan, which last I checked, is not known as a wellspring of democracy but is known for impressing children into suicide bombers, as part of President Bashir's policy of global jihad and arms supply to Hamas and Hizbullah.

Good Job, all factual.
I love it when someone posts the truth in such a concise form. 

Crap job, reposting bullshit (consisting of lies mixed with some real information) found in sewers of internet.

We all are entitled to our opinion.
One thing you and I have in common, we were not there to witness any of the facts.

[wraper]

We all are entitled to our opinion.
One thing you and I have in common, we were not there to witness any of the facts.

Then try to modify such clock into being countdown timer. That's where the credibility of that info ends. Also his father is not Jihadist (if you know what this word means).

[SeanB]

The IIL clock chip has a clock, alarm with snooze and a countdown timer, for it's designed use as a part of a clock radio, where you can have the radio on or off, or use it as a wake up aid ( like I do) or to fall asleep using the radio, while it turns off so as not to awaken you during the night. Hidden feature is that you can get a display of the minutes digit ( not 10's of minutes) and the seconds count on the display, just by holding down a combination of the input selector buttons.

 A lot of clock radios also use the redundant power switch on the small edge variable resistor volume control as a method to inject 100/120Hz or a 400Hz signal into the audio amplifier instead of the radio audio, with the open switch removing a short across a voltage divider, that is fed into the amplifier chip after the volume control at a fixed loud level.

[AF6LJ]

We all are entitled to our opinion.
One thing you and I have in common, we were not there to witness any of the facts.

Then try to modify such clock into being countdown timer. That's where the credibility of that info ends. Also his father is not Jihadist (if you know what this word means).

That shouldn't be so hard, I know where there is one of those clocks, might be fun.....
Most of those clock chips have a countdown timer function, just because it is in a table clock doesn't mean the chip doesn't have the functionality.

[SeanB]

MM5314 is one, amongst others.

TMS3450NL another. LM8560 yet another.

[AF6LJ]

OR;
You can just go to the Dollar Store and buy a kitchen timer.   

[free_electron]

7490 ( for decade dividers ) 7492 ( for /6 dividers ) 7447 or 7448(display decoders) and a few and gates

50 hz :
7490 div 5
7490 div 5
7590 div 2  -> 1hz 50% duty cycle clock

60hz :
7492 div 6
7490 div 5
7490 div 2 -> 1hz 50% duty clock

seconds
7490 one's
7492 tens
minutes
7490 one's
7492 tens
hours
7490 one's
7492 tens     ( you only count to 12 and add an AM pM indicator  . )
7400 NAND gate

[AF6LJ]

This use to be the go-to first project for those of us who wanted to play with digital logic.

[Howardlong]

7490 ( for decade dividers ) 7492 ( for /6 dividers ) 7447 or 7448(display decoders) and a few and gates

50 hz :
7490 div 5
7490 div 5
7590 div 2  -> 1hz 50% duty cycle clock

60hz :
7492 div 6
7490 div 5
7490 div 2 -> 1hz 50% duty clock

seconds
7490 one's
7492 tens
minutes
7490 one's
7492 tens
hours
7490 one's
7492 tens     ( you only count to 12 and add an AM pM indicator  . )
7400 NAND gate

Sounds very much like the one I made in the mid 70s. The tricky thing then for a 12yo was to work out how to make the hour display from a 7492 which is a div-by-6 plus div-by-2, and make it BCD to feed to a couple of 7447s. I designed some logic solution, I can't remember exactly what the design was, but it used two or three 74xx SSI devices to achieve it. That was the hardest part of the design, I spent a long while writing truth tables and dry running it while I saved up my pocket money for the parts.

[krivx]

Dave, did you not use PNPs at the time?

[Dave Turner]

I used to have great fun experimenting with TTL in the early 70's. This biggest problem was obtaining parts. unless you could find bargain bins or rare surplus/damaged equipment (rare because TTL was still relatively new). The cost new was anything upwards from 10 shillings (50p in decimal money) from hobby shops.

IC sockets and continual tear downs was the name of the game.

I spent more time with truth tables than I did with a soldering iron.

[SeanB]

NPN was a lot more common in old equipment, plus it made the logic easier not changing conventions.  It does not work well with DTL to mix PNP and NPN in gates, especially if one is driving the other. It either will be always on or always off, or will have even worse noise immunity than it already has. Worse it can sit in the middle of the indeterminate range and oscillate.

[LabSpokane]

Dave - outstanding!  I hope that a lot of people scrounge up the parts and give that a whirl.   

[apis]

Crap job, reposting bullshit (consisting of lies mixed with some real information) found in sewers of internet.

Yeah, indeed, that's how certain internet groups operate. They keep piling up the lies mixed with a gain of truth.

[apis]

Really cool clock Dave!

Clever using the mains frequency as clock, love the discrete diode/transistor logic and the 0.1 second display was a nice touch!

[Cliff Matthews]

Dave, did you not use PNPs at the time?

That was in the 60's germanium transistor era when PNP's were used more. I built my first 2-tranny kit radio on the 2N107 around the time when SCR color-organs were built from Christmas-tree lights jammed into old speaker boxes covered with Plexiglas diffusers     I'm sure the drugs made them look better (far out man..)

[nitro2k01]

3.  Said clock/timer was executing an active countdown when the incident occurred.

Source?

5.  Ahmed Mohamed's father, instead of consulting with the school board (which is the legal and responsive authority on this matter), has contacted CAIR, which is in the process of manufacturing a racial incident out of this event.
6.  The same father, a known jihadist, has twice ran campaigns for the presidency of Sudan, which last I checked, is not known as a wellspring of democracy but is known for impressing children into suicide bombers, as part of President Bashir's policy of global jihad and arms supply to Hamas and Hizbullah.

As much as Ahmed's father may be a less sympathetic person who took advantage of the situation after the fact, that proves nothing about the events leading up to it. Was Ahmed instructed to wear a NASA t-shirt too, to give that authentic "kid who's into STEM fields" look? In short, don't judge the son by the father.

[AF6LJ]

We do tend to use what we cut our teeth on.
Dave made a comment regarding microcontrollers in the video. We actually had the 8031 and the 8051 before 1980. One place I worked in 1979 was building ticket issuing machines for a number of airlines and their ticket printer used a 9 pin print head and an 8051 to control it. (I could be off on the numbers but Intel was building dedicated controllers back then.) The rest of the machine except for the data modem was controlled by an Intel 8080 SBC. (single board computer). The 300 baud modem was all discreet CMOS all wirewrapped together.

 

[int2str]

Dave,

How do you adjust that clock?
If it's all BCD overflow logic, does that mean you have to run to every 10th of a second to get to the right time?

[SeanB]

Would have been an 8748 for the development, and an 8048 for the production units. Still have some 8748 chips around, and a unit with one in it that I really should tear apart.

Clock setting was described, he had a switch pair that selected either the regular count input, or a 10x faster one, for both seconds and minutes. Reset was by depowering it then count up to right time.

[Howardlong]

The reminiscing over recycled parts took me back. My old recovered IC collection has seen better days.


Things like IC sockets were an unnecessary expense for most parts: still are today, but I gave the right tools now to replace them on the odd occasion it's necessary!

The parts I still have don't quite go back to the 60s, although at one stage in the mid 70s I often substituted DTL parts for TTL parts which were from the late 60s. The oldest here looks like a 74153 from 1970. There are a couple of 2102 RAM chips in there, AY-5-1013 uarts, an 8080, and some recovered TTL with letters like AZ, HZ, or M22 in large characters for example. These were TTL rebadged, the Z meant high speed Schottky. Some of you may remember the sticky paper pinouts you could stick on your chips too.

[zapta]

Really cool clock Dave!

Clever using the mains frequency as clock, love the discrete diode/transistor logic and the 0.1 second display was a nice touch!

I read once that utility companies used to adjust the number of cycles during the day for the benefit of clocks like this.

Anybody knows if this is true?

[Fungus]

How do you adjust that clock?

You have to plug it in at exactly midnight!


(either that or sit and watch the whole first minute of an EEVBLOG video)

[rdl]

Nice video. I have a very similar clock built on a plastic breadboard that I was playing around with because I wanted one that displayed the seconds. Most commercial clocks are 4 digit only. I used the 4026 chips also, though the reset at 6 count is done differently. The only reason I stopped working on it was because I decided I wanted 12/24 hour capability, then I got sidetracked onto other things. I really need to go back and finish it up.