I've recently attempted to convert two cheap LED digital clocks to 24 hour format. One uses the LM8560 and the other has the TMS3450NL. Both chips are similarly configured and call for connecting pin 28 (24hr enable) back to Vss (pin 15). That seems to work OK until I start stepping through the hours in 24-hour format. When I get to 20, 21, 22, and 23 hours, however, only the b segment of the 10's hr digit lights--agd&e are off on both clocks, so I assume there's something else on the board which is inhibiting these segments. Both of these clocks were built to display 12 hr format only and had the 24 hour option disabled with no way to switch it on. The 10's hr pin in both cases is #1, but it doesn't seem to be connected to anything but the LED display. Any idea what I could be overlooking?

 

It sounds like the tens digit for hours is strictly set up for segments b and c (a numeral 1). This could be because the display doesn't have the a, d, e, f, and g segments (what is called a "half digit,") or because those segments aren't connected to anything, i.e., no tracks on the PCB. But I guess you have already checked that?

 

I should have checked all that, but I haven't. Thanks. That's what I'll check next.

 

Apparently, when manufacturers design digital clocks around dedicated clock chips and only intend to display time in 12 hour format, they typically do not fully enable the 10 hr digit. There are 21 pins to the LED display that enable the other three digits, then several unconnected pins, then a few more for the few segments needed in 12 hour format for the 10 hr digit, the colon between hrs and minutes, and the AM/PM indicator. So although the chip itself has 24 hour capability, the display usually does not, at least without some kind of modification.

 

I've recently attempted to convert two cheap LED digital clocks to 24 hour format.

Rule of thumb for most manufacturers: Don't include anything that you can possibly leave out. Some manufacturing engineer somewhere probably got an "attaboy" for that cost reduction.

 

Rule of thumb for most manufacturers: Don't include anything that you can possibly leave out. Some manufacturing engineer somewhere probably got an "attaboy" for that cost reduction.

Quite true. I had to go peek at MY cheapie alarm clock (it's the cheapest one I could find at Walmart) and I was surprised to see the display actually had the outline of the full 10's hour digit, all 7 segments.

But I would not be surprised to learn the 5 unused segments of that digit did not have any LEDs inside. When you are going for lowest base cost you do not include anything that is not absolutely essential, and if the display comes in half a cent cheaper by leaving out a few LEDs you leave them out.

So before you go wiring up the unwired segments of the display check there is actually a LED inside to light.

 

I acknowledge that this is resurrecting an old thread. However, this is a discussion on modifying an 8560 based alarm clock into 24-hour mod.

I too am trying to perform this modification on a number of inexpensive alarm clocks. I got very frustrated when I too learned (by experience) that display did not light up the remaining segments for 20-23 Hours. However, the display aside, shouldn't pin 1 read high, when in the 20-23 hour range?

I noticed that many of the displays do have a xxxx-12 stamped into the back. Perhaps they aren't meant to function in 24-hour mode. I'm still perplexed as to why the IC won't pull this pin high.

 

You acknowledge it, but you did it, which is not allowed at AAC. Start your own thread, that is how it works here.