The following LED driver circuit task is being considered. The task involves 24 LEDs – 12 single color LEDs and 12 multi-color LEDs. Your suggestions would be appreciated.

A portion of the task – the hour portion – involves illuminating one large LED for exactly one hour. When then one large LED is switched off at exactly one hour, a different (second) large LED is illuminated for exactly one hour. When the second large LED is switched off after being on for exactly one hour, another (a third) large LED is illuminated for exactly one hour. When the third large LED is switched off after being on for exactly one hour, another (a fourth) large LED is illuminated for exactly one hour. The same sequence continues for a total of 12 LEDs, each LED being on for exactly one hour, while all other “hour” LEDs are off. The idea is like a common LED sequencer, but greatly slowed. These twelve LEDs will represent twelve hours, the active hour LED being the only “hour” LED illuminated during that hour. This cycle will repeat and continue indefinitely.

Another portion of the task – the 15 minute portion – involves a nearly identical sequence. The 15 minute portion involves the 12 multi-color LEDs. In one twelve hour cycle, each of these “15 minute” LEDs will be illuminated as one color for exactly 15 minutes, then another color for the next 15 minutes, and finally a third color for the next 15 minutes. Each “15 minute” LED will be illuminated for a total of 45 minutes in one twelve hour cycle (15 minutes as one color, then 15 minutes as another color, and finally 15 minutes as yet another color).

Thus, at any given time this LED clock will always show exactly one illuminated “hour” LED. Additionally, the associated “15 minute” LED will be either off (designating; for example, 7:00 to 7:14), or one color (designating; for example, 7:15 to 7:29), or a second color (designating; for example, 7:30 to 7:44), or a third color (designating; for example, 7:45 to 7:59).

 

Nice of you to consider this. Is there a question you need help with?

 

Okay.... so what you are wanting suggestions about? You've laid out what you want to achieve (fairly clearly, I think, so thank you for that -- it's a rarity). But unless you give us a hint as to what part(s) of the project you have concerns about we aren't in a position to offer much.

 

A little long winded in your description of the hour LEDs; I fairly certain everyone knows what you want.
How are you arranging the 12 multi-color LEDs? It would be just as easy to indicate five minute intervals by having the 12 LEDs is a smaller circle than the other 12 (and you would use one color instead).

 

The goal is to make the entire clock ciruit with the fewest number of components (e.g., ICs, resistors, capacitors, etc.). Other than the required 24 LEDs, there are no restrictions on what type of components may be used. Any type of battery (or batteries) may be used to power the circuit and the associated LEDs.

 

Why 12 multi-color LEDs? One is sufficient to show the 15 minute intervals; you wouldn't need to use one per hour. Save a lot of circuitry and complexity.

 

This is sounding like a homework/lab assignment. Is it?

If you want minimum part count, then you might consider an FPGA.

 

Homework - that's flattering
Which specific FPGA would you suggest?

 

It wouldn't take much of one. How much current do each of the LEDs need?

Are the tricolor LEDs ones with two pins where if you apply a voltage one way you get one color, the other way you get another color, and if you alternate you get the third color? If so, then you can run one of these between a pair of outputs to control the color. If you are lucky, you might find an FPGA with a slow output option that would be suitable for limiting the current, but most likely you will need to use current limiting resistors. You can minimize parts count by getting SIP resistor packs. For the hour LEDs, you could play a similar game and put two LEDs back-to-back between two outputs with a single current limiting resistor for the pair. With an FPGA you will have plenty of pins, so there isn't much to be gained by multiplexing them. If you use an MCU, then that would be something to consider since I/O pin count tends to be much less.

You could probably do this with a CPLD as well, which would get rid of the need to use an external ROM for the FPGA configuration (though I wouldn't be surprised if there are now FPGAs with embedded Flash -- it's been a number of years since I played with any).

For the time base you either want to derive a timebase from the mains frequency or use a watch crystal. Using a prepackaged oscillator will reduce parts count.

 

Homework - that's flattering
Which specific FPGA would you suggest?

A FPGA is a field-programmable gate array. It has a bunch of gates connected to the inputs which you can disconnect by programming the array. The number of gates and types of gates are what differentiates the different chips.

I was looking at timer chips a few months ago and noticed there was one that only needed a 32KHz crystal and output a pulse every second.