This is a longterm project of mine, which started in the 360 version of MC. I was always afraid it would be too much for the 360, so I never went all the way with it. It involves hundreds of pistons and redstone components. It's not a new idea, and has been done since years ago in the PC version. The principle involves rotating a "cylinder" of blocks akin to the drum in a music box. Opaque blocks conduct redstone current while transparent blocks do not. Each cylinder is programmed with the data required to drive a 7-segment display, and bump the next cylinder when the display rolls over. The 5th cylinder also drives a "half" digit--a 1 or a blank, so that 10, 11 and 12 can be displayed. Then the final rollover bit drives the AM/PM flip-flop. The 7-segment display works because of the shading when blocks in the white wall get retracted by sticky pistons behind it.
I completed it recently, and then added the ability to run either real time or Minecraft time (72x the speed of normal time). I'm surprised how well it runs. Running normal time is easy, since each tick of a repeater or redstone torch is 1/10th of a second. Running at 72x speed was trickier. Driving the minutes display at the speed of seconds gets it to 60x. Dropping the clock cycle to 8 ticks from 10 gets it to 75x, which is close. To go the rest of the way, I had to figure out how to interrupt one clock cycle out of every 25. That's where this gadget comes in. (I cut away the floor to show it.)
The big L-shape of repeaters is a clock running at 1/25th the speed of the main clock. It sends a pulse to block the output to the display once per 25 main-clock pulses, which is why the display hesitates every so often in MC-time mode. The main clock is a little thing toward the lower-left, with a gold block, a redstone torch, 2 repeaters, and a redstone wire going off to the right. The repeater facing down at 90 degrees to the other repeater (center-bottom) is what locks the redstone at the right time, to interrupt one display pulse.
The clock is synched to the day-night cycle in MC at first. It stays synched for weeks of game time. (I've tested it.) Of course, when I went to show the real-time mode, it got out of sync, which is why dawn came around 3 AM.
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Edit (10/15/2015): I can't leave well-enough alone. I've been doing a lot to this thing. I totally revamped the clocks that drive the display. Now a 1-tick pulse circles forever in a loop of repeaters, and drives the display at the appropriate intervals. The 10-repeater loop feeds into the seconds display, to keep real-world time. The 8-repeater loop feeds into the minutes display, and keeps Minecraft time, with the help of the slow "100-clock"--that U shape of 25 repeaters that sort of looks like a troll mouth. (As I said back in August, the purpose of the slow clock is to block the output of the 8-tick clock every 25 cycles, to keep 72x-normal time, which is Minecraft time.) I moved all these clocks behind the display mechanism, to their own chunks. The result was cleaner digit transitions in the display, and a great reduction in lag when viewing the display. There are much fewer of those annoying black pixels--which are really caused by some lighting bug in MC.
Each of the 3 emerald blocks visible is part of a display-driving clock. The bottom one is the original inverter "5-clock" (one-second cycle). It can still be selected with the lever switch visible to its right.
I also added chimes, complete with half the Westminster tune to lead them off each hour. Haha! I'm using fireworks and beacons in addition to 6 noteblocks to make them so. The mechanism for that is those rows of red and yellow blocks behind the display. There's a 1-12 counter in front of them made possible by repeater locking. Then the pulse generated by each bump to the count meanders through a long twisting path of repeaters, chiming once for each number in the count. When the counter reaches 12, it resets itself (visible as all the yellow pulse-limiter blocks dropping in sucession), ready to restart.
I've also streamlined the rollover process a lot. I'm using piston-popper pulse limiters throughout the display mechanism now. No more icky torch inverters in the time-critical rollover path. The result is a 4-tick rollover time (from data drum to data drum), which is easily compensated for with variable delays in the digit displays. The entire clock display now changes at precisely the same time each iteration, no matter how many digits change.
Here's a lousy vid of it in action. The glitches are in the video, not the clock. You can skip the first 50 seconds or so without missing anything. Upload is being a pain on the Xbox, and it would not let me edit and upload the video. I had to upload the raw footage with the Youtube app. I'm only running Minecraft time here. To see that the clock also works in real-world time, and to get a closer look at the piston arrays in action, see the earlier video above.
This is a longterm project of mine, which started in the 360 version of MC. I was always afraid it would be too much for the 360, so I never went all the way with it. It involves hundreds of pistons and redstone components. It's not a new idea, and has been done since years ago in the PC version. The principle involves rotating a "cylinder" of blocks akin to the drum in a music box. Opaque blocks conduct redstone current while transparent blocks do not. Each cylinder is programmed with the data required to drive a 7-segment display, and bump the next cylinder when the display rolls over. The 5th cylinder also drives a "half" digit--a 1 or a blank, so that 10, 11 and 12 can be displayed. Then the final rollover bit drives the AM/PM flip-flop. The 7-segment display works because of the shading when blocks in the white wall get retracted by sticky pistons behind it.
I completed it recently, and then added the ability to run either real time or Minecraft time (72x the speed of normal time). I'm surprised how well it runs. Running normal time is easy, since each tick of a repeater or redstone torch is 1/10th of a second. Running at 72x speed was trickier. Driving the minutes display at the speed of seconds gets it to 60x. Dropping the clock cycle to 8 ticks from 10 gets it to 75x, which is close. To go the rest of the way, I had to figure out how to interrupt one clock cycle out of every 25. That's where this gadget comes in. (I cut away the floor to show it.)
The big L-shape of repeaters is a clock running at 1/25th the speed of the main clock. It sends a pulse to block the output to the display once per 25 main-clock pulses, which is why the display hesitates every so often in MC-time mode. The main clock is a little thing toward the lower-left, with a gold block, a redstone torch, 2 repeaters, and a redstone wire going off to the right. The repeater facing down at 90 degrees to the other repeater (center-bottom) is what locks the redstone at the right time, to interrupt one display pulse.
The clock is synched to the day-night cycle in MC at first. It stays synched for weeks of game time. (I've tested it.) Of course, when I went to show the real-time mode, it got out of sync, which is why dawn came around 3 AM.
--------
Edit (10/15/2015): I can't leave well-enough alone. I've been doing a lot to this thing. I totally revamped the clocks that drive the display. Now a 1-tick pulse circles forever in a loop of repeaters, and drives the display at the appropriate intervals. The 10-repeater loop feeds into the seconds display, to keep real-world time. The 8-repeater loop feeds into the minutes display, and keeps Minecraft time, with the help of the slow "100-clock"--that U shape of 25 repeaters that sort of looks like a troll mouth.
(As I said back in August, the purpose of the slow clock is to block the output of the 8-tick clock every 25 cycles, to keep 72x-normal time, which is Minecraft time.) I moved all these clocks behind the display mechanism, to their own chunks. The result was cleaner digit transitions in the display, and a great reduction in lag when viewing the display. There are much fewer of those annoying black pixels--which are really caused by some lighting bug in MC.
Each of the 3 emerald blocks visible is part of a display-driving clock. The bottom one is the original inverter "5-clock" (one-second cycle). It can still be selected with the lever switch visible to its right.
I also added chimes, complete with half the Westminster tune to lead them off each hour. Haha! I'm using fireworks and beacons in addition to 6 noteblocks to make them so. The mechanism for that is those rows of red and yellow blocks behind the display. There's a 1-12 counter in front of them made possible by repeater locking. Then the pulse generated by each bump to the count meanders through a long twisting path of repeaters, chiming once for each number in the count. When the counter reaches 12, it resets itself (visible as all the yellow pulse-limiter blocks dropping in sucession), ready to restart.
I've also streamlined the rollover process a lot. I'm using piston-popper pulse limiters throughout the display mechanism now. No more icky torch inverters in the time-critical rollover path. The result is a 4-tick rollover time (from data drum to data drum), which is easily compensated for with variable delays in the digit displays. The entire clock display now changes at precisely the same time each iteration, no matter how many digits change.
Here's a lousy vid of it in action. The glitches are in the video, not the clock. You can skip the first 50 seconds or so without missing anything. Upload is being a pain on the Xbox, and it would not let me edit and upload the video. I had to upload the raw footage with the Youtube app. I'm only running Minecraft time here. To see that the clock also works in real-world time, and to get a closer look at the piston arrays in action, see the earlier video above.
All together now:
!NICE!
Nice job!
Build Planes, Boats, Cars, Airships and fight!
I've updated the OP with the changes I've made to the clock since August. Thanks for the kind replies!