Half Dial Wall Clock

This is wall LED clock. Despite to have only twenty LEDs it is full day clock showing time with 15 seconds precision. Is it really possible? May by these LEDs packs the time in so cryptic code that no ordinary human can read it. I believe the opposite is true: any person, who can read regular mechanical clock, can read it ( after some explanations and examples. of course). One year ago I have published project describing this approach in details: https://www.instructables.com/Half-Dial-Clock-or-Dialog-With-Know-it-all/ . But I believe examples (pictures an video here) would be sufficient enough to understand the principle.

• Atmega328p Microchip processor.
• Seven 10mm green LEDs, seven 10mm white LEDs, four 5mm white LEDs, one 10mm blue LED, one 10mm RGB LED. All LEDs are taken from adafruit.com.
• Twelve silicon diodes 1N4148 and one Schottky diode 1N5817
• One NTE3042 optoisolator.
• One super capacitor 0.5 Mcf 5.5 V.
• Six resistors 55 Ohm, two resistors 100 Ohm, seven resistors 150 Ohm, one resistor 170 Ohm, and one resistor 220 Ohm. All are 0.25 W.
• Crystal 32678 Htz frequency .
• Three mini pushbuttons.
• Syntax prototype board.
• 28 pin dip socket (for processor) and 6 pin dip socket (for optoisolator)
• 6 pin header (to make a connector between AVR ISP adapter and processor.
• Some wire.
• Wooden round 9x12 inches from Michael's
• One foot / 3.4 inches wooden round from Home Depot.

On this and on the next steps there are some pictures illustrating how clock shows the time. Clock is shown next to normal mechanical face with hours and minutes hands so you can see similarity.

First picture is for time 3:15. As one can guess smaller half-circle made out of green LEDs represents hours ( as smaller hand of mechanical clock). Outer (bigger) half-circle made out of white LEDs represents minutes with 5 minute precision (as bigger hand of mechanical clock). Does i look similar?

For time 3:20 next white LED will be lighted. For 3:25 one more. You can look at LEDs as arch started form the top of the clock face going down when time increases. But what happened at 3:30 and after? We are out of white LEDs.

That where we can look at second picture: time 3:45. After minutes reach 30 all white LEDs will go dark apart of one LED at the bottom. That LED represent 30 minutes. And after that white arch starts grow again from the bottom to the top: to show time 3:35, 3 40, and 3:45 as shown on the second photo. You can look at at white arch now as a of mirror reflection of invisible left half dial of the clock (google for reflection symmetry).

If last two paragraphs make a sense to you you understand how this clock work. If in doubt you may visit dedicated simulator at https://tinyurl.com/half-dial-simulator to play with it. Or maybe you decided that the whole idea is stupid. In that case you should stop here and switch to some other authors.

Of third photo we see the same trick as on second photo but performed not on minutes but on hours. Time is 9:15. Last photo shows time 9:45. Here both green and white arches starts from the bottom and grows to the top.

One would say: but so far we see time with precision five minutes. What about promised fifteen seconds. That we will discuss on the next step

To set the time there is need to have a control. For this project control is wired. It has three buttons connected to processor input pins. Black button controls clock mode. Pushing it switches form normal mode (when clock shows time) to set mode. Another push switches it back. When in "set time" mode seconds are cleared, clock are not running. White button increase minutes ad green button increase hours. When two color buttons pushed simultaneously (white first, then without releasing it green) time is reset (to zero hours and minutes). On the clip you can see who pushing white buttons increases minutes. There are four small LEDs on the right side of the face. You should add number of lighted small LEDs to minutes shown by white arch. That way you'll get one minute precision. Pay an attention now to how hours set. When green arch grows from top to bottom, than from bottom to top, blue LED at the left lights up. This is PM LED. It stays dark within first twelve hours of the day and is lighted for the rest of the day. When time is properly set black button is pushed again and clock starts running. You can see that by central LEDs becoming red. Central LED is three- colors. Combination of three basic colors may give a lot of different colors but for this project we need just four:

• Red color used when fraction of minute in second is 1 .. 15
• Green color used when fraction of minute in second is 16 .. 30
• Blue color used when fraction of minute in second is 31 .. 45
• White color used when fraction of minute in second 46 .. 59

Based on central LED color we may evaluate time within 15 seconds (as I promised in introduction),

The last part of the clip shows how clock handles power interruption. When power goes down clock circuit detects it and turns dark all LEDs. But it clock continues to run for some time on the energy saved in capacitor. If power is restored in time less than five minutes, there will be no clock reset and no time lost.

Her is circuit diagram of the clock. Is is built around Atmega 328p microchip. I would like to highlight two non-traditional parts of this diagram .

1. Power backup and power down detection circuit. It is shown at the top left corner of diagram. Super capacitors provides power backup. When external power is down diode prevents capacitor discharge though power supply circuit: all charge should be spent on powering processor.
2. Due to fact that there are not enough pins to handles all LEDs and inputs, twelve LEDs are coupled in six pairs. Single pin control two LEDs. Bottom left part of diagram shows such elements. All white LEDs + blue LED are controlled this way. Instead of traditional balance resistors I used here silicone diodes. Big forward voltage (up to one Volt) of these diodes together with 3 V forward voltage for white LEDs creates enough resistance to avoid any LED lighting when pin is turned into input mode with no pullup resistor activated.

As face of the clock I have used 9x12 inches round from Michael's store.

1. I marked with awls places where holes for LEDs needs to be drilled.
2. For 10 mm LEDs I used 11/16 drill, for smaller 5 mm LED 7/16 drill
3. Holes I treated first with 1/4 round file, than with Dremel tool.
4. I applied some wood glue to pieces of 3/4 rounds and tapped them in to hole ~1/4 inches dip (for smaller LEDs there where pieces of 1/2 round)
5. As soon as glue dried out I cut out tapped rounds with flash cut saw.
6. On the last photo you can see back side of plate after I drilled 3/32 holes for each future place for LEDs.
7. The last step (not shown) was to drill holes for 10mm LED with 7/16 and with for 5 mm drill bits. Later I treated these holes with Dremel round stone.

All components of electrical circuit I soldered into Syntax prototype board. LEDs were inserted into corresponding holes, board attached to back side of the plate.

The last job was to program device. Program was developed with Microchip Studio and uploaded to Atmega 328p processor through AVR ISPII adapter. All done. You can find source code for the project: https://github.com/jumbleview/halfdial

Folder "halfdial" of the project contains C - program and Microchip studio project.