The present drums are 100mm OD so 314mm circumference.  Number of digits/words per drum is 7, 4, 10, 12 (weekday, date tens, date units, month) so worst case is 12 giving a maximum height of 314/12 = 26mm.  25mm high letters for the month then.  For symmetry it probably makes sense the use the same for weekday.  The date digits could be the same size or slightly bigger at 30mm (not a lot of difference really).  The digital clock and calendar has 50mm time digits and 25mm calendar.  The calendar digits are marginal at the distance I normally view it.  OTOH the main problem with the digital clock is contrast which is poor even in daylight.  I have just placed the calendar drum unit on the bookshelf against the wall and a 25mm high digit is pretty clear.

I should have some 25mm or 1" letters and numbers but can't find them so ordered some more.  Self-adhesive vinyl.

Next is the drive.  I could use 100mm and 50mm gears giving a 2:1 reduction ratio.  50mm & 25mm radii.  Trouble with that is the drum gear teeth will protrude beyond the drums.  Also, this leaves virtually no gap between motors and drums.  With the motor against a drum, distance between centres = 75mm.  With say 5mm gap the distance between centres becomes 80mm.  Gears could both be 40mm giving 1:1 and no reduction - worth a try.  To get some reduction the drum gears could be 45mm and the motor gears 35mm giving a 9:7 ratio but that's little better than 1:1 so might as well go for 1:1 and save hassle.

To drive the motors I plan to make use of the electronics recovered from a 3D printer which I've replaced with an upgrade circuit board.  As mentioned above, this consists of an Arduino Mega 2560 plus an Arduino shield called RAMPS (RepRap Arduino Mega Pololu Shield) which provides drivers for up to 5 stepper motors.  It also includes electronics which aren't needed for this project but there is far more wanted than unwanted and it will save me designing and building my own circuit board. 

Stepper motors do not provide any feedback for their position so other methods are used to set a starting position.  In 3D printers these are called EndStops and mostly use micro-switches, though anything that will switch when a certain position is reached will suffice.  For this project I propose to use magnets and Hall effect switches which provide a very effective solution.  The RAMPS happens to provide exactly what we want, allowing for up to 6 EndStops.  This project will use 4 - one for each drum.

I shall not be using the 3D printer firmware (sketch) in the Arduino Mega but a sketch which I shall write specifically for this purpose.  3D printers do not use Real Time Clocks but the RAMPS board does provide for extensions and the RTC module can be added on a small daughter board plugged onto the extension pins on the RAMPS.  There is an Arduino library for use with the RTC which decodes the data to provide year, month, date, weekday, hour, minute and second so we have the data required for a perpetual calendar immediately.  I already have experience of the RTC Library in my Moon Dial Clock to automatically set the time from its RTC.

Now to calculate the number of steps required for driving each drum.  3D printers use 16x micro-stepping whereas full stepping would be better for the calendar.  The RAMPS board has plug in links to set the micro-stepping so it's only necessary to remove these links to change to full stepping.  The stepper motors have 200 full steps per revolution.  For the date this works out as simply 20 steps to advance either digit (assuming the units digits are equidistant around the drum and the tens match).  For the months there are 12 positions corresponding to 200/12 = 16.67 steps if using 1:1 gear ratio - awkward   But with a 5:6 gear ratio one position would be 200x6/(12x5) = 20.  The drum could have a 42t gear and the motor a 35t gear to give the right ratio.

The weekday drum could have the words spaced the same as the date using 7 positions out of the 10.  It isn't necessary to have the spacing equal around the drum - a stepper motor can go in both directions.  That means the weekday drum can use equal 40t gears like the date and 20 steps to advance the weekday.  At the end of the week the drum/motor can either advance by 4 days or go back 6.

If I make the 42t and 35t gears mod 2 their centres would be 77mm apart giving 3mm spacing between motor and drum.  To return this to 5mm means the centres want to be 80mm apart meaning the the radii will need increasing by 80/77 and slightly more than mod 2.  42x80/77 = 43.64mm and 35x80/77 = 36.36mm.

Here are screenshots of the gears in SketchUp.

Printed 40t drum gear and see I've made a mistake   Not much of a problem, can alway redesign the gears.  Adequate centre spacing is 70mm (not 80mm).  Back to the drawing board then...

Can't have 40t and 30t because that gives a 4:3 ratio and 20x4/3 = 26.67 steps. No good!  Checking with the 40t gear I've just printed shows that 45mm radius will be alright giving 45t and 25t at mod 2. Try 45t and 25t ie. 9:5 (that rings a bell )  20x9/5 = 36 (tick).  Using the same gears for the months gives 200/12 x 9/5 = 30 steps (tick).  Q.E.D.

All 4 drum gears printed and attached to drums, now printing the last motor gear.  I'm thinking of printing the casing in black PETG to match the frame of the moon dial clock.  A piece of wood may be the best back board.

Hi Gina,

do you have the complete mechanism design posted somewhere? I would love to either printed it or make some modification for my own design. I am currently in the development part of my design and would like to see some examples to play with.Thanks so much.

best wishes.