Convert a Quartz Clock Module to a Co-axial Stepper Motor

I like to make analogue displays using stepper motors, for example: meters displaying upload and download internet speeds. These have up to now used a single stepper motor per dial. However, there are times when I would like to have two independently moving pointers on a single dial and in that case a co-axial stepper motor design is necessary. These are available but I was looking for something which is trivial to mount in a repurposed clock housing and for which different hands/pointers are easily sourced (ie) a quartz clock module.

As I don't need a large amount of torque (pointers are very easy to move), I came up with a design which rearranges the gears in a non-working quartz clock module with two bipolar stepper motors arranged so one drives the 'minute' hand and the other drives the 'hour' hand.

The conversion uses very little of the original quartz module: only the case, centre gear wheel and the gear wheel immediately meshed with it. The rest can be used for other stuff or disposed of.

The retained gear wheels are extended up through the back of the module case using ABS spacers and connected to the stepper motors via a slot coupling. The stepper motors are fixed to the back of the quartz module using M3 screws and spacers allowing the stepper motors to be removed easily. If you want to avoid tapping M3 holes, you can use nuts or, if you insist, the motors can always be hot glued in place.

Some judgement is necessary to place the motors in a way they both fit on the back of the module, so check twice before drilling any holes!

This design has several desirable features: it's cheap; it reuses something which otherwise would have been thrown away and of course it fits into quartz clock frames, making mounting a doddle.

There are quite a few steps below but I've definitely erred on the verbose side; Overall, I think it took me about two hours to put together including thinking time, so not a huge project. If you were to make a few, the time to construct should get much shorter.

The display shown is my most recent design which uses a clock bought from a local charity shop. It displays how much electrical power I've used today on the 'hour' hand and the 'minute' hand displays the current power I'm using. It uses the coaxial stepper motor design described in this Instructable. I intend to publish construction and software details for the power meter (or something similar) in a later Instructable.

Incidentally, the power data for the display comes from an excellent Raspberry Pi-based home power monitor project, which I recommend highly.

Supplies

Please note this is a list of parts I used to convert the module I used. As there are many slightly different quartz modules out there, please use the list as a guide rather than treating it as definitive - you may need longer screws, different skewers etc.

Parts:

A quartz clock module - any condition, not working is fine! Mine is a Tiancheng TC-666* type.

ABS plastic spacers 7mm OD; 3mm ID; 8mm length eg https://www.ebay.co.uk/itm/194524311350

3mm OD diameter wooden Kebab skewers (or similar)

5.5mm (ish) M1.6 (ish) self-tapping screw (for the hour hand gear wheel pivot post) Chosen to allow the gear wheel to turn freely without much side to side movement - see Step 3.

Small washers (see text)

2x 28BYJ-48 stepper motors eg https://amzn.eu/d/4gf6Ur2

4x 15mm length M3 pan head machine screws (if screw-mounting the motors)

Hot-melt glue

Tools:

Needle file

Junior hacksaw

3mm tap (for mounting stepper motors - see text)

2.5mm drill bit (for the stepper motor mounts tapped holes)

5mm drill bit (for drilling out 'hour' hand spacer - see text)

1mm (see text) drill bit (for the 'hour' hand gear pivot post)

8mm drill bit (to allow the spacers through the module back)

* You may want to rinse the module in holy water if you have this model ;-)

Step 1: Preparation

As there are many different variants of quartz clock module, you will probably have to improvise a little rather than follow this Instructable to the letter.

Selection of the module to use will hopefully be a case of using what you already have. If not, modules are cheap and easily available on eBay and from other sellers. I have a number of non-working modules, most non-working due to corrosion from leaky alkaline cells and it was one of these I used for this project. Other selection considerations are how the module is to be mounted in the display (some modules are clipped in place, others are fixed using a central threaded mount); the pointers/hands you'll be using (spindle sizes vary) and the space available on the back of the module for mounting the motors.

Hopefully reading through this Instructable will clarify.

If you're using an old quartz module, you may need to clean up leakage from the battery - if the battery was an alkaline type, this is easily done with dilute vinegar, washed off with tap water. The leaked chemical is potassium carbonate and not toxic; that said, be careful and wear protective gear. If the battery was a zinc chloride type (unlikely these days), I've read WD40 works but I've no personal experience of this. Again, the leaked chemicals are considered non-toxic (but don't go licking the stuff).

Step 2: Open and Strip the Quartz Module

1. Open up the quartz clock module to expose the gears and electronics. All modules of my acquaintance are held together using clips around the sides and sometimes one adjacent to the battery case. Be careful when prising these open as they snap easily.
2. With the back off, remove the electronics module and all the gear wheels apart from the centre one and the one immediately meshed with it (which drives the hour hand centre gear wheel).
3. Remove the hands setting wheel from the back of the module.
4. Pull out the minute hand gear wheel and remove (if one is fitted) the upper gear wheel. Refit the minute hand gear wheel.
5. Remove the gear wheel which meshes with the hour hand centre gear wheel and flip it so the large diameter part of the gear meshes with the hour hand centre gear wheel.

Step 3: Position the Hour Hand Gear Wheel

1. Position the gear wheel to one side away from the battery compartment. It needs to be positioned to allow space for two stepper motors to be fitted to the back of the module (read through to step 5 to see what I mean).
2. Once you're happy the gear wheel is in the right place, use a pin drill and 1mm bit to make a small hole via the centre of this gear through the front of the case.
3. On my module the hole through the gear wheel is 1.8mm diameter. I therefore selected a 5.5mm long M1.6 screw (obtained from the strip down of an old camcorder) to act as the gear wheel pivot post.
4. Carefully screw the small screw through this hole so it's sticking up perpendicularly (important!) into the case.
5. Put the hour hand gear wheel in place over this screw and check the teeth align vertically with the those of the minute hand gear wheel. If not, use a thin washer to lift the hour hand gear wheel so they do.

Step 4: Add the Stepper Motor Couplers

1. Use an appropriately sized drill bit to ream out one of the spacers so it fits snugly over the top of the hour hand gear wheel. In my case, I needed to widen the spacer inside diameter to 5mm. Only drill part way into the spacer so around half of the centre hole length is left unaltered. This of course is easier said than done as the bits tend to snag on the plastic. I found the best way is to fix the spacer into the drill chuck and fix the drill bits into a vice (like on a lathe). It's then a matter of carefully offering up the spacer to the drill bit with the drill speed set very slow. Start with a 4mm bit, then increase to 4.5mm and finally 5mm.
2. This spacer isn't long enough to poke through the back of the module, so it needs extending (unless you've been cunning and fitted a longer spacer(!). Shorten one of your 8mm long spacers to around 4mm and file a slot in one end; the slot should be about 2mm or 2.5mm wide - it will form one half of the dog clutch (see step 5) and should be sized to match the mating half you'll file on the stepper motor shaft.
3. Glue the first spacer in place on the gear wheel with the hot melt glue, being careful not to get any glue on the pivot post.
4. Insert one of the 3mm diameter kebab skewers into top of the spacer and slide the shortened spacer down until it pushes up against the first spacer.
5. Cut the skewer so it stops at the bottom of the spacer slot.
6. Disassemble and glue the parts together with the hot melt glue.
7. On my module, the top of the minute hand gear wheel has a 5mm diameter extension. Yours may have a different measurement. As done above, ream out one half of an 8mm spacer to match this extension. At the other end of the spacer file a similar slot to that above.
8. Glue a short length of kebab skewer into the centre of the minute hand gear wheel (you may have to sand the diameter down a bit to get it to fit) and glue the spacer in place, again ensuring the end of the skewer doesn't emerge into the slot.
9. Fix a lump of plasticine or similar onto the inside of the back plate and carefully offer it up to the module's front plate. It won't clip together as the spacers now get in the way but try to ensure the front and back plates are aligned as they would be if clipped together - the plastic clips should partly engage and help with this.
10. Carefully remove the back plate and hopefully the plasticine will have a clean impression of where the clearance holes need to be on the plate. Repeat if necessary.
11. Using a pin drill or similar, drill a small pilot hole in the centre of each of these two impressions.
12. Remove the plasticine and enlarge the holes to 8mm or so, using increasingly bigger drill bits to reduce the chances of snagging. The size of each hole should be only as large as it needs to be to allow the motor shaft to turn freely - too large and you could end up with too much wobble on the coupled gear wheel.
13. Fit the back plate and confirm the spacers fit through the plate and are free to turn without too much sideways slop.

Step 5: Fitting the Stepper Motors

1. File the shaft of each stepper motor to engage with the matching slot in the spacers (creating a 'dog clutch'). The shafts appear to be made of brass or similar, so are relatively easy to file. Try to ensure the fit is reasonably snug to avoid mechanical slop when operating the motor in opposite directions.
2. Engage each motor's shaft in the corresponding spacer, arranging the motors in a similar manner as shown in the photograph. The mounting flanges end up pretty close to the edges of the module but with care, you should be able to position both motors so each one is perpendicular to the module and not touching the other.
3. Mark the module where the holes for the mounting flanges are and drill and tap accordingly; If using 3mm screws, your drill size should be 2.5mm. You could use self-tapping screws of course, but a threaded hole is longer-lasting. If you elect to use screws and nuts instead, push each screw up though the back of the module and glue in place. You can of course just glue each motor in place. I wouldn't do this....
4. Mount the motors as shown. You'll probably need the odd washer as well as the spacers to ensure the motor is at the correct height to engage properly with the slots.
5. And that's it. All you need to do now is connect the stepper motors to your drive circuitry and give them a whizz. Note: to drive the module spindles the same way, the each motor is driven in the opposite direction to the other due to the presence of the extra gear wheel on the 'hour' hand.