The Electro-Mechanical Clock

Electronics & Mechanical are the two engineering disciplines that I love the most. So I wanted to create a showpiece that portrays the awesomeness of these disciplines. Also I believe that time is the most important asset anybody can have.

So I wanted make something that portrays the importance of time and also the essence of electronics and mechanical and there came the Electro-Mechanical Clock.

The display shows the minutes and the white arrow on the gear teeth points to the hour. Therefore its Electro-mechanical. In the picture the time is 8:10.

This clock contains almost everything – microcontroller, transistor, IC, LED displays, resistors, capacitors, inductors and circuitry done two ways, both as a PCB and also the traditional wired way.

To go with that, there is also a hand-crafted wooden gear assembly and a roller bearing (two gems from the mechanical world) and not to forget the electrical – a transformer and a DC motor.

NOTE: I have included few links to my other stuff like making PCB's etc. that will be of when doing this project.

The beating heart of the clock is Atmel's ATTiny 85 microcontroller. The ATTiny also serves as the clock for the IC 7493 (this IC is set to count 0-5). 4 of the ATTiny's pins form the 0-9 count and therefore combined we get the minutes.

Everytime the microcontroller counts to 0 after 0-9, the 7493 gets a negative which which increases its count by 1. Then it counts from 0-5 and after an hour it goes 00 after 59. The 4 bit data from the microcontroller and the IC7493 drive two 7447 IC's. We have two LED displays that combine to show the minutes from 00-59.

Every time the minutes goes to the next clock after 59, the motor is driven which through the gears makes the hour pointer move to the next number. The transformer and the PCB form the power supply system for the clock.

I bought a 60 rpm motor. Since its slow there wont be much inertial movement. I decided to run the motor for about 500ms. So, the motor will go half a rotation or 180 deg.

Now for each half rotation of the motor that happens at 1 hour intervals the hand has to move 30 deg (360/12). So our gear ratio is 180/30 or 6:1. Remember, these are my calculations. You can always do your own and it will still work.

A gear looks like a gear only if there are a minimum of 6 teeth. So if the smaller gear has 6 teeth, then by our ratio the bigger gear should have 36 teeth.

I used a software called eMachineShop to generate the gears. In that software you will have to input various parameters of the gear. It is better to select the Pressure angle of 25 deg and a pitch diameter of 2.5 cm.

But no matter what value you select for these two - remember that both the smaller and the bigger gear should have the same values. You can only change the number of teeth.

Print the gears on a sheet of paper and cut them close to their perimeters.

Reproduce the design as exactly as possible on to the plywood your willing to use. I used a half inch thick plywood.

Important: Gears can be made with hand tools and not necessarily lasers or CNC's.

Drill a hole at the point where the two teeth merge. Do this by first making a puncture hole with a nail at the point where you want a hole and then drill.

This should be repeated all along the gear. I used a 4mm bit to do the drilling. These holes will make it easier to cut out the teeth.

Using a saw, cut the rough shape of the gear out from the board. This will enable us to easily handle the intricate cutting of teeth.

Then cut from the outside towards the hole along the 'teeth marking line' we traced. This will produce a perfect gear shape and repeat this throughout both for the bigger and the smaller gear.

This clock features a lot of stuff and we need a platform to hold all that in place. So I used a 21 by 24 cm board of a quarter inch thick plywood for this. I prefer this thickness because then I can easily mount the motor with the nut that comes with it.

Do the marking and cut the board out.

I always prefer the top brown part of my plywood removed. So I heavily sanded the surface of the gears but I left the board as it is.

I painted the board brown but varnished the gears. The trick here is to use the same brush to paint and varinish. I painted the board brown first and then varnished the bigger gear.

So a part of the brown adds on to the bigger gear. Then I varnish the smaller gear which will get even lesser a brown tinge.

Two coats of the same method later, we have a board and two gears that look distinct and visible due to the color contrasts we gave when finishing them.

At the end of the day aesthetics and colour do matter.

I have uploaded the Proteus simulation file of the circuit as well as the screenshots. This will be the circuit diagram for reference.

As explained earlier, we use the microcontroller to count the minutes. Everytime the 0-9 count 4 bit data goes from 9 - 0 i.e. 1001 to 0000. The most significant bit or the left most bit creates a negative edge.

This triggers the 7493 to count up. The 7493 is set to count from 0-5. The set of two 4 bit data individually are given to 2 IC 7447 that control two 7 segment displays.

The microcontroller also gives a finite duration pulse of 500ms to the motor after every hour.

I know the connections in the Proteus file are a little messed up but try to look keenly into where the connection goes.

For more information on counters and how to use 7447 to drive 7 segment displays: https://www.instructables.com/id/Digital-Clock-Sequ...

Note: The simulation is in intervals of seconds, in the microcontroller programming it has to be extended to minutes.

Our main circuit will be directly above a bearing. Since this is all about showcasing stuff, the bearing cannot be hidden under a PCB or a breadboard.

So I used a plastic box like the one in the picture. I separated the lid from the box and I cut out any remaining protruding portions of plastic.

Since we want to be able to wire our circuit here, we will use an interesting arrangement. We will use the DIP holder to place our components. To go with that we will use header pins of the corresponding number in a pin to pin arrangement.

1) A 4 pin holder and two rows of 4 pin headers together for mounting the ATTiny 85.

2) Two 8 pin holders and two rows of 8 pin headers each for the two IC 7447.

3) A 7 pin holder with two rows of 7 pin headers together for the IC 7493.

4) Two 7 pin holder with two rows of 5 pin headers each for the two LED 7 segment displays.

Solder the corresponding pins together and now we get our familiar breadboard like arrangement wherein the pins of the IC or the display can be accessed by inserting wires into the corresponding header pin.

Try positioning the components first to see the most salient way to place them. Mark those locations on the plastic plate.

Cut a 4 pin holder into two pieces and push those two 4 pin holders through some holes in the plastic. Solder the pins underneath to form a rail. The one on the left will be our Vcc rail and the one on the right will be our Ground rail of the circuit.

Superglue our components assemblies into place and seat them using silicone.

I used a BC547 transistor to act as a switch to turn the motor ON/OFF depending on the micro-controller input to the base junction. Make 3 separate holes at the bottom for that.

Also make a hole about 8mm wide in the center for later use.

I used a 9-0-9 transformer. So I used a simple rectifier circuit to go with the transformer.

The circuit is a rectifier followed by a L-C filter which becomes the input for the IC 7809 and 7805 IC. Though these IC can tolerate the heat that will be generated, it is better to use a heat sink especially for the 7805. Attaching the heat sink is basic stuff that I don't think I have to elaborate or instruct.

The three input wires on the left (Red-Black-Red) are the 9-0-9 AC input from the transformer.

The three output wires on the right (Blue-Black-Green) are the 5-0-9 DC supply. 5V is for the digital circuitry and the 9V for the motor with a common ground.

I have uploaded the EagleCAD file of the schematic and the board.The pictures also explain the various parts in the PCB. To see how I make my PCB's: http://hpposts.wordpress.com/2014/02/03/making-pcb...

After the varnish on the gears has dried out, find the center of the gear and puncture a hole there with a nail.

I used a small spare bearing of radius 3 cm and therefore cut out a 1 1/4 inch thick hole. I used some strips of newspaper padding to tightly fit the bearing into the center of the gear.

It doesn't matter how you do it but fix the bearing to the gear's center. Since I used some newspaper padding, the superglue was extra effective and the bearing got strongly fixed to the gear.

Find the diameter of the motor's shaft that you are using. I found mine to be ~4 mm. So I drilled a 4mm hole and pressed the motor shaft through it to see if it fits.

Find the perfect fit and then separate the gear from the motor. Don't superglue it yet.

Now we need something to keep the gear in place. The smaller gear gets its shaft from the motor. But the larger gear still needs one too. So I used the bottom part of a fountain pen. I will call this the stem.

Use one that has a nice bulge in one end and tapers towards the other, like the one in the picture.

The larger diameter of the pen was 1 cm, so I drilled a 1 cm hole in the center of the board. Force the pen through the hole and use some padding if necessary.

Now place the larger gear's bearing in the tapered end of the stem and push it to however far it can get. It looks aesthetically better if you place the larger gear about 1 cm above the board.

Now we need to drill the holes to hold the transformer in place. Also holes for the wires to be pushed through. We will do some wiring underneath board to maintain a level of neatness.

So position your transformer and depending on the length of the wire, find the position of the two holes through which the wire should be run.

On my board the two horizontal holes are for holding the transformer. The two holes above and below are for running the wires through.

Then we need to the same for the PCB. It has three input wires and three output wires. So position the PCB and drill holes of the required width to run the wires.

I drilled two 4mm holes fairly close to each other and chiselled out the little wood in between to get that narrow hole.

Align the smaller gear such that it fits well with the teeth of the larger gear. Then mark the center point of the smaller gear onto the board.

Now my motor has a fastening nut with it around the shaft. Those grooves were about 1.3 cm in diameter. So I cut the required hole, pushed the motor through the hole and fastened it with the nut.

Then push the smaller gear through the motor shaft down to a height where it maintains perfect contact with the larger gear. Only then should the smaller gear be super glued to the motor's shaft.

I didn't do the checking at this step but it is a good idea to check the gear's working correctly now using a battery. I used the on-board power supply that we are gonna assemble next.

Bolt the transformer through the holes to keep it in place. Run the wires through the top and bottom holes and pull them down neatly and gently.

Add silicone glue to the underside of the PCB and fix it in place. Similarly run the wires through the holes meant for the PCB. Seal the holes with silicone on the underside of the board.

So till now, we have made the PCB and placed it along with the transformer. We have run the wires down but we still haven't done the wiring yet.

The larger gear with the bearing is mounted to the central stem. Make sure that fixation is strong with some superglue. Also make sure the larger gear is strictly parallel to the backbone board.

The smaller gear should be in its proper place. And make sure the height of alignment is spot-on between the larger and smaller gear. Then superglue the smaller gear in place.

See to that, the smaller and larger gear interact properly. Only then should the motor be superglued to the back of the board.

Remember it is easier to adjust the smaller gear into position, so tightly fix the larger gear onto the stem and then accordingly position the smaller gear.

The two 9V wires (white in my case) should be soldered to the two red wires of the PCB. The ground black in both cases should be soldered too. I don't prefer soldered wires to be left bare so, I tape them to the board and place them firmly.

Now check for the polarity of the motor. Remember, the larger gear should rotate in a clockwise direction which means the smaller gear should rotate in an anti-clockwise direction.

So I soldered the positive terminal of the motor (positive with respect to my required rotation) to the 9V DC wire from the PCB (green wire) and then taped it. The negative terminal is extended to be connected on our plastic board later.

The blue and the black wires from the PCB are also extended. The extension requires a uni-core wire being soldered to another uni-core wire which in most cases usually breaks off. So I used a multi-core wire as a link to do the extension.

Remember that the taping is to limit the movement of the wires which in turn means the chances of the solder breaking off is less.

Three of our extended wires

1) negative terminal of the motor

2) +5V DC from our PCB

3) Ground wire

are now pushed through the stem and pulled out from the other side.

I soldered the motor extension wires and folded it with the stem (as seen in the picture). The other wires were simply pushed through. Since the wires are being pushed from the broader end to the tapered end, it is better to strip the insulation and push them one by one.

The metal requires lesser space and therefore appears on the other side, where you can slowly pull it out.

Solder the +5V extension wire from the PCB to the positive rail (4 pin holder on the left). The ground wire is soldered to the ground rail (4 pin holder on the right).

Push the transistor BC547 through the 3 holes we made earlier on the board. The transistor was placed such that the base was in the center with the collector on the left and the emitter on the right. Solder the emitter junction to the ground rail on the right hand side.

Now make two more holes above the base junction hole for the resistor.

Now solder the resistor to the base junction. Make one more hole above the resistor and push an uni-core wire though it. Solder the other terminal of the resistor to that wire.

Now the free end of the wire can be connected to the appropriate motor control port on our microcontroller. The transistor acts as a switch that turns ON/OFF the motor controlled through the base junction.

Make a hole to the left of the collector. Push the motor's negative terminal extension wires through the hole and solder the wire to the collector.

Now the board is there to be connected like how we usually do with a breadboard. Refer to the circuit in the simulation that we saw in Step 8.

But for additional information I have given the connections as I have done in the picture. Remember that the connections can change if you change the ports programmed in the micro-controller or if you use other components.

But other than that, the connections are done like those shown in the pictures. Remember to use long wires (this is not where you should be stingy). Carefully, push aside the wires after the connections so that the display can be seen.

We have now completed our basic clock!!!

I cut out small circles of numbers and pasted it at every 3 teeth interval. I also had a small message for myself because I didnt want the bottom right corner to be vacant.

I added a small arrow on one of the teeth to point the hours.

Then the moment of truth, connect the transformer wires to the power cord and watch the showpiece come to life!

This is dedicated to all those electronics or mechanical enthusiasts who just love the sight of IC's or gears in action.