Introduction: A Wood Gear Clock With a Unique Drive Mechanism
Synchronicity is a unique exposed wood gear pendulum wall clock, with a combination of old and new tech.
Many wood gear clocks are driven by weights. These clocks need to be wound periodically, as often as every day. I wanted to build a wood gear clock that did not require that frequent winding.
Some designers use an electromagnetic pendulum to drive their clocks. The pendulum contains a rare earth magnet which passes by a coil or two. As the magnet passes the coil, it induces a current in the coil. This triggers electronics to put a pulse of current into the coil (or a second coil) and repel the magnet, giving the pendulum a push and powering the clock. I chose this method to power my clock.
Often times wood clocks of this type are not very accurate. Wood expands and contracts with changes in temperature and humidity. I wanted my clock to be accurate, so I use a microcontroller in my electromagnetic pendulum drive so that the clock keeps perfect time. My circuit is powered by 4 D cells, which will operate the clock for up to three months.
Here is a video on the design and construction of the clock: Synchronicity
Files are available to machine the wood parts of this clock on a Carvewright CNC. Visit the Carvewright Pattern Depot. (Sorry; Carvewright uses a proprietary file format; no files are available for other CNC machines). A hardware kit of all the wood parts, including the electronics, is also available at carveshop.com.
- 1/4" Baltic birch plywood
- 1/2" hardwood
- 3/4" hardwood
- hardware (see list in image)
- brass tubing (see list in image)
Step 1: Theory
In most pendulum clocks, the swing angle of the pendulum is very small, and so the actual swing angle has no bearing on the period of the pendulum. However, the period of a pendulum does increase gradually with increased swing angle. If the swing angle is relatively large, and is varied between about 15 to 25 degrees, the pendulum period can be altered by over a half percent.
The Synchronicity software and electronics vary the swing angle of the clock's pendulum to slightly speed it up or slow it down as needed to keep perfect time.
Step 2: Electronics
The heart of the electronics for this clock is a Texas Instruments MSP430 microcontroller. It is a low-power device, - in fact, most of the time it is in "sleep" mode, drawing virtually no power. A 32.768 kHz watch crystal provides a timebase for the microcontroller.
The electronics are powered by four D cells supplying 6 volts. A 3.3 volt voltage regulator supplies the microcontroller. The regulator has a very low quiescent current.
A single coil is used to detect the magnet passing by and to repel it. The coil is wound on a bobbin made up of two 1.25" nylon washers with a 1/2" long x 1/4" diameter nylon post glued to the washers. The nylon parts came from my local Ace hardware. It is wound with about 100 feet of 32 gauge magnet wire, yielding a resistance of about 50 ohms.
When the magnet passes the coil, it induces a negative then a positive current and voltage. A simple filter limits the voltage of the signal from the coil when the magnet swings by. This voltage is fed into a comparator on the microcontroller. When the voltage exceeds a threshold, it "wakes up" the microcontroller. An output port drives a transistor pair that injects current into the coil. The current pulse is typically 25 mS long.
A bicolor LED provides a visual signal to help set up the clock. It is connected to two output ports of the microcontroller. The LED lights red with one port high and one low; green with the port levels reversed.
Step 3: Software
The software was developed using an inexpensive MSP 430 LaunchPad development kit from Texas Instruments. The LaunchPad connects to a host Windows computer via USB. A few components such as the transistor drivers were add via a breadboard during circuit and software development.
There are other Instructables showing how to use the MSP 430 LaunchPad with TI's supplied Code Composer Studio to develop code, so I will not duplicate that here. I did use the graphical interface Grace to set up the microcontroller. The attached file main.cfg contains the Grace setup, and main.c is the C source code. Code listings are also provided in pdf form if you just want to look at the code without installing Code Composer Studio.
The software is interrupt driven. Once the microcontroller is set up, it is put into "sleep" mode. It is awakened when one of the interrupt conditions are met.
A timer driven by the watch crystal overflows and generates an interrupt every two seconds. This is used as the basic timekeeping for the clock.
The comparator generates an interrupt when the positive voltage pulse from the coil is generated. An interrupt timer routine set up in state machine fashion
- delays an amount of time for the magnet to move slightly past the coil for optimal push
- turns the coil on
- turns the coil off and allow it to quiesce so that a second false trigger is not detected.
For each state, a interrupt is generated, the action taken, and the microcontroller put back to sleep to await the next step.
The software measures each and every swing of the pendulum, and compares the actual swing time to the desired 1 second, generating an error number. A modified PID (proportional - integral - differential) control algorithm uses the error signal to adjust the pulse duration to the coil. This modifies the swing angle of the pendulum to speed up or slow down the pendulum and therefore the clock. The actual C code is
pulse = NominalPulse - (Kp*error + Ki*i_error);
Where Kp and Ki are empirically-derived constants and error and i_error are the proportional and integrated error respectively (no differential term is needed or used).
A National Instruments data logger was used to help derive the constant values and tune the system. A software routine set up a pulse width modulated (PWM) output port connected to a simple low pass filter and the datalogger. The routine generated a voltage proportional to the pulse width to the coil or the error signal of the PID algorithm. Shown is a log of the error signal closing and locking on on zero.
The software lights a bicolor LED red or green to indicate if the pendulum is moving too fast or too slow. This allows the bob to be adjusted up or down in just a few minutes to set up the pendulum. Once it is set close enough, the microcontroller regulates the pendulum to keep accurate time.
Circuit boards were designed and built after the hardware and software were finished.
Step 4: Ratchet and Pawls Design
Weight-driven wood clocks with a regulating pendulum use an escapement. Because our pendulum not only regulates, but drives, the clock, we use a ratchet and pawl arrangement. A moving pawl moves back and forth with motion by the pendulum, and a locking pawl allows the ratchet wheel to turn in only one direction. Our pendulum has a one second period, so for our ratchet wheel to drive a second hand, we put 60 teeth on our ratchet wheel.
I used a Radial Vector Generator program to generate vectors for the ratchet wheel and gears. It's available for free on the Carvewright user forum. It's pretty easy to use. You just enter a few parameters and the vectors are generated to your specifications. The illustration is an example of a ratchet wheel.
I used the Carvewright Designer software to design all of the parts for my clock. The vectors from the Radial Vector Generator were imported into Designer, and then center holes and spokes and the like added with Designer. Parts such as the ratchet wheel were cut out of 1/4" Baltic birch plywood using a 1/8" cutting bit on my Carvewright CNC machine. Baltic birch is made up of many thin layers of wood with no voids, and is very stable.
Step 5: Pendulum Design
For the ratchet to operate properly, the moving pawl's movement must be controlled. If it moves too little, it won't advance the ratchet wheel. If it moves too much, it will advance the wheel by more than one tooth at a time. To control my moving pawl, I designed a cam for my pendulum.
The cam is at the pendulum pivot. It uses a larger and a smaller radius, with a smooth transition between the two. A weighted lever with a wheel rests against the cam. As the pendulum swings, the cam moves the lever, but only so far. Even when the pendulum swings a great deal, the cam pushes the lever only so far.
At about the middle of the pendulum, a recess is placed that will hold and hide a rare earth magnet.
The lower part of the pendulum uses a brass threaded rod. A wood bob slides over the rod and is secured by a pair of brass knurled nuts. The bob can be moved up and down to adjust the effective length of the pendulum, and therefore its period.
Step 6: Gears Design
The second hand is driven directly by the ratchet wheel. To drive the minute hand, we need a set of gears that divides movement by 60. We need an even number of gear pairs so that the second and minute hands turn in the same direction. I used four pairs of gears, each with a smaller pinion and larger wheel. I used a simple spreadsheet to enter the number of teeth of each wheel and pinion pair and compute the gear ratio to get the desired result. These numbers were then input into the Radial Vector Generator to produce vectors for the wheels and pinions.
(One of the program's inputs is the "module", which is the arbor distance * 2 / (no. of wheel teeth + no. of pinion teeth). I let the spreadsheet compute the module necessary to space the gears on shafts exactly 3" apart.)
The hour hand requires a gear set that divides by twelve. Only two sets of wheels and pinions are needed.
Step 7: Clutch Design
To allow the time to be set, a clutch is used on one of the gear sets. This allows the minute hand to be moved manually without disturbing the rest of the mechanism. I used a ratchet on the face of one of the pinions, and the same mating surface on the wheel. The pinion is pressed towards the wheel with a light spring. This allows the wheel to maintain contact with the pinion to drive the minute hand, but the pinion can slide over the wheel when the minute hand is rotated clockwise.
Step 8: Frame Design
The frame of course holds it all together. The pinions and wheels were designed to fit onto shafts 3" apart, so the frame has a central shaft with three others 3" from the center. The bottom two shafts are also 3" apart. There is a front and rear frame, the front frame integrated with the clock face. Lots of open space ensures that the gears remain visible.
The gears are layered on the shafts so that the respective wheels and pinions mate while not interfering with other gear sets.
Step 9: Base Design
The base or back of the clock houses the batteries, coil, and electronics. The base is made of two peices that get screwed together. The coil is inserted from the back in a recess such that the coil is only a fraction of an inch from the front surface of the base.
Step 10: Machine the Parts
The wheels are machined from 1/4" Baltic birch. Holes must be precisely drilled so that brass tubing may be press fit and used as arbors. Most of the wheels use 1/4" diameter brass tubing for arbors that will ride over 7/32" brass tubing shafts.
The pinions and other parts are machined from 1/2" hardwood such as oak.
The frames, face, base, pendulum, and bob are machined from 1" nominal (3/4" actual) hardwoods. Some of the moving parts, such as the pendulum pivot, ride on ball bearings. 3/8" recesses are cut into these parts to hold the bearings.
Step 11: Drill the Frame and Other Parts
Three holes at the top of the frame are drilled through as illustrated. Two holes are drilled with a 3/8" bit 1/2" deep.
From the front of the frame, drill 1/16" holes through the frame with a drill press at the red circled locations.
From the back of the frame, using the 1/16” pilot holes that you just drilled through from the front (circled), drill recesses for the three shaft retaining rod nuts using a ½” Forstner bit and drilling only ¼” deep.
From the front, at the same positions, drill 7/32” holes ¼” deep. Finally, drill these holes through with an 11/64” bit.
Several parts hold bearings, and these parts need to have recesses for the bearings drilled.
Start with 1/16" pilot holes for these parts. In the case of the pendulum and lever, which will have two bearings each, one on each side front and back, drill these pilot holes through to the other side the part.
Using a 3/8" Forstner bit, drill recess holes in each part 1/8" deep. Drill both sides of the pendulum and lever. Drill the front (flat) side of the minute hand, hub, and frame.
Next, drill a ¼” diameter hole through the center of each these recesses. Drill about ½" deep on the frame; drill through to the other side of the other parts.
Step 12: Assemble Gear Set One
The shaft for Gear Set 1 is a ¼" diameter tube that is 1-3/16" long. Insert the shaft into the 12-tooth pinion that does not have the clutch face carved into it and secure with superglue. Set the shaft flush with the face of the pinion. (I put a couple of drops of superglue inside the pinion hole - the wood will absorb some - and a drop around the end of the shaft. You must assemble quickly and accurately. I insert the shaft by hand a bit, set the pinion down on the bench with the shaft sticking up, use a wood block on top of the shaft and drive the shaft home with a hammer.)
Slide one of the 5-arm 30-tooth wheels on, and secure to the shaft and pinion with superglue. (I put a couple drops of superglue on the face of the pinion and a drop around the shaft, assemble, then clamp the wheel to the pinion with two Irwin Quick-grip clamps.)
Step 13: Assemble Gear Set Two
The shaft for Gear Set 2 is a ¼" diameter tube that is 1½" long. Insert the shaft into the one-way round clutch as shown, so that the end of the shaft protrudes from the wheel exactly 7/8". Secure with superglue.
Next, slide the 32T12 four-armed wheel onto the shaft. (Note that the 32T12 four-armed wheel is smaller in diameter than the 32T8 wheel - use the correct one!) Secure it to the shaft and hub with superglue.
The 12-tooth pinion with the one-way clutch face is the mate to this gear set. The shaft for this pinion is a ¼" diameter tube that is 5/8" long. Insert it into the pinion so that the end of the shaft is flush with the clutch face.
Step 14: Assemble Gear Set Three
The shaft for Gear Set 3 is a ¼" diameter tube that is 2" long. Insert the shaft into a 10-tooth pinion so that 1/16" of the shaft protrudes from the pinion. Secure with superglue. next, slide the 3-arm 30-tooth wheel onto the shaft, and secure it to the shaft and pinion with superglue.
Step 15: Assemble Gear Set Four
The shaft for Gear Set 4 is a ¼" diameter tube that is 3-1/8" long. Insert the shaft into the 8-tooth pinion so that the shaft protrudes ¼" from the face of pinion. Secure with superglue. Next, slide a 5-arm 30-tooth wheel onto the shaft and secure it to the shaft and pinion with superglue.
Step 16: Assemble the Seconds Shaft and Gears
The second hand shaft is a 3/16" diameter tube that is 5½" long. It is supported by three bearings. Start by test fitting bearings on both ends of the shaft. If the bearings don't slide on, reduce the diameter of the shaft slightly. I use fine sandpaper or emery cloth with the shaft inserted into my drill press. You don't need to worry about the center of the shaft, just the ends.
Slide a bearing onto one end of the shaft so that it is at least 1¼” from the end of the shaft. Slide a 7/16" diameter, 1/8" long brass spacer on next. Slide the 10-tooth pinion with the 3/16" hole on next, so that exactly ½” of the shaft protrudes from the pinion. Secure with superglue.
Finally, slide the ratchet wheel on, paying attention to the direction of the teeth. Secure to the shaft and the pinion with superglue.
Step 17: Assemble the Minutes Shaft and Gears
The minute hand shaft is a ¼" diameter tube that is 3¾" long. Start the assembly by attaching the 7/16" thick by 1" diameter hub to one end of the shaft. The bearing recess faces outward from the shaft, and the shaft itself is inserted up to the bottom of the bearing recess. Secure these parts with superglue.
Next, take one of the 5-arm 30-tooth wheels and slide it onto the shaft all the way down to the hub. Secure the wheel to the hub and to the shaft with superglue.
Next, assemble and fasten two ¼" thick by 1" diameter plywood hubs.
Finally, assemble and fasten a 10-tooth pinion as shown.
Step 18: Assemble the Hours Shaft and Wheel
The hour hand shaft is a 9/32" diameter tube that is 1-7/8" long. Attach the 4-arm 32-tooth wheel to one end of the shaft. Set the shaft so that the end protrudes 1/8” from the back face of the wheel.
Next slide a ¼" thick by 1" diameter plywood hub onto the shaft. Secure these parts to each other and the shaft with superglue as you assemble them.
Step 19: Assemble the Frame
Insert a bearing into the center of the frame.
Insert the three 7/32" diameter, 3½" long gear set shafts as shown. When fully seated, 3¼" should be exposed.
At the top right of the frame, insert a 3/16" diameter, 1½" long lever shaft. Before installation, check that a ball bearing will slide all the way from one end to within ½" of the other end. Remove some material from the shaft if necessary. I put the shaft in my drill press and use emery cloth or fine sandpaper.
Step 20: Assemble the Moving Pawl
Insert a 7/23" diameter/ ½" long brass tube pawl bearing onto the moving pawl, with equal lengths protruding from either side.
Insert ball bearings front and back into the lever.
On the back of the lever, insert a 3/16" diameter, 2" long lever-to-pendulum bearing shaft. Before installation, check to see that a ball bearing will slide at least 5/8" onto the end of the shaft, and remove some material from the shaft if necessary. Secure the shaft to the lever with superglue if necessary.
On the front of the lever, insert a 3/16" diameter, 1-15/16" moving pawl shaft. Secure with superglue if necessary. Slide the moving pawl onto the shaft as shown. Secure with a cap and superglue. Make sure that the pawl moves freely on the shaft. You may need to file the inside of the brass tubing with a small needle file to deburr the inside edge.
Step 21: Assemble the Fixed Pawl
Fasten the fixed pawl tip to the pawl arm using a #6 3/4" brass round head wood screw. Insert a 7/32" diameter, 1" long fixed pawl bushing, leaving 1/16" protruding from the front of the arm.
Step 22: Assemble the Pawls to the Frame
Slide a 1/16" spacer (a 7/32" diameter brass tube 1/16" long) onto the lever shaft at the top of the frame. Slide the lever (bearings) onto the shaft. Slide a 1/8" spacer on next. Secure a cap to the end of the shaft with superglue. Make sure the lever moves freely.
Slide the fixed pawl assembly onto its shaft as shown. Make sure that it moves freely. Secure a cap to the end of the shaft with superglue.
Step 23: Assemble the Lever to Pendulum Bearing
Slide a 7/32" diameter, 1-1/8" long brass tube bearing spacer onto the lever-pendulum bearing shaft. Slide a ball bearing on next. Finally, slide a small piece of vinyl tube on to secure the bearing.
Step 24: Install the Second Hand Assembly
Slide a 1/8" long spacer onto the shaft as shown.
Insert the assembly into the bearing in the frame.
Step 25: Install the Gear Sets
Install gear set one, then two.
Install the minute hand assembly, pressing the arbor onto the bearing tat is on the second hand shaft.
Install gear set three. Install the 12-tooth pinion with clutch, clutch face down. Slide a spring onto the shaft.
Install gear set four.
Install the hour hand assembly, placing a nylon washer over the shaft.
Install the face. Secure the face to the frame using the three threaded rods, acorn nuts on front, and nuts in the back.
Place washers on both sides of the face - that is, between acorn nuts and face front, and between face back and gear shafts. Do not over tighten.
Step 26: Check Alignment
Check the alignment of the assembly, making sure that there is sufficient clearance between wheels and pinions. Make sure that the pawls are centered on the ratchet wheel. Operate the gears manually. Make adjustments if necessary.
Step 27: Install the Hands
Install the hour hand on its shaft, securing with superglue as needed. Point the hand to the 12 o'clock position when gluing.
Insert a ball bearing in the minute hand. Install the hand on its shaft, slipping the bearing over the second hand shaft. Again align in the 12 o'clock position while gluing.
Install the second hand.
Step 28: Assemble the Pendulum
Press ball bearings into the pendulum front and back. Install the magnet with superglue. Install and secure the brass threaded rod.
Screw a 10-32 brass knurled nut onto the threaded rod, followed by the bob and another knurled nut. Locate the bob halfway up the threaded rod. Cap the threaded rod with a 10-32 acorn nut.
Step 29: Install Frame Mounting Posts
Insert three 3/8" diameter brass tubing frame mounting posts into their respective holes. Ball bearings slide onto the center post, so reduce the diameter if necessary. Exactly 1" of the posts should protrude, and they must be all exactly the same height.
Step 30: Install the Pendulum
Slide a 1/8" spacer onto the center post.
Slide the pendulum bearings onto the center post. Follow with a 1/16" spacer.
Step 31: Assemble the Base to the Frame
Insert three #6 2½" machine screws into the back of the base. Place one brass washer each onto the screws.
Slide the screws into the posts. Secure with #6 brass acorn nuts on the face, placing washers under the acorn nuts. Tighten securely.
Step 32: Adjust the Pawl
Manually swing and hold the pendulum to the left. The driving pawl should rotate the ratchet wheel clockwise. Look at the locking pawl and ratchet wheel interface. The locking pawl should clear a tooth on the ratchet wheel and land about halfway onto the next tooth with the pendulum swung well to the left. Adjust the angle of the locking pawl on its arm until this is the case.
Next, swing the pendulum and hold to the right. The ratchet wheel should freeze movement, and the driving pawl should skip onto the next tooth. The driving pawl should land about halfway onto the tooth as shown. Adjust the angle of the locking pawl on its arm as needed.
Move the pendulum back and forth several times, and check both the driving and locking pawl for proper action. When satisfied, tighten the screw securing the locking pawl to the arm.
Step 33: Install the Electronics
Install the battery box with double-sided tape Place the coil in its recess. The side of the coil with the wire exiting near the center faces the front of the clock. Place the circuit board in its recess with the LED in its slot. (I secured by circuit board with hot melt glue.) Rout the wires in the channels, and secure with tape if need be. (The battery box wires run straight across the top of the coil, not to the side as in this earlier photo.) Place a square of foam on top of the coil to hold it against the face of the base. Install the back of the base and secure with four #6 wood screws.
Step 34: Setup and Adjust
Install batteries in your clock. When power is first applied, the LED will blink red and then green as a check. A pulse of current will then be delivered to the coil. If the magnet and coil are oriented in the correct polarity, the pendulum will be repulsed and may start swinging on its own. If not, the pendulum will be attracted to the coil and may just shudder. In that case, flip the coil over.
There is no on-off switch to the clock. As long as the pendulum is not swinging, the circuit draws only minimal current. The swinging pendulum triggers the circuit.
Adjust the bob to the center of its travel, and lock it in place with the knurled nuts. Give the pendulum a gentle swing. If it doesn't start and run, consult the Troubleshooting section below.
Check the LED after a minute or so. If it is consistently blinking red, the pendulum is swinging too slowly. Raise the bob to speed up the pendulum by loosening the top knurled nut and then tightening the bottom one, and then try again. Conversely, if the LED is consistently blinking green, the pendulum is too fast. Lower the bob a bit. Keep the pendulum stopped for at least 3 seconds between adjustments to allow the electronics to reset itself.
When the LED no longer consistently lights, the pendulum is initially set using short-term measurement. (It’s OK if the LED occasionally flashes red or green, as long as it is off most of the time.) The LED is disabled after 5 minutes. However, the pendulum speed is then measured with a more dependable long-term measurement. The LED may start blinking again after a period of time. If so, adjust the pendulum slightly, using as little as one turn of the knurled nuts. Keep the pendulum stopped for at least three seconds after adjusting, and restart it. It’s probably best to ignore the short-term measurement blinking of the first 5 minutes after this adjustment. But watch for additional long-term warnings and then adjust again if needed.
Once the LED stays off for an extended period, the electronics will generally compensate, and no further adjustment will be needed.
To set the time, just rotate the minute hand in the clockwise direction.
If you can't get your clock to run or keep running, check the following possible causes:
- Improper magnet or coil polarity
- Weak batteries
- Pendulum contacting frame or base
- Pendulum pivot contacting frame to base posts at large swing angles
- Gears not meshing properly
- Rough or misshaped gear teeth
- Faces of wheels rubbing on another gear or the frame
- Bearings or shafts binding
(How do I know what to troubleshoot? I've built several clocks, and I've had each of these problems at one time or another!)
You may find it helpful to temporarily disassemble parts of the clock to isolate problems. For example, remove the driving pawl to see if the pendulum will operate on its own. If it does, the gear train is probably binding somewhere.
If your clock runs but then stops, carefully examine the clock at the stopping point. It may be helpful to release the pawls and move the ratchet wheel or gears back and forth to locate the source of binding, rubbing, etc.
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