Epicyclic Day Clock

The goal of this clock is to be able to show the time when its environment is bright but stop moving when it is dark. The clock has to adjust itself back to the right time when the brightness rises again. This system has the advantage to be quite when the room is dark and not visible. This solve the issue of noisy clock the prevent people from sleeping during the night.

• 3D printing
• laser cutting
• stepper motor
• Arduino UNO
• photo resistor
• resistor (around 10 KOhm)
• connecting wires

In order to obtain a clock with two hands (hours and minute), I require a ratio of 12 between the two. To achieve that I use two consecutive epicyclic train with respectively a ratio of 4 and 3.

The epicyclic trains are use in a configuration where the outside rime in fixe (and used as the outside of the clock). The central gear is the leading one, and the satellite carrier is the output.

The first train, connected to the motor has a central gear with 48 teeth, satellites with 24 teeth, and an outside rime with 96 teeth.

The first train, connected to the motor has a central gear with 32 teeth, satellites with 32 teeth, and an outside rime with 96 teeth.

The long hand is connected to the first central gear and pass through the second train. The short one is directly placed one the second satellite carrier.

The circuit is a tension divider between the resistor and the photo resistor.

When there is no light, the photo resistor will have a high resistor value. In that case, the voltage accros the resistor will by low. And when there is light, the resistor value will drop down, and the voltage measured will be high.

For the assembly, the two train are set in place first. Then the long hand is placed trough from the front and connects with the moto in the back. Because the motor wasn't strong enough to overcome the friction from the system, I added a simple reduction gear between the long hand and the motor. An other solution would have been to use a stronger one, but for size concern, I choose to keep the small stepper motor.

To encase the motor assembly, the Arduino card, the resistor and the wires, I boxed in all of it between two plate and I connected the two with flexible walls cut with a laser cutter.

The long hand needs to rotate by 1/60 turn each minute. This can be achieve either by calculation if the motor is precise enough or empirically. The second option is how I found 51.5 step per minute.

The half step is achieved by doing 51 and 52 step alternatively.

The second parameter to be adjusted is the light sensitivity. Once again, this is done by testing in different light environment and deciding which threshold is better.

In order for the clock to be on time after it has stop, the Arduino keeps track of the time, even if the motor is not activated accordingly. Once the light comes back, the clock rotate the amount of time it has lost (max 12 turns) and keeps going.

One idea for improvement is to optimise the code. If 11h59 have passed in the dark, the clock will do 719 rotation forward instead of 1 backward. More generally, between 6h and 12h passed in the dark, it is more advantageous to go backward.

Another way of improvement is to have a smoother rotation with less friction.

The clock could also be fitted with an internal battery, instead o f relying on the PC for power.