I built an orrery (a mechanical model of the solar system) using a kit. However, I wasn't happy with the motor that came with it because it was too noisy and ran too fast even at its lowest setting. So, I decided to replace it with a much quieter motor and got a PWM controller to adjust the motor's speed.

PWM stands for Pulse Width Modulation, and here's how it works: Instead of directly controlling the voltage to the motor, it sends the full voltage in rapid pulses. These pulses are at full voltage so the motor doesn't easily stall, but they vary in width, which means they change the time the motor is on compared to the time it's off. By adjusting the width of these pulses, you can control the motor's speed.

The beauty of PWM is that because pulses received by the motor are very rapid, the overall effect is still a smooth motion, and you can even slow it down significantly if you want. The modified orrery can run 4 times slower than the original one. This way, you can enjoy a quieter and more precisely controlled movement of the orrery.

Supplies

pwm controller
slow, silent motor (not really needed, read on)

and of course the orrery but presumably you are only interested in this instructable when you already have it.

Step 1: Motor Noise

First listen to the difference in motor noise. I attached something to the motor on the right so you can see it rotating in total silence. I admit the sound of the original motor on the left is a bit attenuated by the wood it is put on, but the difference is stunning.

Step 2: Slow Speed

Next look at how slow it can go. Great! But also notice that it is nearly silent at slow speeds. As I wanted it to run slowly anyway, I decided not to use the silent motor and 3D print a frame for it, but instead only use the PWM controller with the existing motor. A much easier modification.

Step 3: Cutting and Reconnecting 1

Remove the top of the orrery and open the base. Disconnect the motor. Remove the wire at position 2 in the picture. Then cut the red wire going to the motor at position 1 in the picture and reconnect it at position 2.

Step 4: Cutting and Reconnecting 2

Cut the wires from the power connector at position 3 and remove the remaining wires from the power connector. Note the red wire going to the center of the power connector.

Connect the wires you just cut to the motor output of the PWM controller (the polarity doesn't matter too much) (see #6 in the second picture). Add two new wires from the power connector to the power input of the PWM controller. (here polarity does matter, so make sure you connect the red wire to + and the black wire to -. (See #5 in the picture).

Step 5: Mounting the Knob

With all the wires connected mount the knob into the base plate with the supplied nut. Note there is a little tab that you need to remove to mount it nicely parallel to the plate. Before mounting it, put some insulating tape around it, to prevent shorts. As the backside of the PWM controller touches the metal base plate it also needs some insulating tape at the back. I applied two layers, just to be sure.

Step 6: Enjoy

Put everything back together and enjoy your super slow moving orrery. Of course it can still go as fast as the original, but than it is also as loud ;-) )