Only three simple 3D printed parts link a servo to a stepper motor and make a data plotter. Ideal for 24 hours a day registration of temperature, humidity or whatever. The stepper motor shaft has an adapter so an old CD or DVD can be clamped on it. The stepper motor will rotate at constant speed, depending on the time interval that needs to be covered during one rotation. The servo motor follows the value of the parameter to be measured and drives the pen holder. Not interested in physics? Use it as a spirograph.