This design can be used to teach how springs, pulleys, and energy work in physics.
The lesson plan might go something like this. Using weight sets that are found in standard physics classrooms, students would attach weights (increasing in mass) to the string and record the displacement of the spring. From that information, they can find the slope of the force vs. displacement curve, otherwise known as the spring constant (https://en.wikipedia.org/wiki/Hooke%27s_law). Then, knowing the spring constant, they would put an arbitrary amount of weight in the basket, attach it to the spring, and deduce what the weight is using the spring constant and displacement of the spring. Additionally, you can teach how potential energy is converted to elastic potential energy when the basket is attached to the spring, and the spring displaces as the basket reaches steady state. (http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html#:~:text=Elastic%20potential%20energy%20is%20Potential,well%20as%20the%20distance%20stretched.).
Next, the use of pulleys can be demonstrated. With the pin slide all the way to the left, the second spring can be attached to the string coming from the top spring. When the pin is moved to the right, it can be measured that the two springs have the same displacement. The additional pulley can then be put in the system (as seen in the pictures), and when the slide pin is moved again to the right, it can be seen that the top spring moved twice the distance, but stretched (or displaced) half as much as the second spring. From this students can derive the pulley constraint equation (https://ocw.mit.edu/courses/physics/8-01sc-classical-mechanics-fall-2016/readings/MIT8_01F16_example8.9.pdf).
Using the constraint equation, the basket can be attached to the double pulley system and knowing that the force acting on the top spring is half because of the second pulley, students can measure the displacement, and multiply the found force by 2 to get the actual weight of the basket.
If you want to get really detailed, you could explain errors that happen in the system most likely stemming from pulley friction, and the non-linearity of springs.
Besides the 3d printed parts, here are the additional pieces that you will need.
-string of some sort (I used sewing thread)
-M5x35 with 2 locking nuts for the top pulley
-M4x25 with wingnut for the pin slider, use washers at your discretion
-2 springs, you want something with a very low spring rate if possible and about an inch long, I used 5/32 x 15/16
-Something for weight in the basket, I used quarters, but anything dense will do
Use the hardware as seen in the pictures, and use a needle-nose pliers to make the second pulley hanger and the little S hook.
Here is a comparable toy (https://www.amazon.com/Educational-Demonstration-Technology-Engineering-Dynamometer/dp/B07Y7P1T6B). It is around $25. The total cost of this device including material and extras (excluding quarters :p) was $11.