Showing the Doppler Effect Using a Rotating Arm

We made a real-life demonstration of the Doppler effect using a rotating arm! Beware: this design contains fast moving parts, so be careful operating it!

For this design, you will need the following materials:

• ITEM Aluminum construction profile 20x20mm of the following lengths:
• 1x 100cm
• 7x 50cm
• 3x 30cm
• 2x 20cm
• 4x 10cm
• ~33x ITEM corner piece
• ~6x ITEM horizontal connection piece
• 2x rolling bearing in pillow block, 8mm inner diameter
• 1x 190XL025 belt
• 2x Bluetooth speaker with mountable side piece
• ~2x large tiewraps to mount speaker
• 1x RS775 brushless DC-motor

In addition to this, the following tools are needed to manufacture the design:

• 3D-printer
• Laser cutter
• M3 hex key
• 24V 2A+ adjustable power supply
• Lathe

3D-printing and laser cutting

• Print all STL files once, using a 3D-printer
• Cut the DXF file twice in a laser cutter, using 3mm plywood

Tapping profiles and turning

• For one 10cm profile, tap one side of the profile as shown in the first picture using an M3 thread
• For another 10cm profile, tap both sides of the profile, again using M3 thread
• Using a lathe, manufacture the round piece according to the second drawing, using aluminum

1. Assemble the bottom part of the frame as shown in the first picture (the picture is taken from the bottom perspective). For this part, 5x 50cm Item construction profile and some corner pieces are needed
2. Assemble the vertical part of the frame using 2x 20cm and 2x 50cm profile and some corner pieces as in the second picture

• Install the 2x 20cm vertical profiles and slide the 3D-printed motor housing (Motorhouder.STL) onto the profiles
• Install the 3x 30cm horizontal profiles to lock / adjust the motor in the correct position
• Insert the DC motor into its 3D-printed housing
• Slide the small gear (tandwielklein.stl) onto the motor

• Install the pillow block bearings on the horizontal Item profiles as shown in the first picture
• Take the piece of 10cm profile where both sides are tapped, and first slide one wooden plate onto the profile, then the 3D-printed large gear and lastly the other wooden plate as shown in the picture. Assemble the gear by using 4x corner pieces and M3 bolts
• Using an M3 screw, screw the 10cm profile onto the lower bearing. Then insert the part you turned in step 1 (using the drawing) and screw it onto the top part of the 10cm profile. It will then be locked inside the bearing
• Attach the belt to the gears, as shown in the third picture

• Using a large tiewrap, attach the speaker to a 10cm profile as shown in the first picture. Repeat this step for the other speaker
• Attach both 10cm profiles (with speakers) to the ends ot the 100cm profile using corner pieces
• Find the balance point of the 100cm profile by using your finger
• Take the remaining 10cm profile (which is tapped on one side) and attach it to the arm at the balance point, as shown in the second picture
• Attach the arm to the frame as shown in the third picture by turning the arm

Attach the cables to the motor (as shown in the picture), and insert them into the power supply. Start by turning on the power supply at 12V, and give the arm a swing with your hands. The arm should now start moving. You can turn the motor up to 24V