📡 Spinning Radar: step-by-step build

A motor-driven radar dish that sweeps round and round on a turntable — with a blinking light.

Airports and ships have radar dishes that spin all day long, looking out for planes and boats. Ours is powered by the M motor. The motor is far too fast for a radar, so a tiny 8-tooth gear drives a giant 40-tooth gear: five times slower. The dish sits on a turntable, so it can spin forever without getting tangled. Flip the switch to sweep the other way!

Level 3 of 5 (medium)
About 25 minutes
15 steps, 38 parts
Powered by: Motor
Series: Starter Builds

What you will learn

Turntables: A turntable lets a whole model spin round.
Mounting a Motor: A motor must be pinned tight, or it spins itself instead of your wheels.
Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
Electric Circuits: Electricity flows from the battery, through the motor, and back.
Switching Direction: Flip which way electricity flows and the motor spins backwards.

Build these first

🧪 Gear Speed Lab: Two cranks, two gear pairs: one makes things FAST, one makes things STRONG. (level 2 of 5, about 20 minutes, 13 steps)
🔋 Motor Buggy: Your first car that drives by itself — with a forwards/backwards switch and headlights! (level 3 of 5, about 30 minutes, 20 steps)

Parts you need

1 × Open frame 5×11
1 × Turntable
2 × Axle 5
8 × Bush
1 × Axle 10
4 × Half bush
1 × Giant gear (40 teeth)
1 × Beam 11
4 × Black pin (grippy)
1 × M motor
2 × Axle 4
1 × Tiny gear (8 teeth)
1 × Big gear (36 teeth)
1 × LED lights
2 × Axle 6
2 × Split pin block
2 × Red axle-pin
1 × AA battery box
1 × Long pin-axle block
1 × Direction switch

Steps

Lay the 5×11 frame flat.
Put the turntable on the frame, near one end.
Push a 5-long axle down through the middle hole of each short end of the frame. Slide a bush onto each one, halfway up. These are the table legs.
Push the tall 10-long mast axle into the turntable, slide a half bush and the giant 40-tooth gear down onto it, and lay an 11-hole beam flat on top so the mast and both legs go through its holes.
Lock the legs with a half bush each, and push two black pins up into the 6th and 8th holes of the flat beam.
Turn the M motor upside down and press its front onto the two pins. Its round output sits right over the 7th hole.
Hold the tiny 8-tooth gear and a half bush under the 7th hole, then push a 4-long axle up through them and the beam into the motor. The half bush stops the gear sliding down.
Put the big black gear on top of the mast as the radar dish, and clip the lights on it.
Stand two 6-axles up in the 7th and 9th holes of one long side and slide a split pin block down both of them onto the frame, its cross hole pointing out sideways.
Slide two bushes down each axle, so they sit on top of the block and hold it down.
One more bush on each axle, then slide a second split pin block down onto the bushes, right to the top.
Push a red axle-pin into the side cross hole of each block, pointing outwards.
Press the AA battery box onto the two pins: they go into a low and a high hole in the end of its ear.
Switch holder: push black pins into the bottom and top holes of the box's far ear, on the outside, and press a long pin-axle block onto both. Its cross hole points away from the box.
Slide the direction switch onto a 4-long axle through the cross hole under its lever, then push the axle into the holder's cross hole. Plug the motor into the switch and the switch into the battery box.

Play time

Switch it on and watch the radar sweep the room. Put a toy plane on the table — can the radar "see" it?

Build next

🛵 Front-Drive Servo Trike: One front wheel does everything: its own motor drives it and the Servo swings it round. (level 4 of 5, about 50 minutes, 21 steps)
🎠 Chair-Swing Carousel: Crank faster and faster — the chairs fly out wider and wider! (level 2 of 5, about 35 minutes, 16 steps)
🌬️ Oscillating Fan: A desk fan that turns its head left and right all by itself! (level 3 of 5, about 40 minutes, 18 steps)
🎡 Ferris Wheel: A big wheel turns slowly while its gondolas always hang the right way up. (level 3 of 5, about 45 minutes, 28 steps)
🌀 Spirograph Drawing Machine: A machine that draws flowers! The paper spins while an arm swings the pen. (level 3 of 5, about 45 minutes, 19 steps)
🌍 Sun-Earth-Moon Orrery: Push the arm round: the Earth circles the Sun, spinning as it goes, with the Moon in tow. (level 4 of 5, about 35 minutes, 12 steps)
🦾 Crank Excavator Arm: Crank the worm gears to swing the digger round and lift the boom — then dig! (level 4 of 5, about 50 minutes, 22 steps)