Using cheap servo in Lego-arduino builds.
Newer version, with less parts: https://www.instructables.com/id/Servo-SG90-With-Unmodified-Lego-V2
There are several guide how use SG90 servo with lego. Most of them require customization of lego/servo or 3d printed case.
This example is using original LEGO pieces. I tried to use parts that easy and cheap to get. Some variations are possible if you don't have exact parts.
Part list (with lego ids):
SG90 servo 2 screws
Brick, Arch 1 x 4 (3659) x2
Plate 1 x 4 (3710)
Plate 1 x 6 (3666) x7
Technic, Axle 3 (4519)
Technic, Axle 4 (3705)
Technic, Axle Connector 2L (6538)
Technic, Brick 1 x 4 with Holes (3701) x 5
Technic, Brick 1 x 6 with Holes (3894) x 15
Technic, Bush 1/2 Toothed (4265a/4265b) x2
Technic, Bush 1/2 Smooth (4265c)
Technic, Bush (3713)
Technic, Gear 12 Tooth Bevel (6589)
Technic, Gear 16 Tooth (Old Style with Round Holes) (4019)
Technic, Pin (2780/4459/3673) x 12
Step 1: Servo
Use 2 screws to attach gear to white part that come with servo. Cut corners with e.g. knife.
Step 2: Middle Section
Step 3: Attach Servo.
Put servo under black arches. Fix it with 1x6 brick (red).
Prepare other axle with gear.
Step 4: Attach Gears.
Step 5: Fix Servo Axle.
Fix servo axle with 2 bricks and plate. This will prevent axle from leaning to sides, or detaching from servo.
Step 6: More Durability.
This will add extra durability.
Step 7: Variations.
Examples if you want to put servo on side.
Or if you want to attach something to servo closer to the axle.
Step 8: In First Robot Build.
Video show how it could be used to make simple robot with arduino.
It could answer yes/no to your questions (controlled by 2 buttons).