This DIY tachometer uses optical sensing to compute the rotational speed of a rotating object and displays the speed on an LCD display. It is Arduino-based project and can measure RPM over 20K.
Digital tachometer
This Bluetooth-controlled door lock, named The Bean Lock, is built around an electronic wireless deadbolt and is controlled by the LightBlue Bean, which is an Arduino compatible microcontroller that is programmed wirelessly over Bluetooth Low Energy so that it can be reprogrammed even after it’s been placed in to the project.
The Bean Lock
This Cassette Tape MP3 player is built by embedding an inexpensive USB MP3 player inside an audio cassette. Miniature switches are implemented to use the cassette tape wheels as control switches for selecting the songs and adjusting the volume.
Audio cassette mp3 player
This Instructable describes how to build your own persistence-of-vision (POV) clock using Arduino and the motor salvaged from an old hard drive. A small cut made on the one of the disks along with an optical sensor arrangement is used for precise positioning and timing of the rotating disk, which is the key in the operation of any POV based display.
POV clock using hard disk motor
HC-SR04 is an inexpensive ultrasonic sensor module readily available in market these days and is extensively used in robotic applications for sensing and ranging the obstacles. With a proper arrangement of multiple ultrasonic sensors, it is possible to detect motion and its direction too. This motion-controlled ultrasonic lamp project uses three HC-SR04 sensor modules in an array to detect a waving hand in front of them and rotates the lamp to adjust its position according to the direction of motion of the hand.
Motion-controlled lamp