Passive Infra-Red (PIR) sensors are used to detect motion based on the infrared heat in the surrounding area. This makes them a popular choice when building a system to detect potential intruders or people in general. These sensors can take for 10-60 seconds to warm up, so try to avoid motion during that time.
Basic motion detection.
This is a very small light sensor. A photo resistor (also known as a photodector, or photocell) changes resistance depending on the amount of light it is exposed to.
The H8054 PIR motion detection module provides basic motion detection in a small package. Compatible with the Arduino and Raspberry Pi
The InvenSense MPU-6050 sensor contains an accelerometer and a gyroscope in a single chip. It is very accurate, this sensor is used in self balancing robots, drones, smartphones, etc. The sensor uses the I2C-bus to interface with microcontroller.
This is the chain and sprocket kit for the IG52-DB4 robot platform. It includes 10 tooth and 15 tooth sprockets, 5 feet of #25 chain, and 4 connecting links.