he 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package. Introduced in 1971 by Signetics, the 555 is still in widespread use due to its ease of use, low price, and stability. It is now made by many companies in the original bipolar and also in low-power CMOS types. This module is use as a square-wave signal generator.  It generates square wave signals used for experimental development. It also generates square wave signal driving motor drive.  With adjustable pulses generated for use by the MCU and adjustable pulse control related circuits.

This is a breakout board for Freescale’s MMA7361L three-axis analog MEMS accelerometer. The sensor requires a very low amount of power and has a g-select input which switches the accelerometer between ±1.5g and ±6g measurement ranges. Other features include a sleep mode, signal conditioning, a 1-pole low pass filter, temperature compensation, self test, and 0g-detect which detects linear freefall. Zero-g offset and sensitivity are factory set and require no external devices.

The Keyes-039 or KY039 Heart rate monitor consists of two things, an Infrared LED, and an Infrared Phototransistor.  The IR LED should come on whenever the sensor has power and stay on (because it is infrared, you won’t be able to see that it is on, but if you look through your cell phone camera you will).  The IR photo transistor causes the voltage to change on a “sensor” wire, and this should be connected to one of the Arduino’s analog pins.