The DFRobot heart rate sensor is a thumb-sized heart rate monitor designed for Arduino microcontrollers.

This heart rate monitor sensor is a pulse sensor which is developed based on PPG techniques. This is a simple and low-cost optical technique that can be used to detect blood volume changing in the microvascular bed of tissues. It is relatively easy to detect the pulsatile component of the cardiac cycle according to this theory.
The sensor has two holes that you can use to attach to your belt. You can wrap on your finger, wrist, earlobe or other areas where it has contact with your skin.

A Gravity Interface is adapted to allow plug&play to ease the barrier of usage.  The IO sensor shield is the best option to connect this heart rate sensor with your Arduino or other microcontrollers. Besides, this sensor can also be compatible with Raspberry Pi, intel edison, joule and curie by means of 3.3V Input Voltage. 

IoT Based Heart Rate Monitoring System is a very basic project on sending the pulse rate to Blynk Application and monitoring it from anywhere.

NOTE:
1. This product is NOT a professional medical device and should not be used to diagnose or treat medical conditions.
2. This sensor is designed to work when the user is not moving. If used while moving it will give inaccurate results.

• Input Voltage (Vin): 3.3 - 6V (5V recommended)
• Output Voltage: 0 - Vin (Analog), 0/ Vin (Digital)
• Operating current: <10mA
• Dimension: 28 x 24(mm), 1.102" x 0.945"(in)
• Interface Type: PH2.0-3P

• Product WIKI (Heart Rate Monitor Sensor)
• Related Documents 

• Heart Rate Monitor Sensor For Arduino x1
• Wrist belt x1
• PH2.0-3P cable x1

Project 1. How to make a Heart Rate Monitor
This project makes a Heart Beat Detection and Monitoring System using Arduino that will detect the heart beat using the Pulse Sensor and will show the readings in BPM (Beats Per Minute).
Hardware needs:
Arduino UNO Rev3
I/O Expansion Shield for Arduino
Heart Rate Sensor
ESP8266 WiFi Bee

Project 2. How To Make A Micro:bit Heart Rate Monitor
The following tutorial will show you how to set the micro:bit beating together with your heart using the optical heart rate sensor:
Component List:
micro:bit - An Educational & Creative Tool for Kids
Gravity: Heart Rate Monitor Sensor for Arduino

Project 3. How to Make a Running Heart Alarm T-shirt

A DIY heart-rate wereable in a running t-shirt that vibrates when your heart-rate is high, using DFROBOT components:

Component List:

Arduino Nano

vibration motor module for arduino

beetle- the smallest arduino 

heart rate monitor sensor arduino