Make a Heart Rate Monitor

Remember to refer to this guide if you are not familiar with the engineering design process. You will go through the first several steps of the process before you purchase any materials and start building something. Then you will go back to the Materials tab and decide what parts you need to buy. Figure 3 shows some examples of what you might build in this project.

 
Figure 3. Top: an Arduino Uno connected to a pulse sensor and an LED on a breadboard. Bottom: a LilyPad Arduino with a pulse sensor and LEDs sewn onto a headband.

1. Define the problem. As explained in the Introduction, there are different types of heart rate monitors, used for different purposes. Before you can build one, you need to decide who will use it and under what circumstances. Someone wearing a heart rate monitor while mountain climbing will need a very different product than parents who want to monitor their newborn baby's heart rate while she is sleeping.
2. Do background research. This step could involve interviewing people who use, or want to use, a heart rate monitor. They are your "customers." What do they want in a heart rate monitor? You can also do research on existing heart rate monitors and technologies. If you have not already, make sure you review the information in the Bibliography before you continue. You need to understand how the pulse sensor you will use in your circuit works, and the basics of using an Arduino. Note that the specific pulse sensor recommended in the materials list is designed to be clipped onto a person's fingertip or earlobe.
3. Specify requirements. Based on your problem definition and background research, specify the requirements for your heart rate monitor and the constraints you have to work with. Here are some suggestions to get you started:
   1. How much can it cost? What is your budget?
   2. How much time do you have to build it? Especially if you need to learn new skills (like building circuits or programming), you should take those into account.
   3. How big or heavy should it be? Does it need to be portable, or will it be in a fixed location?
   4. How durable or sturdy does it need to be? Does it need to withstand someone dropping it, or wearing it while moving around?
   5. How easy and convenient should the monitor be to use? Is it designed for someone to use by themselves (a self-sufficient adult), or someone who will need help (a baby, or a person in an ambulance)?
   6. What information will it convey, or how will it communicate with the user? For example, will it blink LEDs with each pulse? Will it display the heart rate on a screen? Will it sound an alarm if the heart rate goes too low or too high? Will it do more than one of these things?
   7. Note: the example code and circuits on the pulse sensor website shows you how to blink a single LED with each pulse, and how to display the heartbeat on a computer monitor (by connecting your Arduino to a computer with a USB cable). If you are not sure where to begin, you can use that as a starting point.
4. Brainstorm solutions. Based on your requirements, start thinking of different ways you could build your heart rate monitor circuit and program your Arduino. Do not worry about details (like exactly what Arduino board you will buy) at first. Try to come up with general ideas for at least three different solutions. The brainstorming phase can include jotting down ideas and making sketches of possible designs, or drawing a flowchart that describes how your program will work. For example, if you know you want to do a wearable design, you could think of different ways the user could wear the monitor (wrist strap, attached to a piece of clothing, etc.).
5. Choose the best solution. To decide which of your ideas is the best solution, now you will need to work them out in more detail.
   1. This is where the Choosing an Arduino for Your Project resource will be useful. The type of Arduino (and accessories) you select will depend on the desired size, cost, and use for your heart rate monitor. They will also depend on your level of skill/comfort with circuits and programming. For example, some Arduino boards may require soldering, and some accessories may not have as much example code readily available. A complete list of materials for each of your ideas will allow you to calculate their cost.
   2. This is also where the engineering design process can start to become iterative. For example, you might find that you cannot buy parts to meet all your requirements and still fit within your budget. Now you will have to go back to step 3 (specify requirements) and make a decision: do you increase your budget, or change your requirements? You could even go back to step 2 (brainstorming) and come up with some totally new ideas.
   3. Figure out which of your designed solutions best fulfills all your requirements. You might have to make some trade-offs if a single solution is not the best option for every category. A decision matrix worksheet can be useful for doing this. Once you have narrowed it down to a single solution, order the parts you need for your circuit.
6. Build a prototype. Once you have all your parts, prototype your circuit and program your Arduino. You will need to follow the appropriate tutorials or getting-started resources depending on what type of Arduino you ordered. Solderless breadboards are a great way to prototype circuits. You can also use alligator clips to temporarily connect sewable circuit components. That way it is easy to redo connections if you make a mistake. It will be easier to test your circuit before you build a case/enclosure for it, in case you need to change something.
7. Test your solution. At first, it will probably be easiest to test your heart rate monitor on yourself, while you are near a computer and have all your tools and circuit parts handy. That way you can quickly debug and make changes to your code or circuit if something does not work properly. After you have your circuit working, try testing it in "real world" usage situations, or with volunteers. For example, for a wearable heart rate monitor, try wearing it while you go for a walk, or ask a friend to wear it. Do you run into any problems? For example, do any wires come loose when the user moves around? Is it difficult to keep the pulse sensor attached to your fingertip or earlobe? Are the pulse sensor readings affected by ambient light? Based on your observations, you may need to make changes or improvements to your design. You might even need to order more parts. Make sure you document all your observations and design changes.

Keep iterating and improving your design until it satisfies all your requirements. However, you do not have to stop there—there is always room for improvement! If your time and budget allow, can you make your heart rate monitor even better? Even if you are out of time or money, your final design report can discuss how you would change the design if you had more resources available. Can you design a "version 2.0" of your monitor?