Understanding MAXREFDES117: A Tiny Optical Heart-Rate Sensor

For wearable projects, having the right sensor module makes all the difference. As you all know, this year’s contest theme is Smart Home and Wearables, so we came up with another sensor module, the MAXREFDES117 optical heart-rate module. This device comes with integrated red and IR LEDs, making it an ideal choice for developers and enthusiasts alike.

This tiny board, perfect for wearable projects, can be placed on a finger or earlobe to accurately detect heart rate. This versatile module works with both Arduino and mbed platforms for quick testing, development, and system integration. That’s why we chose this as one of the development boards for our upcoming Smart Home and Wearable Project Challenge, where you can win prizes up to Rs. 7,00,000. Not only that, you can even win a development board and other exciting goodies just by submitting your project ideas. 

Be sure to explore the Smart Home and Wearable Project Challenge for complete details and participation guidelines. Now, In this comprehensive guide, lets cover the features of the module, the hardware, methods of interfacing, and examples. Let's see what MAXREFDES117 Module has to offer.

What’s Included in the Package?

Since this is a sensor evaluation module, it’s made to be as small as possible, so no fancy accessories are included. Still, you can check out the unpacking image of this module below.

More than just hardware, Analog Devices Inc./Maxim Integrated provides excellent resources to help us get started easily. We’ll see more about that in this article. For now, let’s take a look at what this module comes packed with.

MAXREFDES117 Module’s Features

The MAXREFDES117 reference design is a low-power optical heart-rate module complete with integrated red and IR LEDs, along with a power supply. Perfectly suited for low-power wearable projects, here’s the list of features and benefits:

Features & Benefits

• Heart-Rate Monitor and Pulse Oximeter Sensor in
• LED Reflective Solution
• Highly-integrated, small-size sensor - 12.7mm x 12.7mm (0.5in x 0.5in) Board Size
• Non-chest-based heart rate/SpO2 detection
• Ultra-low power consumption (1.5 mA at 3.3 V input)
• Multiple Input Voltage Level (2v to 5.5v)
• Device Drivers
• Free Algorithm
• Example C Source Code for Arduino and mbed Platforms

Application of MAXREFDES117

Below are some of the possible applications of this module.

MAXREFDES117 Hardware Overview

Now that you’re familiar with the features of the module, let’s take a detailed look at the hardware. In general, this module is a two-sided PCB with components soldered on both sides, keeping the main heart rate/SpO₂ sensor on one side and a level translator IC plus a step-down converter on the other. This design is the main reason why the module is made more compact.

You can check out the part marking image above for a better understanding.
This minimal approach also makes the overall block diagram simpler.

Three main ICs handle all the core operations.

They are,

You might already be familiar with the MAX30102, as it’s a more popular option for heart-rate/SpO2 sensing that comes as an all-in-one package. The other two ICs have dedicated jobs to do, making this module more versatile.

Pinouts of the MAXREFDES117 Module

The pinouts are a little unusual compared to regular modules, but they really come in handy for real-time projects. Surprisingly, this module has pinouts on all four sides, two sides for I2C and the other two for power. Refer to the image below for a better understanding.

Looking at the pinouts, you can easily see that this is not a breadboard-friendly design. That’s one of the downsides for regular breadboard users, but it can be solved with the help of simple jumper wires, as shown in the image below.

Resources Available for MAXREFDES117

By referring to the official resource site, you can get the module’s complete guide for both hardware and software aspects.
By downloading the 19.03 MB design files, you will receive:

• Bill of Materials (BOM)
• PCB CAD (Cadence)
• PCB Fab Package
• PCB Layout
• Schematic
• Arduino Files
• Mbed Files

With these files, we can completely recreate this module with minimal effort. For software development, there is dedicated Arduino support along with support for other mbed cores as well.

Interfacing the MAXREFDES117 with Arduino

Hardware

On the hardware side, the connections are very simple. But as I already mentioned, the module is not breadboard-friendly, so I’m using a perfboard to make it breadboard-compatible. That’s because I only use this module for interfacing tutorials. Otherwise, you can go with direct soldering.

The connections are straightforward. Let’s see them in detail:

Power:

Since there is an inbuilt step-down converter, this module supports various voltage levels from 2V to 5.5V without any issues. This makes it adaptable for both 3.3V and 5V power inputs.

In this case, Arduino uses 5V, so we can directly connect the MAXREFDES117 to the 5V and GND of the Arduino Uno. The connection looks like this:

Sensor Module → Arduino UNO
VIN → 5V
GND → GND

I2C Connection:

We don’t need to worry about the voltage level of the signal pins because there is an in-built voltage level shifter IC (MAX14595). The connection looks like this:

Sensor Module → Arduino UNO
SCL → SCL [A5]
SDA → SDA [A4]

Now, let’s get into the actual hardware connections, where I use the perfboard to connect the sensor to the breadboard and then connect the Arduino to the breadboard as well.

Above, you can see how I made the hardware connections.

Software

In terms of software, you have two options:

1. Use the default software support provided by the manufacturer.
2. Use any other library available for the MAX30102 from the Arduino Library Manager.

Whatever your choice, I decided to go with the second option.

Simply searching in the Library Manager, I found a library by SparkFun and decided to use it for quick testing.

Opening up its built-in examples, I selected the one with the SpO2 reading feature and uploaded it to the Arduino. You can then see the live readings in the Serial Monitor while keeping your finger above the sensor on the module.

With this, we got to know more about the MAXREFDES117 sensor module. Hopefully, you got better insights into this module and will be able to make the best use of it.

Ready to get started? Don't forget to check out the attached video demonstration and explore the wealth of resources available to maximise your development experience with this sensor module.

Reference Sources
1. https://www.digikey.in/en/products/detail/analog-devices-inc-maxim-integrated/MAXREFDES117/6165562?s=N4IgTCBcDaILYEMAeAnApgMwCZoM4EZ8B2AYhAF0BfIA 
2. https://www.analog.com/en/resources/reference-designs/maxrefdes117.html#rd-overview 
3. https://www.analog.com/en/products/max1921.html 
4. https://www.analog.com/en/products/max30102.html 
https://www.analog.com/en/products/max14595.html