Heart rate monitoring

Overview

In 2011, I created a circuit for detecting heart beats using an infrared ear clip as found as accessory for sports equipment like rowing machines or cycle ergometers. It converts the very small analog signal into a low impedence digital signal using filters, amplifiers and a Schmitt trigger. This signal can be fed into an Arduino or similar device for further processing.

In 2020 a guy named Burkhard contacted be, because he built his own fitness device from spare parts and was lacking a heart rate monitor. He came up with the idea of interconnecting my detector circuit with a Beat Counter and asked me for help. We discussed a bit and decided on designing a new standalone device. I did the electronics design and board layout, he did the assembly, mechanical design and power supply research.

Heart rate detection

The cardio ear clips I have seen so far consist of an infrared LED and a phototransistor, which basically form a fork light barrier with your earlobe inbetween. Depending on the amount of blood in the earlobe and its oxygen level, the amount of light being absorbed differs. Using a simple driving circuit you can see a small signal on an oscilloscope that directly correlates with the heart beat. This small signal can be conditioned and digitized to turn into a low impedence square signal.

Test schematic Raw signal Processed signal

My design uses a LMC6484, but any CMOS Operational Amplifier should work.

Schematic

Monitoring device

For the actual heart rate monitor, Burkhard's request was "the downright simplest thing that does the job; no bells and whistles". We ended up with a three digits 7-segment display, a status LED and two jumpers for configuring a heart rate threshold above which the display should start flashing. As microcontroller we decided on using an ATtiny2313A (8 bit AVR, 2 KB program memory, 128 byte RAM, 20 pins).

Display front Display back Device 1 Device 2

As power supply, Burkhard decided on using a USB power bank which provides stable 5 volts while encapsulating all the charging circuitry. I remember this was bit quirky, because the power bank automatically shut down if there was too little load. To be honest I don't recall how this was addressed, I think he added a small load resistor.

References