Op Amp Circuit Custom PCB

Custom signal amplification circuit to scale audio signal for a MSP 432. The goal was to make a small wearable device, which could filter audio to read a heartbeat. This could have some advantages over simple athletic EMG heart monitors.

Since the project options were pre-defined for a course, the mechanism of using a mic to gather heart sounds was dictated in the project description. Because of this, the concept development primarily consisted of component searches for a small microphone and researching the types of circuits that would be required to scale that mic output for the microcontroller we were given for the course. This was more of an engineering assignment than a design problem.

Design

The circuit design work was incredibly collaborative, with Dom and me switching back and forth on our work. Using reference circuits, I designed the initial amplifier circuit which scales the mic input to the microcontroller's analogue to digital converter (ADC). Dom then re-modelled the circuit in LTspice with the components I was finding for us to use. Once He converted the schematic to Eagle, I designed the component layout on the PCB to minimize the size.

Fabrication

Both Dom & I took turns soldering the PCB components. After running into issues trying to solder the mic's hidden pads traditionally, we were able to get access to a hotplate fromt the electrical engineering department. Both of us also worked on the code which sent the audio signal from the microcontroller to Matlab for filtering.

The final PCB performed well. However the initial amplification was a little too conservative. The data sheet warned of the potential for large outputs, which we were never close to reaching, regardless of input volume. Because of this, we had scaled the signal to protect the microcontroller. Fortunately, it was easy to switch out a resistor to provide reasonable amplification. Additionally, we weren't easily able to pick out a heart beat, even when recording in a sound-proof room. Dom and I actually somewhat expected this outcome. Another professor confirmed our thoughts, by suggesting that to pick up a heart beat, we wouldn't be able to use live filtering, but would need to take in a much larger amount of data (30+ seconds). Having reached the end of the course, Dom and I agreed that the project with the same motivation could probably work in the future if a graduate student decided to take long samples of the data along with EMG data. Using both data sets, a student could probably figure out a method to reliably filter the audio data to find a heartbeat.

Acknowledgements

The project was a collaboration with Dominic LoPinto, another Virginia Tech graduate student. During the Process, we were aided by Dr. Alan Asbeck, Hani Awni, and Dr. Alfred Wicks.

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