The final project in our sophomore electrical engineering lab ELEC 240 had to do with building an electrocardiogram, and connecting the circuit to a speaker to “hear your muscles”.
It was a pretty cool and involved project, putting together everything we had learned throughout the semester.
Naturally, I had to build a PCB for this with Surface mount parts and see how small I could make it.
Below is the circuit which we were given. I adapted the parts on Eagle and made the circuit drawings.
The circuit works by taking a differential signal of your muscles into an instrumental amplifier, and amplifying it through 3 stages, with reference to Vdd/2. The signal is finally outputted through a audio amplifier to a speaker. You attach leads to your arm, and when you flex, you are able to hear the circuit.
below is the first revision of the board:
I had the board produced by seeed studios, and I soldered it together and showed it off for our final lab.
A couple weeks after the lab was done with, I had the itch that I could have made the board smaller. I had found out about a new PCB manufacturer, JLCPCB, which produced PCB boards for $2 (with shipping included, no joke). So I made this board as small as I could and spun it once again.
In total, this has been the smallest board I have ever made, even going for thinner thickness than normal.
In the end, all the revisions worked! The biggest challenge was finding a 200 uF 0805 capacitor. They dont exist, so I just used a couple of 10 uF in parallel to acheive this.