Hello everyone! this is my very first project that i wish to share with you... (it's not the best one, but the results were really amazing!) before we begin, i recommend you to have some basic skills in analog circuits and LabView, or join someone who already know both things.
The analog stage consists only of Opamp substages like filters or amplifiers, and the Digital one consists in a simple analog read operation in LabView using an Arduino UNO board with a basic GUI showing the heart frequency and the signal coming out from the electrodes.
If you're ready to go, let's get started!
Step 1: Making the Electrode Wires
We need 3m of RCA cable, i recommend this cable because it's hehaviour is similar to a coaxial cable due to the outer conductors, they give more stability to the signal coming out from the electrodes. We're going to use only the inner conductors.
Weld 3 size 3 snap buttons to each inner conductor. Then weld the other ends to a 3 channel audio jack.
Remember to isolate every bare wires.
The electrodes that i used are shown in the picture. The're exclusively for ECG/EMG
Step 2: First Analog Stage (Instrumental Amplifier)
1.- in the schematic, E1 and E2 are the electrodes (both are connected to the chest, the third one is connected to GND and it is usually placed onto the heel). The opamps that i used are TL081, which i noticed, were more stable than 741.
You can see the resulting signal in the scope
Step 3: Second Analog Stage (60 Hz Notch Filter)
The notch filter is very important in the circuit, because it cleans up the frequency coming out from the AC voltage source.
Step 4: 3rd Analog Stage (low Pass Filter)
Add another filter in case of lower frequencies are detected like the movement of the electrodes
Step 5: 4th Analog Stage (inverting Summing Amplifier)
This stage is very usefull because i detected that small dc components (but greater than the electrode signals) are altering the output signal of the instrumental amplifier, so we need an offset stage to compensate it.
Step 6: 5th Analog Stage (Inverting Amplifier)
Finally, the last stage of the ECG, the inverting amplifier will magnify enormously the small signal of the last stage, making it easier to see.
Step 7: Breadboard/pcb Design
Sadly i didn't have enough time to make a fancy pcb design. i took out the components of the breadboard and placed it just like that onto a perforated breadboard-like plate. The circuit works with a separated dc voltage source (+-9V)
Connect the output signal of the ECG to the A0 Arduino pin. Connect the adruino GND to the circuit's GND.
Try to drill holes in the box for the opamp voltage sources, the audio jack, the pot and the usb cable.