Design and Build an ECG Circuit

The electrocardiogram (ECG) shows general behavior, typically for a human heart. By observing the voltage over time of the heart, doctors can get a general sense of the patient’s health, as many respiratory and cardiac problems appear in and potentially distort the ECG signal. Here, we explain the steps of each stage that is required to construct your own ECG circuit and then record an ECG signal.

Step 1: Instrumentation Differential Amplifier

First, an Instrumentation Differential Amplifier should be created to account for a gain of around 1000. A gain is important in amplifying the signal to ensure the more clear, readable signal. This specific amplifier will give you two inputs which will allow you to properly set up the electrodes at the end of the construction and read the ECG signal.

Components:

- (3) uA741 Op amp

- (4) 10 kohm resistors

- (3) 5 kohm resistors

Step 2: Buffer Addition

Between each stage, it is important to add a Buffer to preserve the signal leaving each stage. This will help to reduce noise throughout the construction of the circuit.

Components:

- uA741 Op amp

Step 3: Bandpass Filter

The construction of a Bandpass Filter is important in allowing only a certain range of frequencies to pass through the circuit to the output. For an ECG, a range of around 0.1 Hz to 250 Hz is ideal. The Low Pass Filter will allow signals below 250 Hz through and the High Pass Filter will allow signals above 0.1 Hz through. The cutoff frequency equation fc=1/2piRC can be used to calculate the resistor and capacitor values.

Components:

- (1) uA741 Op amp

- (1) 6.8 kohm resistor

- (1) 160 kohm resistor

- (2) 0.1 uF capacitor

Step 4: Notch Filter

A Twin Notch Filter should then be constructed to block the frequency 60 Hz from going through the circuit. This frequency should be excluded because it is commonly associated with power lines and so it could potential cause interference with the ECG signal. In order to select the components, the equation 1/4piRC can be used.

Components:

- (2) 27 kohm resistor

- (1) 13 kohm resistor

- (2) 50 nF capacitor

- (1) 100 nF capacitor

Step 5: Build Your Circuit

Finally, connect all of the stages together! Remember the addition of the buffer between each stage to ensure preservation of the signal. The construction may take some trial and error in ensuring that all of the components are properly placed in the breadboard. It may help to test each individual stage on an oscilloscope to see if each stage is producing desired results.

Step 6: Test the ECG on Human

You can then test your constructed ECG circuit using an oscilloscope. Attach two electrodes to your ankles and one to your right wrist. The positive lead goes to the left ankle, the negative lead goes to the right ankle, and ground goes to the right wrist. Be sure to check if your wires are working that you are using to power the circuit as well as the wires connected to output.