Introduction: Electrocardiogram Circuit

Hello! This is written by two students who are currently studying Biomedical Engineering and taking a circuits class. We have created an ECG and we are very excited to share it with you.


The basic supplies that will be needed for this project include:

- breadboard

- resistors

- capacitors

- operational amplifiers (LM741)

- electrodes

You will also need the electronic equipment listed:

- DC Power Supply

- Function Generator

- Oscilloscope

Step 1: Differential Amplifier

Why is it necessary?

The differential amplifier is used to amplify the signal and to reduce the noise that can occur between the electrodes. The noise is reduced by taking the difference in voltage from the two electrodes. In order to determine the resistor values necessary, we decided we wanted the amplifier to create a gain of 1000.

How it is built?

To achieve this, the gain equation for a differential amplifier was used, the math can be found in the image attached. When calculating, it was found that the resistor values should be 100Ω and 50kΩ. However, since we did not have a 50 kΩ resistor, we used 47 kΩ. The set up of the differential amplifier for both LTSpice and the breadboard can be seen in the attached photo. The differential amplifier requires a breadboard to connect it to, 1 x 100Ω resistor, 6 x 47kΩ resistor, 3 LM741 operational amplifiers, and plenty of jumper wires.

How to test it?

When testing in LTSpice and on the physical device, you want to make sure it produces a gain of 1000. This is done by using the gain equation of gain = Vout/ Vin. Vout is the peak to peak output and Vin is the peak to peak input. For example, to test on the function generator, I would input 10 mV peak-to-peak into the circuit, so I should get an output of 10V.

Step 2: Notch Filter

Why is it necessary?

A notch filter is created to eliminate noise. Since most buildings have 60 Hz AC current which would create noise in the circuit, we decided to make a notch filter that will attenuate signal at 60 Hz.

How to build it?

The notch filter design is based off the image above. The equations to calculate the values for the resistors and capacitors are also listed above. We decided to use a frequency of 60 Hz and 0.1 uF capacitors since it is a capacitor value that we had. When calculating the equations, we found the R1 & R2 to be equal to 37,549 kΩ and the value for R3 is 9021.19 Ω. In order to be able to create these values on our circuit board, we used 39 kΩ for R1 and R2 and 9.1 kΩ for R3. Overall, the notch filter requires 1 x 9.1kΩ resistor, 2 x 39kΩ resistor, 3 x 0.1 uF capacitor, 1 LM741 operational amplifiers, and plenty of jumper wires.The schematic for the setup of the notch filter for both LTSpice and the breadboard are in an image above.

How to test it?

The functionality of the notch filter can be tested by doing an AC sweep. All frequencies should pass through the filter except 60 Hz. This can be tested on both LTSpice and the physical circuit

Step 3: Low-Pass Filter

Why is it necessary?

A low-pass filter is needed to reduce the noise from your body and the room surrounding us. When deciding the cutoff frequency for the low-pass filter, it was important to consider that a heart beat occurs from 1 Hz- 3Hz and the waveforms that make up out ECG are near 1- 50 Hz.

How to build it?

We decided to make the cutoff frequency 60 Hz so we could still get all the useful signals but also cut out the unnecessary signal. When determining the cutoff frequency would be 70 Hz, we decided to pick the capacitor value of 0.15uF since it is one we had in our kit. The calculation for the capacitor value can be seen in the image. The outcome of the calculation was a resistor value of 17.638 kΩ. We chose to use an 18 kΩ resistor. The low pass filter requires 2 x 18kΩ resistor, 2x0.15 uF capacitor, 1 LM741 operational amplifiers, and plenty of jumper wires. The schematic of the low pass filter for both the LTSpice and physical circuit can be found in the image.

How to test it?

The low-pass filter can be tested using an AC sweep on both LTSpice and physical circuit. When running the AC sweep, you should see the frequencies below to cutoff are unchanged, but the frequencies above the cutoff start to be filtered out.

Step 4: Complete Project

When the circuit is complete, it should look like image above! You are now ready to attach the electrodes to your body and see your ECG! Along with the oscilloscope, the ECG can also be displayed on Arduino.