This project will teach how to build a simple EKG on a breadboard. The circuit is made up of 3 stages, an instrumentation amplifier, a notch and low pass filter. This can be constructed all on one breadboard with three main component types, resistors, capacitors and LM471 op amps.

Supplies

resistors (1k-1M)
5 LM471 op amps
Capacitors (.033u-.1u)
breadboard
wires (various lengths
function generator
power source
oscilloscope
3 BNC cables ( two that have alligator clips at one end and one double-sided BNC connectors
3 cables with alligator clips on one end
DMM
three EKG leads plus electrode patches
cable splitter

Step 1: Step 1: Simulating INA

First, you need to calculate your resistor values using the equation above. The left hand side of the equation is your gain which for this project you want to be 1000. That means the two multipliers on the right hand side are equal to the square root of 1000. By choosing values for R1 and R3 you can solve for R2 and R4.

Next, you want to build your circuit in LTSpice with your calculated resistor values. You may need to download the LM471 op amp as it is not the LTSpice library. For the op amp power supply each op amp will get 15V. For simulating output of the circuit ground one of the power sources and send a sine wave from the other one. note, the amplitude of the sine wave should be small given the circuit has a gain of a 1000 i.e. a 1mV input will have 1V output. Also, your output should be smaller than 15V so the output is not cutoff so your input should be smaller that 15mV. Run the simulation for 0.1 seconds and then measure the input by clicking the wire coming from the voltage source and then measure the output by clicking the wire after coming out of the op amp. Your output should look like the figure above. Since the input so small compared to the outputs it will appear as a straight line.

Step 2: Step 2: Simulating Notch Filter

Similar to simulating INA you first need to calculate your resistor values using the equations above. The frequency you are trying to filter is 60Hz as to eliminate noise caused by the AC power supply. For this circuit you also what a quality factor of at least 8 but 10 is preferred. By choosing a capacitor value (0.1 uF makes the resistor values within a reasonable range) you can calculate your resistor values.

Next, build your circuit in LTSpice and include and intermediary power source (this power source will be removed when the entire circuit is put together). set the source to 1 AC, and the op amp will get 15V and instead of running a transient simulation, the notch filter requires an AC sweep. When prompted change the simulation settings to AC sweep, Decade, 100, points and then run the AC sweep from 1 Hz to 1k Hz. To get a better view of the cutoff frequency run the simulation from 50 Hz to 70 Hz.

Step 3: Step 3 :Simulating Low Pass Filter

Calculate Resistor values using the equations above, for this circuit K is our gain and since our INA is doing all the amplifying K=1, this means that R3 is replaced with by an open circuit and R4 is replaced by a short circuit. Our cut off frequency is 150 Hz. using this you can calculate the capacitor values and then the resistors.

Next, Build your circuit in Ltspice and have a 1 AC power source. for this circuit we are also running an AC sweep from 1 Hz to 1k Hz. you results should show that the circuit has a gain of 1 until is gets closer to 150 and past 150 it should decrease to zero.

Step 4: Step 4: Building and Testing INA

First, build your INA on the breadboard according to your schematic. Connect your cable splitter to your function generator. then connect one end the BNC cable with two BNC connectors to the function generator and the other to the oscilloscopes. Connect one of the cables with the alligator clips to the function generator and then connect the positive alligator clip to the input of the INA, ground the negative alligator clip. ( The easiest way to ground this is by dedicating on the positive or negative strips on either side of the board to ground another two should be dedicated to either the positive or negative supply to the op amps). Next, connect three cables to your power supply, one to +25V, -25V and ground. get +25V to 15V and -25V to -15V. Connect all cables to their dedicated strips. Next, connect your last BNC cable to the second port on the oscilloscope and the output of the circuit (red to output wire and black to ground). Set your function generator to a sine wave with the lowest amplitude and what ever frequency you want. Your output should look like the figure above (yellow is input and green is output)

Step 5: Step 5: Building and Testing Notch Filter

Leave your INA on the breadboard and op amp power supply cables but do not connect your INA to your notch filter and start to build your notch filter according to your schematic. Use the same positive or negative strips you used for the positive and negative supplies to the op amps and the strip you used as ground. connect the function generator to your input of your circuit and connect the oscilloscope to the output (leave the cable that connects the function generator to the oscilloscope). On the function generator leave the setting as a sine wave but change the amplitude to 1V. To create a magnitude response plot record the voltage of the input and voltage of the output at different frequencies, 20 to 150 Hz in increments of 10 when far away from 60 Hz and increments of 1 Hz when close to 60 Hz should be enough. next calculate magnitude by dividing output by input. then plot magnitude versus frequency (putting the x axis on a log scale might make it easier to read). Your circuit and magnitude response should looks like the figures above.

Step 6: Step 6: Building and Testing Low Pass

Leaving everything you have built so far on the board, build your low pass filter according to your schematic. connect the function generator to your input and oscilloscope to output. Like the Notch filter you will need to mag a magnitude response plot but this time from a range of 50 to 1k Hz (take enough points to see where the graph levels out after the cutoff frequency. your circuit and output should look like the figures above.

Step 7: Step 7: Connecting the 3 Stages

Now connect the output of the INA to the input of the notch filter and the output of the notch filter to the input of the low pass filter. Before connecting leads to a patient, send a simulated signal into your signal. switch the function generator to ARG and set it to the smallest amplitude and a frequency of 1.2 Hz. If that looks correct, disconnect the function generator from your circuit. Optionally connect a lead from the output of the INA to the oscilloscope to se what the input looks like before filtering. Next, Connect your leads to a patient, the positive lead (G1) goes the the left ankle, the negative lead (G2) goes to the right wrist and ground (COM) goes to right ankle. Your circuit and output should look like the figures above. Optionally you can attach an Arduino to show the signal .

Introduction: Intro: Breadboard EKG