Harness your biopotential, or: How to measure a heartbeat in approximately twelve steps
The heart in your chest beats every minute of your life.1 Accompanying each new beat is an electrical dance whose waves ripple throughout your body. The same is true of every single human being you will ever come across. What you will do is measure those waves between two points in space and thereby detect the exact moment a heart’s muscles contract for a few minutes of someone’s life one fine day.
To do this, you will construct a system, an electrocardiogram, comprising the following:
Specifically, you will (1) power up a prototyping electronics board to create the system, (2) build an instrumentation amplifier to amplify a signal, (3) construct a filter to remove unwanted noise from the signal, (4) detect a heartbeat from a willing participant, and in the process (5) become a biomedical engineer.
1. Except in those rare instances of individuals with artificial hearts or transplants, techniques being ever more perfected by biomedical engineers and their collaborators.
Review material inventory
Begin by reviewing the inventory of your kit. Ensure it contains the following materials:
- Twenty (20) jumper wires, ~5 cm each (4 red, 4 black, 4 green, 4 yellow, 4 blue)
- Three (3) long wires, unstripped, ~15 cm each (1 red, 1 black, 1 green)
- Three (3) resistors
- RG: 1.5 kΩ; Actual value: ________________ Ω
- R1: 330 kΩ; Actual value: ________________ Ω
- R2: 10 MΩ; Actual value: ________________ Ω
- Two (2) capacitors
- C1: 10 µF Actual value: ________________ F
- C2: 100 pF Actual value: ________________ F
- One (1) AD620 instrumentation amplifier (Figure 1. AD620)
- One (1) LM741 operational amplifier (Figure 2. LM741)
[Note: You will need to measure each of the passive components (i.e., the resistors and the capacitors) to determine their value. To do this, a volunteer will show you how to measure these values with a multimeter.]
[Note still further: Sometimes smaller valued capacitors can be difficult to measure precisely.]
Review workstation equipment
Please also check your workstation for the following equipment:
- A breadboard (Twin Industries TW-E41-1060, Figure 3. A breadboard with posts, rails, trench, and terminals)
- A power supply (Agilent E3630A, Figure 4. A power supply with (A) voltage adjustment, (B) port selection, and (C) output ports)
- An oscilloscope (Tektronix TDS 2012C, Figure 5. An oscilloscope with (A) position and vertical scaling, (B) input, and C) horizontal scaling)
- Three (3) 2m banana cables (one red, two black; with connectors, Figure 6. Banana cables (2m))
- Three (3) 1m banana cables (one red, two black, Figure 7. Banana cables (1m))
- One (1) BNC-to-splitter cable (Figure 8. BNC-to-splitter cable)
1. POWER A BOARD
The instructions that follow assume you build your circuit in a vertical orientation. That means centering work around a breadboard “trench” (Figure 3). To follow along, locate one trench on the breadboard. From here on, everything to the right side of that trench is on the “right” and everything to the left side is on the “left”. Feel free to consult the Reference sheet or to ask any of the volunteers any questions you may have.
Step 1. Connect a power supply to a breadboard
- ¡Turn on! the power supply
- To adjust a particular output port voltage, push in the “METER” button to select that port voltage, and adjust the voltage via the “VOLTAGE ADJUST” knobs
- Set the +20 V port voltage to be approximately +5 V
- Set the –20 V port voltage to be approximately –5 V
- Attach a 1m red cable from the +20 V port of the power supply to the red breadboard post
- Attach a 1m black cable from the –20 V port of the power supply to the black breadboard post
- Attach another 1m black cable from the COM port of the power supply to the green breadboard post, this will serve as a reference point, where voltage is 0
Step 2. Wire the breadboard posts to its power rails
- Wire the red breadboard post to the red (+) column of the right rail
- Wire the black breadboard post to the blue (–) column of the left rail
- Wire the green breadboard post to the blue (–) column of the right rail
- Where on the board is it +5 V?
- Where is it –5 V?
- What voltage is it at the reference point?
Before you continue: ¡Turn off! the power supply (for now)!
2. BUILD AN AMPLIFIER
In your kit there should be two eight-legged chips that look quite similar. They both should have a little notch at the “top”. Only one of them will have a polished circle next to that notch. That is the LM741 and it will be used to CONSTRUCT A FILTER. For this step, find the AD620, the instrumentation amplifier. It will not have a polished circle.
Step 3. Place an instrumentation amplifier on the breadboard
- Orient the instrumentation amplifier to align with the top of the board and place across the trench so that each of the “legs” of the amplifier is electrically independent
- Center the amplifier between the powered rails
Step 4. Wire the instrumentation amplifier for power
- Wire the positive rail (the one with +5 V) to Pin 7 of the amplifier (+Vs)
- Wire the negative rail (the one with –5 V) to Pin 4 (–Vs)
- Wire the reference rail (the one with how many volts?) to Pin 5 (REF)
Step 5. Create gain, inputs, and an output for the instrumentation amplifier
- Use RG to connect Pin 1 to Pin 8
- This will enable the amplifier to increase the difference in voltage by a predetermined amount known as gain
- Create two loose wires and insert one into each Pin 2 and Pin 3
- These two points will be the inputs across which you DETECT A HEARTBEAT
- Wire Pin 6 to a free space to the left of the trench and below the amplifier
- This output will contain the amplified signal
- The gain, G, of the amplifier is defined by the following formula
G = (49.4 kohm / RG) + 1
What is the gain of your signal, G, given the measured value of your RG resistor?
3. CONSTRUCT A FILTER
The LM741, the chip with the polished circle on its upper-left hand side, is a generic operational amplifier, “op-amp” for short. The particular wiring used here allows one to construct a “filter” that gets rid of parts of the signal that are not wanted. Specifically, we are creating a “bandpass filter” that will get rid of low frequencies and high frequencies.
Step 6. Place an op-amp on the breadboard
- Place the op-amp across the same trench as the instrumentation amplifier, oriented such that the op-amp’s top side aligns with the top of the board
- Wire R1 and C1 in series from the output wire of the instrumentation amplifier to Pin 2 (–IN) of the op-amp
[Note: electrolytic capacitors have a specific direction they are designed to work in. Connect the big leg of C1 to R1 and its little leg to Pin 2]
Step 7. Wire the op-amp for power
- Wire the positive rail to Pin 7 (+Vs)
- Wire the negative rail to Pin 4 (–Vs)
- Wire the reference rail to Pin 3 (+IN)
Step 8. Finish the filter
- Use a parallel combination of R2 and C2 to connect Pin 2 (–IN) to Pin 6 (OUTPUT)
- Create a loose output wire and connect it to Pin 6; this will be our “system output”
- The “lower corner frequency”, C.F.L is the region below which we attenuate low frequencies and the “higher corner frequency”, C.F.H is the region above which we attenuate higher frequencies. These are calculated by
C.F.L = 1/(2*pi*R1*C1)
C.F.H = 1/(2*pi*R2*C2)
Using your values of R1 and C1, what is your lower corner frequency?
Using your values of R2 and C2, what is your higher corner frequency?
4. DETECT A HEARTBEAT
In this portion of the activity, you will actually be measuring actual heartbeats from an actual person. To observe the heart’s signal, you will use an oscilloscope, an instrument that allows for the precise measurement of electrical signals detected by our system – a humble electrocardiogram. Before moving on to the next steps, work with a volunteer to ensure your circuit is correct, then ¡turn on! the power supply and the oscilloscope.
Step 9. Connect an oscilloscope to the output
- Using a BNC-to-splitter cable, attach the BNC-end of the cable to the Channel 1 terminal of the oscilloscope
- Using the same cable, attach the black portion of the splitter to the reference post on the breadboard
- Connect the red portion of the splitter (the one with the alligator clip) to the loose wire with the system output
Step 10. Connect a participant to the inputs
- Attach two electrodes to the wrists of a (willing) participant
[Note: the electrodes are sticky, but not too sticky. So if one of them falls off, just replace it with another!]
- Connect one of the electrodes via a 2m cable to one of the inputs of the instrumentation amplifier (loose wire at Pin 2 of the AD620)
- Connect the other electrode via a 2m cable to the other input of instrumentation amplifier (loose wire at Pin 3 of the AD620)
- Attach a third electrode, a “reference electrode” to an electrically neutral part of the participant’s body (generally a body part like your elbow or your ankle)
- Connect the reference electrode to breadboard’s reference post via a 2m cable
Step 11. Ask someone to let you measure their heartbeat
- Use the oscilloscope to observe the signal coming from the participant’s heart (it may require adjustment of the oscilloscope’s abscissa and ordinate scales)
- Can you think of a way to determine a heart rate?
- What is the participant’s heart rate (in beats per minute) right now?
5. BECOME A BIOMEDICAL ENGINEER
Step 12. Experiment!
- Think of a way to make the heart rate of the participant go lower. How low do you think the rate can go? Test your hypothesis if the participant is still willing and record the results
Lowest heart rate observed?
What did you do to make it go so low?
- Think of a way to make the heart rate of the participant go higher. How high do you think the rate can go? If the participant is still willing, test your hypothesis and record the results
Highest heart rate observed?
What did you do to make it go so high?
- ¡Turn off! all equipment. Return everything to the order you founding it in,
if not a little better
- The filter you built actually snuck a little extra gain into our system. The filter amplifies by an amount H = R2/R1, thereby making the Overall Gain
Overall Gain = G x H = ( ) x ( ) = __________
- If the signal you are measuring has been amplified by the Overall Gain, what is the size of the original signal? [Hint: The measured signal is the original signal multiplied by the Overall Gain.]
Next steps. Share what you learn, learn how to share
- Write down one or two things you learned today.2
- Is it important to ask someone’s permission before measuring their heart beats?
- Can you think of other biomedical data whose integrity, security, and privacy we want to guarantee?
- How could we secure such guarantees?
- What can we do to make healthcare better?
2. Are you interested in learning more about biomedical engineering? (If so, consider visiting https://bme.umich.edu.)