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The George Washington University School of Engineering and Applied Science Department of Electrical and Computer Engineering ECE 2110 – LAB Experiment: Introduction to Soldering Principles and Biomedical Engineering Equipment: Name

Specifications

Digital Oscilloscope

Agilent DSO1024A

(1) BNC Cable

NONE

(1) BNC-to-mini-grabber test lead

NONE

Prototype Breadboard

NONE

Printed Circuit Board

GTA – Request this from Equipment room 1 week prior to LAB

Soldering Iron

NONE

Bioamplifier Kit

NONE

Disposable Electrodes

NONE

Multi-strand wire

NONE Table 1: Equipment

Components: Kit Part #

Part Description

Symbol name (used in schematics throughout this lab manual)

Resistor

Limiting resistor

Rlimit

AND130CR

Red LED

N/A Table 2: Components

Objectives:      

Learn how an ECG works Learn how to use an ECG amplifier Learn how to set up a 3-lead ECG Use the digital oscilloscope to observe an ECG waveform Construct a simple LED circuit to monitor your heart rate Learn how to solder and de-solder components onto a PCB

Introduction Electrocardiography (ECG or EKG from the German Elektrokardiogramm) is a transthoracic (across the thorax or chest) interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body. The recording produced by this noninvasive procedure is termed an electrocardiogram (also ECG or EKG). Most EKGs are performed for diagnostic or research purposes on human hearts, but may also be performed on animals, usually for research.

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The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat. At rest, each heart muscle cell has a charge across its outer wall, or cell membrane. Reducing this charge towards zero is called depolarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat a healthy heart will have an orderly progression of a wave of depolarization that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through "intrinsic conduction pathways" and then spreads all over the ventricles. This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart which is displayed as a wavy line either on a screen or on paper (Figure 1). This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.

Figure 1. A sample ECG readout.

Figure 2. Enlargement of one “PQRST” wave from figure #1. It is a single PQRST pulse for a normal ECG. The normal ECG (EKG) is composed of a P wave, a QRS complex and a T wave. The P wave represents atrial depolarization and the QRS represents ventricular depolarization. The T wave reflects the phase of rapid repolarization of the ventricles.

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Figure 3. Possible three-lead ECG configurations. (Earth refers to Ground).

Figure 4. LED circuit schematic.

In this lab, you will be recording ECG signals from your heart. In order to safely do so, you will be using an isolated bioamplifier to prevent possible electrical shock. You will also learn proper soldering technique and its importance for your future senior design project. From the output of the bioamplier, you will construct the circuit in figure #4. Figure #4 doesn’t show the biomamplifier, it shows your heart as the voltage source for the circuit. This will be replaced with the bioamplifier when you construct the circuit. The purpose of the circuit is for the the LED to ‘light up’ when it receives a positive voltage from your heart. For the prelab, you will construct the circuit in figure #4. Prelab: In PSPICE, construct the circuit in Figure 4.  Use the Vsin component as your voltage source (offset=0V, amp=1V, freq=1kHz),  Use the D1N4002 as the red LED,  Use a 1 kOhm as Rlimit.  Place a voltage probe after the LED and run a transient simulation to monitor the output voltage across the resistor. Questions:  What do you notice about the output voltage?  Describe the behavior of the LED in terms of open-circuit or short-circuit for the positive and negative portions of the signal. Turn In:  Your PSPICE schematic  Your simulation output  Answer to the questions above, refer to your simulation data to justify your answers GWU SEAS ECE Department ©2012 Page 3 of 5 ECE 2110 – Experiment: Introduction to Soldering Principles and Biomedical Engineering

Lab: Part 1 – Setting up the ECG amplifier a) b) c) d) e) f) g) h) i) j) k) l) m) n) o)

Read the user manual and familiarize yourself with the controls and method of operation. Connect the amplifier to an AC outlet and flip the “ON” switch. Turn the “Balance” dial until the front panel meter needle is centered. Set the “amplification” to max on both dials. Set the “high” frequency roll-off to 50Hz and the “low” frequency roll-off to 0.1Hz. Connect the lead box to the 6-pin socket in the back of the amplifier. Connect the three leads to the lead by matching their respective colors. Referring to Figure 3, pick a configuration and place electrodes at the designated locations on the subject. Connect the leads to their respective electrodes based on their respective colors (Positive, Ground, Negative). Turn on the digital oscilloscope. Connect a BNC-to-BNC cable from the AC socket located in the back of the amplifier to the digital oscilloscope. Adjust the voltage/div and time/div until you see a signal that resembles Figure 2. You may need to reduce the amplification if the signal is clipping. Show the signal to the GTA. Record the peak voltage of the ECG waveform. Save the waveform either as .PNG or .CSV onto a USB flash drive.

Part 2 – Building the LED circuit a) Given a 1.4V drop across the LED and the peak voltage recorded from Part 1, calculate the value of Rlimit required to obtain 20mA through the circuit. b) Construct the circuit in Figure 4 onto a breadboard using the value you obtained for Rlimit. (Note: The longer pin of the diode is the positive end.) c) Disconnect the BNC-to-BNC cable from the back of the bioamplifier and replace it with a BNC-tobanana mini-grabber. d) Connect the mini-grabber ends to the positive and negative terminals of your circuit (denoted by the heart symbol in Figure 4.) e) The LED should now blink at the same rate as your heart. f) Show this to the GTA. g) Disconnect the mini-grabbers. Part 3 – Soldering the LED circuit a) b) c) d) e)

The GTA will give a lecture on how to solder. Transfer the circuit onto the PCB and solder the components. Reconnect the mini-grabbers to verify that the circuit works. Show this to the GTA. De-solder all your components from the PCB.

Part 4 – Soldering multi-strand wire a) The GTA will discuss the importance of soldering in the senior design project and explain the technique of tinning. b) Peel apart the multi-strip wire to form a fork and strip both strands. c) Tin each strands and show your work to the GTA. d) Have a lab partner hold onto both ends of the multi-strand wire and solder the two tinned ends together. e) Have the GTA approve your work. f) De-solder. GWU SEAS ECE Department ©2012 Page 4 of 5 ECE 2110 – Experiment: Introduction to Soldering Principles and Biomedical Engineering

Post-lab analysis:     

Calculate your heart-rate in beats-per-minute from the ECG waveform. Based on your peak output voltage, what is the peak voltage of the raw signal voltage prior to amplification? Compared to the P and T waves, is the QRS complex a high or low frequency signal? Explain. If you only wanted to view the QRS complex, how would you adjust the frequency roll-off values to attenuate the P and T waves? What is the purpose of the limiting resistor in the LED circuit?

References: Thomas, R. E., A. J. Rosa, and G. Toussaint, The Analysis and Design of Linear Circuits (Prentice Hall: 2009). https://secure.wikimedia.org/wikipedia/en/wiki/Electrocardiography

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