MASSACHUSETTS INSTITUTE OF TECHNOLOGY 6 071 Introduction to Electronics Signals and Measurement Spring 2006 Design Build and Test a Heart Rate Monitor Objective The objective of this laboratory is to design build and test a device that measures your heart rate The device will be based on a non invasive sensor that detects the variation of the blood flow in your finger due to the heart action Expectations There are two parts to this project 1 Design build and test the instrument 2 Write a detailed report describing your design and the project in general The report is due on Friday May 12 You will be working with your lab partner in all aspects of the project except for the writing of the lab report which must be done independently by each student Evaluation and grading will consider both the design and implementation of the circuit and the written report It is expected that the report contain in the minimum the following information A brief review of the physics of the problem and not just a reproduction of the description given here We are focusing on electronics not physiology and thus the extent of this review should be appropriate Systems description of the solution using block diagrams show how the device works Detailed presentation of each circuit clean hand drawn circuits are acceptable but I prefer to have circuits drawn with spice or similar software Detailed list of all components used in each circuit Plots of the signals at various crucial nodes of the circuit you may copy screen data and use in your report List of References Laboratory Schedule It is expected that all students attend the regular scheduled times MWF 12 30 2 30 in the laboratory Additional times when the laboratory will be open will be posted on the class web site 6 071 22 071 Spring 2006 Page 1 Project Description Physical description As the heart beats the pressure wave that is generated propagates along the arteries Even though the pressure wave attenuates as a function of distance along the arterial system it is still very strong and may be felt at the extremities It is very easy to for example sense the beating of the heart by simply touching the artery at the wrist These pressure fluctuations result in a volumetric change of the blood in the tissue The pulse that we feel at the wrist affects the blood volume in the fingers although we do not perceive it with our sense of touch Due to the changes in the blood volume in the tissue of a finger the transmission and the reflection of light through and from the tissue is altered By placing a light source on one side of the finger and an appropriate light detector on the other side it is possible to detect changes in the transmitted light due to changes in the blood volume We will build an apparatus to detect these changes and thus measure the heart rate System Description The system schematic of the instrument you will be building is shown on Figure 1 Finger ACH0 Circuit here to power the IR emitting diode D Q DPC Diode Power Circuit Circuit here to detect and process the signal SPC Sensor Vhr ACH0 Signal Processing Circuit Heart Rate vi DAC0 Test your signal See your heart rate beats minute Speaker Hear your heart rate Display Instrument Figure 1 Complete system schematic You will build and interface the various subsystems shown on Figure 1 The sensor shown separately on Figure 2 will be constructed using an infrared IR light emitting diode LED D and a phototransistor Q For D you will use the device QED123 and for Q you will use QSD124 Both of these devices are available in the laboratory In addition the data sheets for them are available for viewing and downloading from the class web site The mechanical characteristics of the sensor should be such that the finger the diode and the phototransistor do not move during the measurement Movement will result in signal fluctuations and up to a certain level your circuit will or should account for them 6 071 22 071 Spring 2006 Page 2 Finger D Q Figure 2 Sensor schematic The radiant intensity of the IR diode is proportional to the forward current of the diode You are expected to design the Diode Power Circuit DPC by taking into consideration the characteristics and ratings of the IR diode too much current and the diode is destroyed too little and the diode does not radiate enough energy for your sensor Therefore the operating characteristics of the diode should be optimized for the desired performance You may find additional information about the design of LED circuits in the various documents listed at the class web site The phototransistor receives the radiant energy and the output signal will depend on the type of amplifier circuit used You should describe in detail and characterize the behavior of the phototransistor interface circuit before proceeding The goal of the Signal Processing Circuit CPC is to accept as input the phototransistor signal and return signal Vhr at its output that has the following characteristics Square wave ranging from 0 Volts to 5 Volts Voltage tolerance 0V 200mV 5V 200mV Duty cycle dc as close to 50 as possible but will accept it if it is in the range 35 dc 65 In order to achieve these characteristics CPC should contain various sub circuits each performing a specific task A schematic with some candidate SPC sub circuits is shown on Figure 3 Aside from the obvious position of the phototransistor interface circuit no order is implied or suggested for the other sub circuits 6 071 22 071 Spring 2006 Page 3 Phototransistor interface circuit Rectifier circuit Amplifier circuit Filter circuit Comparator circuit Figure 3 Possible sub circuits of SPC The output voltage Vhr of SPC will in turn be read by the LabView instrument Heart Rate vi from channel ACH0 This instrument will evaluate the signal Vhr based on the requirements listed above If the Vhr passes these tests the heart rate in beats per minute will be calculated and displayed on the screen Also the signal will be send back out to DAC0 so that you can connect it to your speaker for auditory feedback of the signal That s All Have Fun 6 071 22 071 Spring 2006 Page 4