Vital Signs Tracking Device

1. Vital Signs Tracking Device Dan Chen Matthew Lee

2. Introduction • Each year, on average, 150 people are killed from avalanches. • Three quarters of the victims are climbers, back country skiers and snowmobilers. • Due to the boom in mountain recreation and resorts there has been an increase in avalanche fatalities in the past few years

3. Introduction • Currently, the most commonly used rescue device is an avalanche beacon (transceiver). • In order for it to work, user must switch beacon to “transmit” to allow rescuers to receive the rescue signal. • Unfortunately, most avalanche deaths have occurred because the victims do not switch beacon to “transmit”. • Beacons are difficult to use because it only sends out a signal for the rescuer to detect. Rescuers must search in increasingly smaller circles to find victim. • Time is crucial in avalanche rescue. Victims that are buried for more than 45 minutes have a very small chance of surviving.

4. Objectives • Create a portable device for climbers and skiers to wear that will allow quicker avalanche rescue. • Even with the rescue signal being transmitted to rescuers, it is still difficult to find the victim in time. • Use GPS to obtain a more precise location of the victim. • Rescuers will be able to find the victims much quicker allowing a higher probability for survival.

5. Objectives • Create a portable device that can will transmit vital signs to rescuers. • People using avalanche beacon cannot be constantly transmitting signals. • When caught in an avalanche, the victim might be unconscious or unable to switch to “transmit”. • Transmitting vital signs to rescuer will allow rescuers to determine if a person is in need of aid. • heart rate, blood pressure, oxygen level, movement, and body temperature

6. Original Design • Initially was going to monitor heart rate, oxygen level, blood pressure, body temperature and movement in vicitims. • Signals would be sent via a transmitter and received by a receiver. • On the receiving end, signals would be analyzed by a program where the person vital signs would be displayed • If any vital signs were to go into critical levels, the program create a warning sign.

7. Original Objectives • Provide safety to skiers in dangerous environments • Non-obtrusive to skiers performance • No maintenance • Usable in extreme environments and durable enough to survive moderate impact

8. Original Block Diagram

9. Project Build: EKG • Heart Rate • Used an EKG to detect ventricular depolarization (one heart beat). • EKG consists of 4 Op-Amps • First two are simply voltage followers. • Third is a differential amplifier. • Fourth is a band pass filter (~70Hz) • Gain of entire circuit • Gain = R4/R3 (1+2R2/R1)

11. EKG • EKG was offset by ~-10V • Used Summer circuit to so that ventricular depolarization pulse would consistently cross zero. • Vout = Vin + Vref (no gain: used same resistors) • Vref determined by changing potentiameter • Summer circuit causes EKG to be inversed, so used inverter to make normal. • Vout = -Vin (no gain: used same resistors) • Used zero-crossing comparator to create a 5V square wave. • Basic Stamp unable to take negative inputs, so used potentiameter to inverse square wave and have peaks consistently fall under zero.

12. EKG

13. EKG after Summer/Inverter

14. EKG after comparator/2nd Summer

15. BASICSTAMP 2 • Used BS2 Carrier Board. • Power Supplied by a 9V battery attached to board • Count the number of pulses over a six second period • Using command: Count • Multiplied # of pulses by 10 to convert to Beats Per Minute. • Encoded number into binary • Output square wave corresponding to binary number • In order to compensate for the lag time of transmitter, had an initiation pulse of 10101 (each with pulsewidth of approximately 2ms) • Also had an end sequence of 0101010101 (each with pulsewidth of approximately 2ms) to allow microcontroller on receiving end to know when pulse sequence has ended. • Pulsewidth of binary data: ~1.5ms

16. BASICX-24 • Signal received by transmitter was inverted and offset. • Used summer circuit to fix problem. • No development board • Used a 100Ohm resistor in series with a 10uF electrolytic capacitor following power supply to conteract decoupling. • InputCapture • Function on BasicX that determines the length of a specified number of pulses and stores the numbers into an array. • Will not start until a transition has occurred (0 -1) • Will not end until the entire array has been filled • Array length used: 16 • Convert Pulsewidths from InputCapture to binary, then to decimal. • Output data onto screen.

17. Signal Encoding/Decoding • Signal received by BasicX • Pulsewidths stored into array [PSequence()] of length 16 • Units of pulsewidths were in units of 136ns. Divided by 7532 to convert to milliseconds. • PSequence(1) = 2ms, PSequence(2) = 2ms, …. • PSequence(6) = 4.5ms, PSequence(7) = 6ms,…. • Heart Rate = 60bpm(binary: 000111100)

18. PSequence analyzed from end. • If pulsewidth was ~2ms, disregard. • Any other pulsewidth was divided by 1.5, to find the number of 0’s or 1’s in series and stored into another array [Binary()] of size 9 • All heart rates were in intervals of ten (because multiplied by 10 before signal was sent out), so last bit of data was always zero. • First pulsewidth not ~2ms, subtract 2ms then divide by 1.5. • InputCapture doesn’t end until array is filled. • That’s why such a long end sequence, to make sure array is always filled (for e.g. HR = 0)

19. Binary() • Binary(1) = 0, Binary(2) = 0, Binary(3) = 0 • Binary(4) = 1, Binary(5) = 1, Binary(6) = 1, Binary (7)=1 • Binary(8) = 0, Binary(9) = 0 • Converted to Decimal and displayed onto screen • HR = 60 bpm • This method of encoding was excessive for only having heart rate being sent. • With other devices, encoding/decoding data converted to binary would be very easy with this code. Simply have to add more bits to the data portion of the transmitting signal, and knowing how the data was sectioned.

20. HP-2 Transmitter/Receiver • Transmitting frequency of 903.37 MHz • Power Supplied • Transmitter • Voltage = 5V, current limit = 26mA, current drawn = 17mA • Receiver • Voltage = 5V, current limit = 20mA, current drawn = 21mA • Antennae • JJB Ultra-Compact ¼ Whip Antenna (916 MHz)

23. Functional Test • Initially when testing circuit used a square wave as input into Basic Stamp microcontroller • Later used subject hooked up to the EKG • Resting heart rate was fairly constant, usually reading 80BPM by Basic Stamp, but sometimes fluctuating to 70 and 90 BPM. • After Subject exercised for 10 minutes Stamp was reading about 130/140 BPM. • The HR obtained by STAMP was consistent with HR received by BasicX.

24. Successes and Challenges • EKG had good readings when subject was sitting completely still. • If the subject moved, the EKG output was erratic • Movement of muscles causes depolarization of muscles and firing of neurons. These electric signals interfere with EKG signal • Possible solution: • Using a 2-lead EKG • Use a 12-lead EKG and more filters would eliminate a lot of the other electrical signals • 12-lead EKG is cumbersome.

25. Blood Pressure Monitor • Two most accurate and popular ways to measure blood pressure • sphygmomanometer • Building an electronic one would be difficult. • Could be a project within itself • Buying one, dissembling it and feeding it to the microcontroller would be difficult

26. Oximeter • Building one would be difficult • Could be a project within itself • Purchasing a portable one would cost around $400. • Dissembling a purchased one and feeding it to microcontroller would be difficult and could risk losing $400.

27. Accelerometer • Purchased one for $40. • Very small chip with no pins. Had to solder wires onto it. • In the process, broke the accelerometer • Could have used GPS instead • By finding change in distance over time, could use GPS to determine if the subject is in motion.

28. GPS • Did not have time to set it up • Programming the data from the GPS to the BASIC Stamp would have been very difficult. • Planning to use a MUX instead that would switch between data of the microcontroller and GPS to be sent out by the transmitter. • Selection of the data from MUX could easily be controlled by STAMP. • GPS Data received by BasicX could be much easier to analyze than with using STAMP. • BasicX documentation provided code to analyze data, allowing output location and acceleration (and/or velocity).

29. Body Temperature • Didn’t have time to set up. • Planning to use a thermistor connected to a plastic sheet that could be placed under armpit. • Used a Wheatstone bridge, placed both ends of thermistor (250 Ohm) into a beaker of water at a known temperature. Calculated the voltage across thermistor for many temperatures. Found an equation relating temperature and voltage. • V = .0016*T + 0.9955 • In trying to implement into circuit, realized that STAMP, unlike the BasicX, is unable to read voltages at I/O pins.

31. Thermistor readings

32. Another solution was to use the thermistor in series with a capacitor and find the RC time constant. • STAMP had a function that could record time at I/O pin between a change in voltages. • First, have to charge both sides to +5V so that there is no capacitance. Can do this by first putting the I/O pin to +5V for about 7ms. • RCTime will record the time it takes for the I/O pin to go from +5V to +1.5V (logic treshold of STAMP). • With known capacitance, could use equation RCtime = R * C* ln(5/1.5) to calculate resistance in thermistor. • From previous equation of temperature and voltages, could calculate equation relating temperature and resistance.

34. Skin temperature sensors are known to be fairly inaccurate during quick changes in temperature. • Thermistor is a 1st Order Device • With our project being for avalanche victims, an external body temperature sensor would be impractical. • Could use internal temperature sensor • Too invasive

35. Acknowledgements • Thanks to Gareth for all the help • Thanks to Professor Bopart in assistance with EKG