Task: Use 2 sensors from class (Biomedical Engineering Measurements) to create a biomedical invention.

1. Automated Life Alert
By Derek Dodge and Ryan Benante
Introduction:
“Life Alert” is a system that elderly people can use when they live alone. The idea is that
if an elderly person “falls and can’t get up” or has an emergency, they can push the button
hanging around their neck and it will connect them to 911 [1]. Figure 1, below shows the Life
Alert Necklace with the button to push in emergency situations.
Figure 1: Life Alert hanging around someone’s neck [3]
This system is nice for elderly people who are not in life threatening danger. Although,
Life Alert would not work in situations where the elderly person becomes unconscious or has a
seizure. This is where an automated Life Alert system would be much better and safer for the
elderly person.
To automate our system we used a tilt sensor and biopotential electrodes to measure ECG
signals. Using the tilt sensor, we created a circuit along with Arduino code that lit an LED when
a person had fallen over for a certain amount of time. This LED symbolized calling 911. Using
the electrodes, we also created a circuit along with LabVIEW code that lit a boolean LED when
the heartbeat became dangerously high or low. This LED also symbolized calling 911.

2. Materials and Methods:
List of Materials:
 Tilt Sensor
 Arduino Microcontroller
 Resistors
 LEDs
 AD620 OP AMP
 Power Supply
 Electrodes
 Wires
We built two circuits to complete this project. One used a tilt sensor and the other used
ECG electrodes. According to Lab 9, the tilt sensor gives different outputs when it is held at
different orientations (as long as it is wired correctly). Figure 2, below shows the four situations
that the tilt sensor detects.
Figure 2: Tilt sensor outputs for different orientations [2]
The tilt sensor looks like an op amp and has six terminals. These terminals can be shown
in Figure 3 below.
Figure 3: Tilt sensor terminals where 1 and 6 are the outputs that correspond to Figure 2.

3. Assuming that the tilt sensor will be resting upright just like the current Life Alert
Necklace system, then Situation A is the only position that corresponds to standing or sitting
upright. This means that any other position besides Situation A, would mean that the elderly
person had fallen over. If the elderly person fell over and cannot get up for a certain amount of
time, then our system will call 911 by lighting up an LED. Below, Figure 4 shows our tilt sensor
circuit diagram.
Figure 4: Tilt Sensor Circuit
As seen in Figure 4, our circuit is controlled by an Arduino Microcontroller. Without the
proper coding for this circuit, this circuit would be useless. Our Arduino code can be seen below
in Figure 5.

4. Figure 5: Arduino Code to read values of Tilt Sensor
Our code uses an “if” statement to read the position of the tilt sensor. The output pins on
the tilt sensor will output either a zero or a one depending on the position it is in. The Arduino
code reads these values and either keeps the LED off or turns it on. As seen in figure two, there
is only one position on the tilt sensor that corresponds to standing upright, the position in which
both the output pins output zero. In this case, the code sees the first step in the “if” statement to
keep the LED turned off. However, if the tilt sensor is in any other position, that is either output
pin outputs a one (or they both do), the code will turn the LED on. The LED turning on is

5. supposed to correspond to calling 911 because the person has fallen over, but calling 911 every
time that the tilt sensor changes position is unnecessary. For example, 911 should not be called if
the person is just bending over to pick something up. Thus, a time delay is incorporated in the
code. This time delay uses the “millis” function to time how long the tilt sensor is not in the
standing position. When the value of the “millis” function reaches a certain value, in this case ten
seconds, then 911 will be called instead of calling 911 instantly.
The other circuit in our project corresponds to an ECG circuit. Using the techniques and
circuit from Lab 5, we were able to create a heart monitor that would light a boolean LED when
the patient’s heartbeat became dangerously high or low. Figure 6 below shows a simple ECG
circuit.
Figure 6: Crude ECG Circuit Diagram with Implied LabVIEW filtering [4]
Once we wired our circuit to the breadboard, it could be seen in Figure 7 below.

6. Figure 7: ECG Circuit on Breadboard
As seen in Figure 7, our output terminal of the AD620 op amp feeds into the DAQ
Assistant for LabVIEW. Our LabVIEW code reads the output data and filters it into a waveform
chart that can be seen on the front panel. Using this graph and a peak detection algorithm, we
could determine the patient’s heart rate in beats per minute. If the BPM fell too low or became
too high for what is considered safe, then an LED would light up on the front panel of the
LabVIEW program which symbolizes calling 911. Below, Figures 8 and 9 show our LabVIEW
code.

7. Figure 8: Front Panel of LabVIEW Code
Figure 9: Block Diagram of LabVIEW Code
As seen in Figure 9, our circuit measurements are read by the DAQ Assistant and then
processed by 2 filters. These are the same filters used in Lab 5, the first being a bandpass filter
with a range of .1 to 100 Hz, and the second being a bandstop filter with a stop range of 59 to 61
Hz. These filters were both third order butterworth filters. The bandpass filter reads the

8. meaningful heart activity and the bandstop filter got rid of the electrical noise at 60 Hz. Once the
signal was processed neatly, it was displayed on a waveform chart. A peak detector algorithm
from Lab 5 to get the BPM and it can be seen in the block diagram [4]. We then created an “or”
statement saying that if the heart rate became dangerously high or low, then the system would
call 911.
Results:
Our Tilt sensor results from two situations can be seen below in Figure 10.
Figure 10: Tilt sensor upright (LED off, LEFT) and Tilt sensor on side for more than 10 seconds
(LED on, Right)
On the right hand side of Figure 10, the tilt sensor had been on its side for more than 10
seconds so the LED turned on. This satisfied our design requirements. The results from the
ECG portion of our circuit can be seen below in Figure 11.

9. Figure 11: Top: Dangerously low/ irregular heartbeat and 911 is being called. Bottom: Regular
heartbeat, 911 is not being called.
Figure 11 shows that the circuit can successfully differentiate between safe and unsafe
heartbeats. Further analysis of these results can be found in the discussion section below.
Discussion:
Although our tilt sensor circuit worked successfully for one 911 call, it did not work
again once 911 was called. If the LED came on and then we tilted the board back to the upright
position, then the light would turn off but it would immediately turn back on when the board was

10. tilted. This means that after the LED was lit, it would not wait for the time delay to turn on
again. This means that if this circuit was actually implemented, we would need to add more
loops to the code, or we would need to have a reset button operated by the paramedics.
Our ECG circuit also had a few issues. The electrodes were often in poor contact with
the skin so meaningful signals were difficult to read. Another problem we faced was the fact that
the threshold value for the peak detection algorithm was not consistent. We needed to change
this value multiple times to correct for our signal measurements. Other than these bugs, the
circuit and code for the safe and unsafe heartbeat conditions worked just fine.
A possible source for error when using the tilt sensor is when the person is sleeping. The
patient may sleep on their side, which would cause the tilt sensor to call 911. A possible solution
to this would be incorporating a kill switch into the tilt sensor, which would turn off only the tilt
sensor part and would leave the heart monitor on. However, using a kill switch could be
dangerous in certain situations. For example, if the person has to get up in the middle of the night
and forgets to turn the tilt sensor back on, this poses the risk that they might fall and not have the
tilt sensor to call 911 for them. A possible source for error with the heart monitor comes from
using electrodes. The patient having to wear the electrodes all the time may cause the connection
from the electrodes to wear down, making it harder to pick up a signal. Also, the electrodes may
be disconnected, making it impossible to read a signal at all. To reduce chances of error for the
Life Alert system, additional studies would need to be done to figure out a better threshold value
to use when reading heartbeats. Also, more studies would need to be done to find the “safe”
heartbeat range, as well as the time delay to call 911 for someone who has fallen over.

11. Future work: If this design was to be created, it would probably be best if it was a small
device that attached to the patient’s chest in an upright position. This way it would read ECG
signals and use a tilt sensor of the torso all in one small device.
References:
1. http://www.lifealert.com/
2. Lab 9, Bronwell 212
3. http://www.ispot.tv/ad/7kK8/life-alert-fires-and-falls
4. Lab 5, Bronwell 212
Appendix:
Appendix 1: AD620 OP AMP
Appendix 2: National Instruments DAQ Assistant machine
BME 3500 - 003L