Portable Digital Blood Pressure Monitor

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Portable Digital Blood Pressure Monitor

• Introduction
• Our final project is to design and build a
  portable blood pressure monitor device that can
  measure a user's blood pressures and heart rate
  through an inflatable hand cuff. The device is
  consisted of three main parts external hardwares
  (such as cuff, motor, valve, and lcd), analog
  circuit, and microcontroller. The anolog circuit
  converts the pressure value inside the cuff into
  readable and usable analog waveforms. The MCU
  samples the waveforms and performs A/D convertion
  so that further calculations can be made. In
  addition, the MCU also controls the operation of
  the devices such as the button and lcd display.
  Since we have the word 'portable' in our title,
  for sure all of the components are put together
  in one package which allows a user to take it
  anywhere and perform a measurement whenever and
  wherever he/she wants.
•  It is undeniable that nowadays people are more
  aware of the health conditions. One of the most
  widely used methods to test the health conditions
  of an individual is to measure his/her blood
  pressures and heart rate. We, as ones of those
  who are concerned about their health, decided to
  work on this subject matter because we would like
  to build something that is useful and useable in
  real life.
•           

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How blood pressures are measured

• Usually when the doctor measures the patient's
  blood pressure, he will pump the air into the
  cuff and use the stethoscope to listen to the
  sounds of the blood in the artery of the
  patient's arm. At the start, the air is pumped to
  be above the systolic value. At this point, the
  doctor will hear nothing through the stethoscope.
  After the pressure is released gradually, at some
  point, the doctor will begin to hear the sound of
  the heart beats. At this point, the pressure in
  the cuff corresponds to the systolic pressure.
  After the pressure decreases further, the doctor
  will continue hearing the sound (with different
  characteristics). And at some point, the sounds
  will begin to disappear. At this point, the
  pressure in the cuff corresponds to the diastolic
  pressure.

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The air will be pumped into the cuff to be around
20 mmHg above average systolic pressure (about
120 mmHg for an average). After that the air will
be slowly released from the cuff causing the
pressure in the cuff to decrease. As the cuff is
slowly deflated, we will be measuring the tiny
oscillation in the air pressure of the arm cuff.
The systolic pressure will be the pressure at
which the pulsation starts to occur. We will use
the MCU to detect the point at which this
oscillation happens and then record the pressure
in the cuff. Then the pressure in the cuff will
decrease further. The diastolic pressure will be
taken at the point in which the oscillation
starts to disappear.
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Hardware Diagram
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Hardware Diagram
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NOTES

• The output voltage of the pressure sensor will be
  proportional to the difference between the
  pressure in the cuff and the air pressure in the
  room.
• The output voltage from the pressure sensor is
  very small (in mV),so we used a DC Amplifier
  (AD620) that amplifies according this equation
  RG49.4KOhm/(G-1).

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NOTES

• We used a Band-Pass filter to red of the DC
  voltage and amplify the AC voltage.
• We used an AC coupling stage to provide the DC
  bias level. Given this bias level, it is easier
  for us to process the AC signal using the on-chip
  ADC in the microcontroller.

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Software Flowchart
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