06-12-2012, 11:39 AM
PATIENT MONITORING SYSTEM USING PIC 16F877
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INTRODUCTION
Measurement of Body Temperature and Respiratory Temperature:
Thermistor is used for the measurement of body temperature and respiratory
temperature. This Thermistor is a passive transducer and it’s resistance depends on the heat
being applied on it. We have arranged the thermistor in the potential divider circuit. This
thermistor exhibits a large change in resistance with a change in the body temperature. The
respiratory temperature is determined by holding the thermistor near the nose. Initially the
thermistor is calibrated to normal body temperature. The thermistor part is attached to the
patient whose temperature has to be measured, which changes the resistance value and thus
the corresponding change in the temperature is displayed on the monitor graphically . Also all
temperature measurements are updated in the patients database. Here in our project we use
bead thermistor.
Electro Cardiogram (ECG):
This is a four lead ECG monitoring system. Four sensors are kept at various parts of
the body ; the two arms and the two legs. All the signal outputs from the sensors are
conditioned by an external circuit consisting of an instrumentation amplifier and a trimpot
and is given to the PC through a PIC controller. An interactive program in Visual Basic is
developed to read the voltage signals and display a waveform pattern. The four leads used are
of silver electrode.
Power Supply Circuit:
All the electronic components only work with a DC supply ranging from –12V to
+12V. We are using the commonly available energy source of 230v-50Hz and stepping
down , rectifying, filtering and regulating the voltage.
Step Down Transformer:
When AC is applied to the primary winding of the power transformer it can either be
stepped down or up depending on the value of DC needed. In our circuit the transformer of
230v/12-0-12v is used to perform the step down operation where a 230V AC appears as 12V
AC across the secondary winding. The current rating of the transformer used in our project is
250mA.. Apart from stepping down AC voltages, it gives isolation between the power source
and power supply circuitries.
Rectifier Unit:
In the power supply unit, rectification is normally achieved using a solid state diode.
Diode has the property that will let the electron flow easily in one direction at proper biasing
condition. As AC is applied to the diode, electrons only flow when the anode and cathode is
negative. Reversing the polarity of voltage will not permit electron flow.
A commonly used circuit for supplying large amounts of DC power is the bridge
rectifier. A bridge rectifier of four diodes (4*IN4007) are used to achieve full wave
rectification. Two diodes will conduct during the negative cycle and the other two will
conduct during the positive half cycle. The DC voltage appearing across the output terminals
of the bridge rectifier will be somewhat lass than 90% of the applied rms value. Normally one alteration of the input voltage will reverse the polarities. Opposite ends of the transformer
will therefore always be 180 deg out of phase with each other.
For a positive cycle, two diodes are connected to the positive voltage at the top
winding and only one diode conducts. At the same time one of the other two diodes conducts
for the negative voltage that is applied from the bottom winding due to the forward bias for
that diode. In this circuit due to positive half cycle D1 & D2 will conduct to give 10.8v
pulsating DC. The DC output has a ripple frequency of 100Hz. Since each altercation
produces a resulting output pulse, frequency = 2*50 Hz. The output obtained is not a pure DC
and therefore filtration has to be done.
ECG Section:
The muscular contractions necessary to maintain the hearts’ pumping action are
initiated by depolarization and repolarization of the SA node and then the depolarization and
subsequent repolarization of the AV node. This electrical activity of the heart generates
external action potentials. Hence by measuring these potentials at the surface of the body the
electrical activity of the heart can be alalyzed end the technique used for this is called
Electrocardio Graphy.
This is achieved by applying electrodes to certain positions on the body and record the
potentials generated between various combinations of these electrodes with an amplifier and
the computer screen display. We have used 4 electrodes for the Left Arm, Left Leg, Right
Arm and Right Leg. Silver electrodes are preferred as they do not irritate the body and also
the contact impedance of Silver is Minimum.
The output from the ECG electrodes are fed to a signal conditioning circuit. This
circuit uses high gain amplifiers OP07 to achieve required signed gain. The gain of amplifier
is set to meet the minimum input signed requirement of the PLC microcontroller. The OP07
belongs to the family of differentiate amplifiers which can very efficiently reject the
interference of signals which are of a non biological origin. The offset voltage is overcome
by using a 500KW trim pot connected for offset balancing of the OP07.