Week 8 - Testing Plethysmograph circuit

As salam..

This post is continued from the previous post.
The research on the Plethysmograph is helped
for me to test the circuit that shown in the schematic
below:

Pulse Plethysmograph Amplifier Circuit

Sensor

The sensor consists of a light source and photodetector.

Whereby the light is shone through the tissues and 

variation in blood volume alters the amount of light falling 

on the detector. The light source and detector can be 

mounted side by side to look at changes in reflected light 

or on either side of a finger or earlobe to detect changed 

in transmitted light. The infrared filter of the phototransistor 

reduces interference from fluorescent lights, which have a 

large AC component in their output.

Amplifier

The amplifier uses an LM358 dual op-amp to provide two 

identical broadly-tuned band pass stages with gains of 100.

The circuit runs from a single 6V battery and the output 

zero is offset by about 1V by referring everything to an 

internal common line at a voltage set by a pair of 

forward-biased silicon diodes. This is convenient for 

interfaces with a 0-5V input.

Components are not critical but the two 2.2uF capacitors 

must be able to stand some reverse bias so they should be 

non-polarized. To get the expected result, the potentiometer 

is adjusted, so that the output is about 2V peak. The trace 

should look like this:

Heart rate trace

Pulse Plethysmograph results during deep breathing

Actually this experiment's result is achieved by using the 

PicoLog software. From this software the expectation 

waveform will display clearly as required. 

So, I'm just trying the circuit by using the oscilloscope 

in lab. The sensor I changed to LDR and Red LED.

The result I obtained is shown as the pictures below:

When the sensor not touched

When the sensor is touched

The first waveform is produced is quite same with

the expected result just had the noise.

The problem I had is, when I'm connected to 

ground, the waveform is not displayed at above.

It was displayed like too much of noise and 

no pattern of waveform can be seen. When I'm 

not connected the ground the result is displayed 

at above.

The potentiometer I'm already adjusted but the 

waveform is still not give any pattern if I'm 

connected to the ground. If I'm disconnected the 

ground the waveform is displayed. 

The sensor I'm already change to the 

IR transmitter and receiver. But the result still the 

same.

The circuit was constructed

Week 7 - Research on the Plethysmograph

As salam..

As the previous post, I'm still on the research the
circuit. For this week, I'm proceed to do a research
for the next circuit that might be suitable for the
measurement of heart rate and pulse oximeter.

I'm also asked the senior that had an experienced

in a project of pulse oximeter and heart rate. Mostly 

few senior that I met, told me that they are used 

plethysmograph. That was my first time heard that kind

of name. ISo that, I'm doing my research on that 

plethymograph circuit. What I found is :

Plethysmograph is an instrument for measuring changes in

volume within an organ or whole body (usually resulting 

in the amount of blood or air it contains). 

A photoplethysmograph (PPG) is a plethysmograph that 

uses optical techniques. A pulse oximeter measures oxygen 

saturation level (SpO2) and is also a PPG.

It can measure the change in the volume of arterial blood

with each pulse beat. This change in blood volume can be 

detected in peripheral part of the body such as the fingertip

or ear lobe using a technique called photoplethysmography.

The pulse oximeter that detects the signal is called a 

plethysmograph (or 'Pleth' for short).

The blood flow is normally shown as a waveform using a 

graph. It can provide useful information regarding the heart 

condition. Below is the different waveform with the status 

of heart beat.

Heart Rhythm

Pleth is useful to detect irregular heart rhythm. The regular

beating or contraction of the heart moves the blood throughout

the body. The heart beating pattern is clearly displayed by 

watching the pleth waveform. This is important to observe the 

pleth waveform because from that the doctor can know 

the condition of the patient's heart.

Signal Strength

Weak signal is indicated by the amplitude of the waveform. 

If the signal is too low, it would affect the accuracy and 

functioning of the pulse oximeter. If your oximeter is not 

giving the correct result, check if the signal strength is too 

low.

There are several causes for the weak signal:

• Low blood perfusion
• Dirty sensor or LED lights
• Improper positioning of the oximeter

All the info I found is help to understand what is actually 

Plethysmograph.. Next post will be about the circuit of

plethysmograph.

(Week5 - Week 6) Research on circuit

As salam..

Since Week4 is the Eid holiday..
I'm continued my research and testing the circuit in
Week5 and Week 6.. :)

As the previous post I had mentioned that my pulse
sensor is already failed in functioning.
This post will focus on the a few circuits that I had
done that is compatible to measure the heart rate
and pulse oximeter. So, I had make some research
about some circuits. Below is the first circuit that I
had tried that used Arduino programme.

Schematic circuit of Pulsometer

The Op-amp that used in this circuit is LM324.
IR led and LDR are used in this circuit.

LM324 is a 14 pin IC consisting of four independent 

operational amplifier (op-amps) compensated in a 

single package. Op-amps are high gain electronic 

voltage amplifier with differential input and a single-ended 

output. The output voltage is many times higher than the 

voltage difference between input terminals of an op-amp.

Theses op-amps are operated by a single power supply 

LM324 and need for a dual supply is eliminated. They 

can be used as amplifiers, comparators, oscillators, 

rectifiers etc. The conventional op-amp applications can 

be more easily implemented with LM324. Here are the
LM324 pin diagram and description. 

LM324

Pin diagram

Pin description

Below is the video that shows successful of the circuit.

As shown, there is working prototype that graphs pulses 

on computer that based on data from serial port. Work 

is going on signal processing to output pulse data, not only 

digital pulsations.

The goal is to build a separate digital/analog sensor for

arduino with possibility  to log data for future reference.

Below is the video of the result that I obtained from 

the schematic circuit that shown in early of this post.

As you can see the graph not stable like the one that
success. When I touched the LDR sensor, it gave the
shape of waveform. When I am not touched it, it became
a flat graph. I had tried to adjust the variable resistor,
but the result still the same.

When I'm looking on the serial monitor, the result in
analog supposedly about 400++ to less than 600.
But the number displayed is fixed to 1000
and that is when I'm not connected to ground.
When I'm connected to ground the reading is going
down to 0.

I'm also tried by change the sensor the LDR to the
IR transmitter and receiver. There is no result is
obtained.

Then, I'm proceed to try other circuit. That one will be
posted for the next post because I'm still in research
for the compatible with measuring heart rate and
pulse oximeter.