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