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Using 741 op amp and LDR to monitor light level

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biomedhed

New Member
Hello all,

I'm a PhD Physiology student from Manchester University and have an idea for measuring tissue density using a laser, a light dependent resistor and a 741 op amp.

It is probably worth saying that although I am fairly familiar with electronics, I am by no means competent....and basically I would be very greatful to know your thoughts on this idea and whether it would work....

In essense i intend to shine a 1mW laser through very thin sections of tissue on to an LDR. If tissue density decreases, more light will get through and therefore the LDR resistance will decrease. I want to amplify this change in resistance/current somehow and and am thinking of a 741 op amp.

The output of the 741 would then go into a computer via Powerlab which would record the fluctuating output.

The laser and LDR will remain in a fixed position and the tissue would be moved within the beam. Any change in tissue density within the sample should I guess result in a fluctuating current input into the 741

I have a few questions:

1)Would an LDR between the +ve rail and 741 input work?
2)Will the output from the 741 directly correlate with the input?
3)Will this work?!

Sorry if I sound a bit thick, cheers in advance.
 

Diver300

Well-Known Member
Most Helpful Member
Please use a good op-amp and not the 741, which dates back to 1968. There have been improvements in 41 years.......

LDRs are quite sensitive, and you might not even need an op-amp at all.

If the change in light is very small, you can amplify it with an op-amp, but you need to know what the zero point is. In the simplest form you feed the inverting input of the op-amp from a potentiometer and manually adjust the zero point.

You need some negative feedback on an op-amp or it becomes a switch with either zero or maximum output.

I suspect LDRs are fairly temperature dependent.

For best sensitivity, you need to turn the laser on and off (an optical chopper can do that) and you have a phase sensitive detector to give the output signal.
 

biomedhed

New Member
Please use a good op-amp and not the 741, which dates back to 1968. There have been improvements in 41 years.......

LDRs are quite sensitive, and you might not even need an op-amp at all.

If the change in light is very small, you can amplify it with an op-amp, but you need to know what the zero point is. In the simplest form you feed the inverting input of the op-amp from a potentiometer and manually adjust the zero point.

You need some negative feedback on an op-amp or it becomes a switch with either zero or maximum output.

I suspect LDRs are fairly temperature dependent.

For best sensitivity, you need to turn the laser on and off (an optical chopper can do that) and you have a phase sensitive detector to give the output signal.

Many thanks for your reply - I will definitely get a more modern chip!

Regarding the zero point, I think Powerlab will sort that out as we can set a base line on the program plus the experiments will always be carried out in same conditions i.e. in the dark - probably.

So far I have got to the stage below - i am quite positive I need some more components in there but really havent a clue what - any suggestions?

R3 is supposed to be the LDR (wasnt a symbol for it on the program)

Cheers
 

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