help with a IR circuit please!!!

[vish_beatle]

Hey!

I'm working on a photoplethysmogrpah circuit.

I have an IR Transmitter-Receiver pair, and the output is either zero or a fixed value, i.e just two states. I want the output to vary.
For instance, if I were to insert a piece of paper, or my finger, I want a corresponding change in output. Rather like a finger plethysmograph.

Can anyone suggest to me what kind of biasing I shall have to provide.
Any further suggestions would be welcome of course!
Anybody?

 

[Nigel Goodwin]

What type of receiver is it? - a photo-diode, photo-transistor, or integrated circuit.

 

[Russlk]

You will have to change the receiver. Do a Google search for "analog IR receiver".

 

[vish_beatle]

thnx...ckt better!

thnx a lot for those replies...well actually i was using a photodiode.

now however, i have switched to a phototransistor.
i have an ir led-phototransistor ckt now.

i have to follow it up with an amplifier and a bandpass filter. i guess a gain of between 80-100 would be adequate for the amplifier. as i want to use opamps to deisgn them, could you suggest to me any precision opamps?

at present i have IC741, a very basic 8-pin opamp which has an offset voltage problem. so i guess i shall need a precision opamp...

 

[Russlk]

Offset voltage can be trimmed out, it should not be a problem. Can you post your circuit?

 

[vish_beatle]

well i went back to my old photodiode ckt...it worked fine!
i'm still getiing very small changes in voltage at the output so am planning on designing a filter with a gain of about 10...i'm doing it with an opamp...my lecturer sez it wud be gr8 if i cud design a s/w for it...
the idea is like this:

i hafta use the ckt on several ppl and make a sorta database...classify the ppl based on age, size of finger, etc. so in case of occlusion, some erratic reading will be observed in the output.

for that i need some s/w...i have heard of a few, diadem for instance...
any idea what i could use? i'm an undergraduate student, so buying some licensed software could be out of the question.
would it be possible for me to analyse the o/p using matlab? my college has the s/w...

thnx!