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By raf
#65553
Hey guys,

I'm trying to make a galvanic skin response sensor to get some biometric data. I stumbled upon this schematic:

http://web.media.mit.edu/~msung/VitaMon ... matics.htm

It is using a OPA4366ea amp. I'm wondering if I can use something else that will do the job...
I have LM741cn - just a normal "radio shack" opamp. Will it do the job?

Sorry if I sound totally idiotic, I'm not too savvy with all this stuff!

Thanks guys
By VStar650CL
#65614
Depends how accurate you want your reading to be, and on what will be powering your circuit. The OPA4366 is micropower for battery operation and has extremely low bias and offset. But if you're only looking for differential response (i.e., like a polygraph) and running off a wall-wart, then the 741 would probably do it. You might need to scale the R's to lower values and drive a bit more current. Be careful to provide a nice, flatline supply no matter what op amp you use... ripple will probably have a major effect on a circuit like that.
User avatar
By bigglez
#65704
raf wrote: I'm trying to make a galvanic skin response sensor
...
I'm wondering if I can use something else that will do the job...
I have LM741cn - just a normal "radio shack" opamp. Will it do the job?
Probably not. The LM741 has been superceded by
many op amps that have superior performance.

Before constructing any 'found' circuit hoping it
will work, you could try a few experiments with a
DMM and a human volunteer.

Notice in the linked circuit that the subject is biased
with a regulated DC voltage and 470k resistor?
This creates a reference current in the subject, which
is then converted to a voltage and fed to the
differential amplifier circuit.

To work the circuit requires a high input impedance
(1Meg or greater), and some means of cancelling the
common-mode voltage from the 'earthy' probe.

A FET input (Operational) amplifier would be better suited.
By VStar650CL
#65743
Good advice on all counts. You may not need anything fancy anyhow... an active GSR device is nothing but a specialized ohmmeter, and there are plenty of "build an ohmmeter" app notes out there. It takes no high voltage and a surprisingly small amount of current to pass through human skin. Some years back I designed a maxillofacial nerve stimulator for a client, used by surgeons to locate nerves in the face during cosmetic surgery so as not to sever them. The current limits were 900uA on "low" and 3mA on "high," and I can tell you the 3mA setting was enough to "jump" your finger nicely when you hit the nerve, even on bone-dry skin. So I think any ohmmeter design you find that drives under 1mA will be quite safe and work nicely. You might want to browse the TI and National websites, both have a great many useful op amp designs.
User avatar
By leon_heller
#65745
Many years ago I was asked to design an EOG system (detecting eye movement potentials) and used a pair of 741s for my initial tests. The system I prototyped used very expensive medically-approved isolation amplifiers with silver/silver chloride electrodes and an isolated power supply.

Leon
By teprojects1
#200483
raf wrote: Fri Feb 06, 2009 7:15 pm Hey guys,

I'm trying to make a galvanic skin response sensor to get some biometric data. I stumbled upon this schematic:

http://web.media.mit.edu/~msung/VitaMon ... matics.htm

It is using a OPA4366ea amp. I'm wondering if I can use something else that will do the job...
I have LM741cn - just a normal "radio shack" opamp. Will it do the job?

Sorry if I sound totally idiotic, I'm not too savvy with all this stuff!

Thanks guys
Hi,

LM714 is an op-amp used mostly in audio amplification appliances. If you wanna work on it then you should have a look at LM741 Pinout and also download its Proteus Simulation. That will help in understanding it.

Thanks.