Question

We want to build a circut that measure the resistance of the human body and convert it to voltage and the circut should have a led that work when the resistance of the body is low. And also we want to use a 741 op-amp

And the measured resistance of the body can be represented by a variable resister(the concept of our circuit is to measure galvanic skin response )

Answer 1

First of all, you need to measure skin resistance in normal conditions. Suppose it's value fluctuate between 40k-50K Ohm.

And in any abnormal condition resistance suppose value drops below 5K ohm.

In the above circuit "Rh" is the human skin resistance. And R1, R2, R3 need to be calibrated.

OP AMP is working in open loop condition. Whenever non-inverting voltage v1 is greater than v2 there will be output Vcc which is +5volt taken.

In Normal Condition:

Let R1=20k Ohm

Rh=40K ohm

Using Voltage division rule:

V1=5*(20/20+40)=1.67v

Let R2=R3=10k

V2=2.5v

V1<V2 => output will be zero/ LED will be off

In Abnormal Condition:

Let R1=20k Ohm

Rh=5K ohm

Using Voltage division rule:

V1=5*(20/20+5)=4v

Let R2=R3=10k

V2=2.5v

V1>V2 => output will be +5v/ LED will be ON

NOTE: The above circuit will work properly. You just need to calibrate the resistance values as your requirement or resistance

of skin. If there is any doubt please ask in the comment section.