The circuit uses skin resistance to detect whether a subject is under stress or not. When a person is under stress (i.e. from lying), there is a noticable change in skin resistance that can be detected. If the person does not care about being found out, and doesn't care that they are lying, it will be difficult to be able to detect the lie.

Schematics

Parts

Component	Name/Description	Quantity
IC1/IC2	741 Op-Amp	2
	MicroAmmeter (Needle type with zero at centre)	1
Resistors	See diagram
	Probes - See Notes	2
	Conductive Paste - See Notes

Notes

You need to construct suitable probes. You can create probes by soldering a piece of wire to the pin on a large brass drawing pin, and using the smooth top as the contact with the person. You will need to make two of these. Also, you need to make some conductive paste. This can be done with:

2 teaspoonfuls of flour
1/2 teaspoonfuls of salt
8 tablespoonfuls of water

Put the flour and salt in a cup and add a little of the water. Stir vigorously to make a stiff paste. Then boil the rest of the water and add it to the cup. Stir the mixture well as you pour on the boiling water. Pour the mixture back into the saucepan and boil it for a few minutes, stirring it continuosly to avoid burning. Then pour it back into the cup and allow it to cool before use. It should set to a stiff jelly-like consistency.

To use: Let the subject sit in a comfortable position with an arm resting firmly on a cushion or table. Attach the probes about 5cm apart on the skin of the forearm. Put a small quantity of conductive jelly on the electrode. Press the electrode firmly on the skin so that the jelly spreads out over the whole area of contact. Use sticky plaster to strap electrodes to the skin. It is important the subject sits still. Adjust the variable resistors until the needle is as close to zero (central) as possible.