Lie Detector

Circuit : Andy Collinson
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This circuit monitors the skin resistance of a person holding the two probes. If they are asked questions and you get an emotional response then the subjects skin resistance will drop and is indicated by the meter. lie detector circuit

This circuit is used to monitor very small changes in skin resistance or 'galvanic skin resistance'. Skin resistance with dry hands varies from person to person and can be as high as 100 kohms with dry skin to as little as 1k with wet hands. If a person is interrogated or asked questions that cause an emotional response then a small amount of sweat will produce a change in skin resistance. Commercial lie detectors monitor pulse and respiration as well, so don't expect too much from such a simple circuit.

The circuit is a basic two stage dc current amplifier. The probes need to be large enough to hold in each hand. You could use crocodile clips attached to old spoons (please don't use your best silver or you could be the one facing questions!) A larger contact area also alters measured skin resistance as does the subjects grip.

Once a person is holding both contacts adjust the controls so that the meter reads half scale. This may be difficult so this is why coarse and fine controls are used.

The circuit is battery powered and VR1, VR2 , R1 and the subject holding the probes form a resistance bridge. Current entering Q1 base via R2 is amplified and is fed to Q2 base. R3 is the load resistor and also the input impedance to Q2. The small changes in current are further amplified by Q2 and passed to the meter. The emitter resistor R4 stabilises the circuit against changes in temperature and R5 prevents excessive current flow in the meter. M1 is the meter and due to fast changes an analogue signal meter must be used. Although I have stated a full scale deflection F.S.D of 100uA, meters up to 1mA can also be used but with reduced sensitivity.

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