# Electronic Lie Detector Circuit

This electronic lie detector circuit project will give two readings: one for difficult questions for the subject and another to show its emotional state in general.
The emotional states are detected not only by heart beat accelaration and trembling hands but also an increase in skin humidity whose resistance decreases causing the entry into operation of the lie detector.

Two electrodes can be used as a flexible wire, bare, wrapped around fingers or wrist.
In order to not influence the measurement result the device must be powered from two 9 Volts batteries.

Each change in resistance, and therefore the voltage at the input circuit will be amplified by operational amplifier A1, which also serves as separator. The output signal will determine, by R3, a deviation of the measuring instrument.

General emotional state of a person can be assessed by measuring the average resistance of the skin over a period of time. The indication is provided by an indicator instrument connected to point B of the circuit. Operational amplifier A2 is connected as an integrator and allows the circuit to automatically adjust according to the average resistance of the skin.

Length of time to measure the skin resistance is determined by R5, C2 and C3. Until such time elapses, the lie detector gives no indication although diodes D1 and D3 provide a rapid response of the circuit.

## Human skin lie detector schematic

Potentiometer P1 allows you to adjust the time delay of the circuit. Since skin resistance varies from one person to another, may be necessary to change the resistance value R1. This resistance can be replaced with a potentiometer.

Reading a great value to the instrument connected to the output of B indicates that subject’s skin resistance is low (which is a characteristic of people with sticky hands) and it is advisable to reduce the value of R1.

lie detector components list
R1 = 47K
R2 = 1M
R3 = 3.3K
R4 = 10K
R5 = 10M
P1 = 10K
C1 = 100n
C2 = C3 = 470n
D1 = D2 = 1N4148
IC1 = IC2 = LF356

## Related Tutorials

• akhilesh

can u tell me after success full febricaion according to circuit diagram it will work ??

• Chris

I built my first electronic project at age 11, about as ‘complex’ as the above project. Reading the comments here, I can’t believe that some of you guys, cannot build the project from the diagram, but seem to be wanting a pcb or veroboard layout done for you.
From age 11 I was building projects from electronics magazines, and in most cases, even after careful construction, the projects did not work. I would then have to go through the project step by step, myself, to get it working, which I invariably did.
And it was the best way to learn.
I suggest some of you guys just try and put a bit of effort in, you can do it!

• ross

my cock stings is this the site were i get help?

• will

youre all retarded

• karen

what’s the use of the 10nF capacitor
reply ASAP my project’s due in an hour

• juti kankana mazumdar

can you please explain the working of the circuit in details?

• Jim Keith

Good suggestion!

Hi all you guys doing this as a school project :
I have a suggestion for you.
make the project and see how well it works, experiment withsome of the components and chart the variations – then – ask the writer to help you out with suggestions relating to what you saw.
Otherwise you are just connecting components together and not learning much. I do not mean to lecture you but try it – thats the joy of Electronics asa Hobby.

• kkrish

hai….i’ve designed circuit as per the given circuit diagram by u…..but i can’t understand from where can i give the power supply…..??and how can i get the reading…..??and where to connect an LED and a buzzer to be blown when the persons says lie…….??plz give rply to my mail…….

• charles

plz can u send me the objective

Looking for the latest from TI?