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# How does this circuit work?

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#### thetmagic

##### New Member
Hi, can someone help me understand how this circuit works, I know what transistor, collectors, and resistors are, but what are their function in this circuit, and how does it work part by part? And how accurate is it? I am doing project on this, to find out if skin perspiration changes when one lies. I also dont understand what the difference between an electrode and a regular wire is, please help.

https://www.electro-tech-online.com/threads/simple-lie-detector.40/

Thank You!

#### Dragon Tamer

##### Member
the average skin resistance of a healthy human is between 1.5 and 2.3M ohms. what this circuit does is when the person is lying their skin resistance goes down, which causes the current through the meter to go down. the reason it goes down is because Q1 is being opened further when the skin resistance goes down so less current is going through the ammeter. R2 is an adjusting resistor, and R1 is a current limiting resistor, maybe some of the other site members can do a better job at explaining than I can.

#### audioguru

##### Well-Known Member
Most Helpful Member
The resistance of the skin of each person is different and their reaction to lying or telling the truth is different. Many people sweat when they tell the truth. Some people do not sweat anytime.
The resistance of the skin of a person is not a reliable test of whether a person is lying or is telling the truth.

I had a quack doctor (Chiropractor) do a resistance test on my back. Where he pressed the probes hard then the computer showed a low measurement and he said I had a problem there. Where he pressed the probes lightly then the computer showed a high measurement and he said it was good there.
In the end he wanted $1500.00 to "fix" me. I ran away. Doors open and close, transistors don't. Transistors conduct more and conduct less or then turn on and turn off. #### thetmagic ##### New Member I had a quack doctor (Chiropractor) do a resistance test on my back. Where he pressed the probes hard then the computer showed a low measurement and he said I had a problem there. Where he pressed the probes lightly then the computer showed a high measurement and he said it was good there. In the end he wanted$1500.00 to "fix" me.
I ran away.
haha
the average skin resistance of a healthy human is between 1.5 and 2.3M ohms. what this circuit does is when the person is lying their skin resistance goes down, which causes the current through the meter to go down.
couldn't you just use an ohm directly instead? If you are measuring in ohms.

the reason it goes down is because Q1 is being opened further when the skin resistance goes down so less current is going through the ammeter. R2 is an adjusting resistor, and R1 is a current limiting resistor, maybe some of the other site members can do a better job at explaining than I can.
I need more help understanding. And also I need to understand how the circuit works urgently. I need to understand how this circuit works in and out by Wednesday, this is urgent, please help. I am not very experienced with electronics, so please bare with me. I really need help.

Thank You!

#### thetmagic

##### New Member
And what grade level do you think that this is for science fair?

#### thetmagic

##### New Member
Are you sure this would work?

#### audioguru

##### Well-Known Member
Most Helpful Member
Of course it "might" work. Some people sweat when they lie and other people sweat all the time, even when they tell the truth. Other people sweat only when they tell the truth because they usually lie all the time. Some people never sweat.

#### thetmagic

##### New Member
Of course it "might" work. Some people sweat when they lie and other people sweat all the time, even when they tell the truth. Other people sweat only when they tell the truth because they usually lie all the time. Some people never sweat.
Thank you, but can you help me understand how the circuit works part by part also?
And how would I go about building it?
Thank You!

#### audioguru

##### Well-Known Member
Most Helpful Member
You need to learn about resistors and transistors before you understand what each part does.

Build it on a small piece of perforated circuit board. You also need to learn how to solder electronic parts and wires.

#### thetmagic

##### New Member
You need to learn about resistors and transistors before you understand what each part does.
I already learned about them.

Thank You!!!!

#### thetmagic

##### New Member
Build it on a small piece of perforated circuit board. You also need to learn how to solder electronic parts and wires.[/QUOTE]
I will research about this.

Thank You!!!!

#### thetmagic

##### New Member
How does the circuit work?

Thank You!!!!

#### u4gubbins

##### New Member
in a nut shell,
The electrodes are just wires or conductive handles or something like that. basically you have a battery and a meter the resistors are there to balance out where the electricity goes
and the capacitor helps control the transistor.

#### audioguru

##### Well-Known Member
Most Helpful Member
How does the circuit work?
You need to learn about resistors and transistors before you understand what each part does.

EDIT: Guess what? It is an extremely simple circuit. Maybe you should study gardening instead of electronics.

Last edited:

#### thetmagic

##### New Member
You need to learn about resistors and transistors before you understand what each part does.
I understand about each part, but not how it all comes together. Please help me.

Thank You!!!!

#### audioguru

##### Well-Known Member
Most Helpful Member
At rest when the victim is not lieing, their skin resistance might be high so the skin resistance, R1 and R2 apply base current for the transistor to turn on a little. The trimpot R2 is adjusted so that the collector of the transistor is the same voltage as the junction of the two batteries so the meter reads zero.

When the victim sweats then their skin resistance becomes less which allows more base current to the transistor and the transistor turns on more which causes current in the meter.

#### Fayazahmed

##### New Member
Dear your circuit is very simple and if u know about the working of capacitor and transistor then you can easily understand the working principle of this lie detector.but you should not worry because now I am going to explain the working of this circuit.As u know that in the circuit the voltage applied to the capacitor is 4.5v dc but the voltage applied to the transistor is 4.5+4.5=9v.you know that capacitor always block Dc voltage and there fore in this circuit initially when the capacitor is charged by the 4.5 dc then there will be flow of electrons from the negative terminal of the 4.5 dc and enters into the emitter of the transistor and go out from the base of the transistor and come to the one plate of the capacitor and in this way there will be accumulation of the electrons on one plate of the capacitor.In the same way from the other plate of the capacitor the electrons move towards the positive terminal of the 4.5 v dc and in this way there will be lack of negative charge or access of positive charge on the other plate.So in this way one plate has negative charge and second one has positive charge and therefore within few movements all the voltage equal to he 4.5 v will store in the capacitor and then current will stop to flow.now you must remember that after charging the capacitor has a 4.5 dc voltage across it so now if any kind of resistance is placed between the electrodes or terminals of wires then capacitor will try to discharge through that resistance with the voltage of 4.5v.but here in the circuit the resistance will be the resistance of any human body.an other thing you must remember that is when capacitor was charging then some electrons coming from the negative terminal of 4.5 volt dc were also going towards the collector of the transistor and in this way those electrons passes through the meter and due to that reason initially needle of meter deflects when those electrons flow through the meter and come to the positive terminal of the 4.5 volt dc( down most voltage source). Dear you should remember that transistor only works in the condition if there is base current at the base of the transistor because transistor current (collector current) is controlled by the base current.if no body touches the electrodes then there will be no current at the base of the transistor and therefore the transistor will not passes the electrons coming from the down most supply.when any body touches the electrodes then base current flow which causes the flow of the electrons from emitter to collector and when about 98% electrons flows through the collector then some electrons passes from meter and deflects its needle.if the needle deflects away from the fix point or normal current at normal human resistance then we can estimate that the man who is touching the wires is telling a lie.Dear I try my best to explain.I pray that you will get the actual point.
Thanks
Fayaz Ahmed Memon
BS Electronics
from Pakistan

#### audioguru

##### Well-Known Member
Most Helpful Member
Fayaz,
The capacitor at the input has nothing to do with the circuit. It is just a filter.
If there is no strong radio station or mains hum nearby then the circuit will work without the capacitor.

If there is no resistance at the input then the transistor has no base current and is turned off. Then the meter reads backwards.

#### thetmagic

##### New Member
Thank you soooo much for your help!
Dear your circuit is very simple and if u know about the working of capacitor and transistor then you can easily understand the working principle of this lie detector.but you should not worry because now I am going to explain the working of this circuit.As u know that in the circuit the voltage applied to the capacitor is 4.5v dc but the voltage applied to the transistor is 4.5+4.5=9v.you know that capacitor always block Dc voltage and there fore in this circuit initially when the capacitor is charged by the 4.5 dc then there will be flow of electrons from the negative terminal of the 4.5 dc and enters into the emitter of the transistor and go out from the base of the transistor and come to the one plate of the capacitor and in this way there will be accumulation of the electrons on one plate of the capacitor.In the same way from the other plate of the capacitor the electrons move towards the positive terminal of the 4.5 v dc and in this way there will be lack of negative charge or access of positive charge on the other plate.So in this way one plate has negative charge and second one has positive charge and therefore within few movements all the voltage equal to he 4.5 v will store in the capacitor and then current will stop to flow.
I am okay up until here, but when you say "current will stop to flow", do you mean through the capacitor?

now you must remember that after charging the capacitor has a 4.5 dc voltage across it so now if any kind of resistance is placed between the electrodes or terminals of wires then capacitor will try to discharge through that resistance with the voltage of 4.5v
How does it help in the circuit when it discharges?

Dear you should remember that transistor only works in the condition if there is base current at the base of the transistor because transistor current (collector current) is controlled by the base current.if no body touches the electrodes then there will be no current at the base of the transistor and therefore the transistor will not passes the electrons coming from the down most supply.
Why no current? But doesnt some current pass there before touching the collector?

I understand the rest, but what is the use of the resistors?

Why do you need this circuit, why can't you just measure it directly with the meter?

And what grade level do you think this is (for Canada)?

I appreciate your help.
Thank You!!!!

#### thetmagic

##### New Member
Fayaz,
The capacitor at the input has nothing to do with the circuit. It is just a filter.
If there is no strong radio station or mains hum nearby then the circuit will work without the capacitor.
Now I don't know who is right :-(

I appreciate your help also.
Thank You!!!!

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