How does this circuit work?

[thetmagic]

But isn't biasing if you put negative in the n lead and positive in the p lead, how does a resistor help with that?

Thank You!!!!

 

[BrownOut]

I don't see any point making this more complex. It's basically a demonstration project, and it works on a physiological principle that might very well vary between subjects. A real lie detector would include calibration facilities and probably measure more than just skin resistance. I don't know of any other circuit that does the same thing better. This is most likely sufficient for you project.

 

[BrownOut]

But isn't biasing if you put negative in the n lead and positive in the p lead, how does a resistor help with that?

Thank You!!!!

The resistors change the voltag according to the equation V=I*R where V is the voltage drop caused by the resistor, I is the current and R is the resistance. You're going to have to read some information on DC circuits and transistor biasing. Then come back and ask questions about anything you don't understand from your reading.

Good luck

 

[thetmagic]

I just wanted to make something that would impress the judges for science fair, but maybe what would impress them is me talking about transistors, pn junctions and etc. What do you think I should do? I will also talk about the sympathetic nervous system, and the phisiological side of this project, which I think will impress them, but I also wanted to impress them with the electronics, how should I? Do you think tha I should talk about siconductors, pn junctions and etc? Or what?

Thank You!!!!

 

[thetmagic]

The resistors change the voltag according to the equation V=I*R where V is the voltage drop caused by the resistor, I is the current and R is the resistance. You're going to have to read some information on DC circuits and transistor biasing. Then come back and ask questions about anything you don't understand from your reading.

Good luck

I have researched about it before this discussion, but now I did a little more. I still don't understand how the resistor helps the biasing.

Thank You!!!!

 

[Gary B]

Ok, so let’s see if I can explain the resisters clearly enough for you.
R1 in series with the victim’s skin provides the forward bias for the base lead of the transistor. R2, the resistance of the meter and R3 together provide the collector bias. R2 is adjustable so you can set a “truth” reference at about half scale before asking real questions.

I believe the resisters were the only outstanding question you had about this circuit.

 

[thetmagic]

Ok, so let’s see if I can explain the resisters clearly enough for you.
R1 in series with the victim’s skin provides the forward bias for the base lead of the transistor. R2, the resistance of the meter and R3 together provide the collector bias. R2 is adjustable so you can set a “truth” reference at about half scale before asking real questions.

I believe the resisters were the only outstanding question you had about this circuit.

Hi, thank you for your help! you are right the resistors are the only outstanding question that I have, but I thought resistors had no polarity, so how can they forward biase the transistor?

Thank You!!!!

 

[audioguru]

The transistor is an NPN. N for collector, P for base and N for emitter.
The base-emitter is a PN diode that is forward biased so it conducts current when the resistors make the base more positive than the emitter. The resistors that feed the base limit the current so the transistor does not blow up if the input is shorted.

 

[thetmagic]

The transistor is an NPN. N for collector, P for base and N for emitter.
The base-emitter is a PN diode that is forward biased so it conducts current when the resistors make the base more positive than the emitter. The resistors that feed the base limit the current so the transistor does not blow up if the input is shorted.

How does the resistor make current more positive?

Thank You!!!!