lie detector test

[langging]

**broken link removed**

 

[Reloadron]

It's a novelty type kit. So what's the question?

Ron

 

[cowboybob]

Sweat detector.

 

[audioguru]

Some people sweat when they tell the truth. Many are wrongly put in prison or are hanged.

 

[Mr RB]

Lie detectors don't measure lies they measure stress. Some people stress from certain conversations even if they are innocent. Some people can be without stress even if they are very guilty. So lie detectors need a trained psychologist preferably with many years experience. And even then they will get false positive or negatives from people with unusual stress responses. There is a good reason that a courtroom to decide innocence or guilt is not just a little room with a lie detector in it.

 

[langging]

can you help me explain this circuit.?

 

[DerStrom8]

can you help me explain this circuit.?

The transistors act as a sort of comparator. The whole thing monitors the resistance between the probes. If the "comparator" senses that the current is above a certain value, TR2 conducts and the green LED is on. If the resistance drops, due to sweat (supposedly a sign of stress due to lying), TR3 conducts and turns on the red LED. I have made this before. It is not very accurate, as you can sweat for reasons other than lying. Professional ones measure heart rate, blood pressure, eye movements (sometimes), and several other things, and are still only correct about 60 or 70 percent of the time. I wouldn't put much, if any, faith in this circuit.

Regards

 

[langging]

ron, can you help me explain this circuit?

 

[langging]

what causes the tr1 to turn off when it detects sweat?
is it true that base voltage of tr1 must be greater than its emitter voltage to make tr1 turn on?

 

[langging]

thank you for your help..

 

[DerStrom8]

what causes the tr1 to turn off when it detects sweat?
is it true that base voltage of tr1 must be greater than its emitter voltage to make tr1 turn on?

No, that is not true. A common bipolar transistor requires 0.7 volts on the base in order for it to conduct. It has nothing to do with the amount of voltage through the device.

TR1 takes the amount of current on its base (which is determined by the resistance between the probes), and controls the amount of current that can flow between the collector and emitter. When the current is low (high resistance), it is only able to switch on TR2 (and turn on the green LED). However, when the current rises (low resistance on the probes), more current is allowed to flow, and if there is enough, it is able to switch on TR3 (and the red LED).

 

[Reloadron]

Hi Again,

OK, the following is taken from your link:

How the Lie Detector Circuit Works

The circuit diagram of the Lie Detector is shown above. It consists of three transistors (TR1 to TR3), a capacitor (C1), two lights or LEDs (L1 & L2), five resistors (R1 to R5), and a variable resistor (VR1). Suitable transistors to use are BC547, BC548 or BC549, or any other small NPN transistor.

The Lie Detector circuit works based on the fact that a person's skin resistance changes when they sweat (sweating because they're lying). Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more.

Resistors R1 and R2 form a voltage divider. They have resistances of 1 000 000 ohms (1 mega ohms) and, because their values are equal, the voltage at the upper probe wire is half the battery voltage (about 4.5 volts).

A person holding the Lie Detector probe wires will change the voltage at the upper probe wire depending on their skin resistance. The skin resistance is in parallel with R2 and, because it is likely to be similar to or smaller than R2, the voltage at the probe wire will fall as skin resistance falls.

Capacitor C1 functions as a smoothing capacitor and removes the 60Hz induced mains hum that is found on a person's body.

TR1 and R3 form a buffer circuit (called an emitter-follower). The voltage at the emitter of TR1 follows the voltage at the probe wire and is now able to drive transistor TR2.

Transistors TR1 and TR2 act as a voltage comparator. If the voltage at the base of TR2 is higher than at the base of TR3 then the green LED (L1) will come on. If the reverse is true then the red LED (L2) will light.

To test the Lie Detector hold the probe wires. Adjust VR1 until the green LED is just on and the red LED is just off. This is the point at which the voltage at the base of TR2 is just greater than at the base of TR3. Now use moist fingers to hold the probes. This lowers the skin resistance and causes the voltage at the base of TR2 to fall. The voltage at the base of TR3 is now greater and the red LED comes on.

I added a link for emitter follower. What other parts do you not quite understand? Any specific part of the circuit?

Ron

 

[langging]

, why is it that when i test this circuit the green led is always on even if my hands are wet?
when my hand is dry, green led is on!! but when my hand is wet red led is on and also the green led is on!!
how can i fix this so that only the red led will on when my skin is wet?
tnx

 

[DerStrom8]

, why is it that when i test this circuit the green led is always on even if my hands are wet?
when my hand is dry, green led is on!! but when my hand is wet red led is on and also the green led is on!!
how can i fix this so that only the red led will on when my skin is wet?
tnx

In this circuit the green LED will always be on, even if there's "a lie". You will only be able to tell if the subject "is lying" if the red led turns on. You could change this if you used a dual comparator sort of circuit. If you connect two coparators together (or use one LM393 or LM358 op amp), you can connect one output to the green LED, and the other to the red LED, and criss-cross the inputs. Unfortunately I do not have a schematic at this time, but I will look for one. I've seen and built them before.

Regards

 

[langging]

thanks!! i will wait for the schematic diagram!! thanks a lot !

* regards pud ko!! haha pero atik rah!

 

[langging]

what is the function of the three resistors R3, R4 and R5?
i'm just a little bit confused!! especially R4 (470 ohms)!

* thanks

 

[langging]

what is the function of the three resistors R3, R4 and R5?
i'm just a little bit confused!! especially R4 (470 ohms)!

* thanks

 

[DerStrom8]

what is the function of the three resistors R3, R4 and R5?
i'm just a little bit confused!! especially R4 (470 ohms)!

* thanks

R3 and R5 are used to limit the current to the base of the comparing transistors. They make it so that they will only conduct when there is just the right amount of current. They help determine the balance between the proper ("non-lying") resistance and the lower ("lying") resistance of the subject's skin. R4 is the current limiting resistor for the LEDs. They make it so that the LEDs don't burn up by passing too much current through them. You will almost ALWAYS need a resistor to limit current to LEDs, but it depends on the source. You can use Ohm's Law, the source voltage, the LED voltage drop, and the LED's required current to calculate the necessary resistance. The formula is R = (Vs - Vled) / Iled, where R is the resistance in Ohms, Vs is the source voltage, Vled is the forward voltage (AKA voltage drop) of the LED, and Iled is the LED's required current.

 

[DerStrom8]

By the way, here is a schematic showing how to connect an op-amp as a comparator. This thing is very poorly designed, though. I don't know why he put those resistors in parallel, and there are some other things that don't really make sense. I would use this ONLY to see how to wire the op-amp:

View attachment 61222

I'll see about designing one properly, but it may be a little while. I'm fairly busy today

Regards

P.S. You should be able to replace R3, R4, and R5 with a single 1K resistor. Not sure why there are 2 1Ms and 1 1K in parallel....

 

[langging]

thank you very much sir!! thanks !! its really a big help to my project!
* and about your design sir, ill just wait for it !!
i know your busy but thanks for effort in answering my questions!! thank you