Simple Lie Detector

[Roff]

Thanks for the reply, Ron - I like your schematic alot. My class will be building these and your version will allow us to have smaller groups working on this since we will now have more power supplies to work with. I see how your particular circuit can be re-drawn to essentially be a wheatstone bridge circuit. However, I am still a bit stuck on how the original circuit can be represented this way with those two power supplies. Why did the designer feel that two power supplies were needed in this circuit?

They are equivalent as shown below. The designer may have used two batteries because he didn't feel any need to reduce the number of batteries, or didn't know how.

PS as Audioguru says, you can use 9V batteries.

 

[sippi3000]

can some tell me why onc i power up my lie detector my meter is at full power so i cant measure resistance

 

[Roff]

can some tell me why onc i power up my lie detector my meter is at full power so i cant measure resistance

Post your schematic and we'll take a look at it. What is your meter's full scale current?

 

[ismlamour]

thans>>>>>>>>>>>>>>>>>>>>>>>>>

 

[Science Mac]

hello fellow people i wanted to see how to build the circuit because of the connection between the second resistor and the analog meter

 

[Roff]

hello fellow people i wanted to see how to build the circuit because of the connection between the second resistor and the analog meter

What does this mean? If you can't read a schematic, describe what you need to know in detail.

 

[cheystar]

a noob question here. this circuit is placed on a board right? the ones with holes? Also wires used to connect each component is stranded 18 gauge insulated wire ok?

 

[audioguru]

18 gauge wire is too big for the small holes in a circuit board.
Use 22 gauge solid tinned copper insulated hookup wires.

 

[CuentaChocula]

Sources for the meter

**broken link removed**

https://www.allelectronics.com/cgi-bin/item/PMD-1MA/385/1_MA_DC_PANEL_METER_.html

Please let me know if anyone found one cheaper.

 

[nightcatz]

can I use any npn replacing the 2n 3565

hi ...electro master can I use s9014 in lieu of 2n 3565 ..pls help

 

[Roff]

hi ...electro master can I use s9014 in lieu of 2n 3565 ..pls help

S9014 will work just fine.

 

[Peter_wadley]

Great project!

 

[nightcatz]

hi guys can anyone tell me If I can use digital ammeter of the Digital Multimeter in lieu of the analog meter .......I dont have a analog meter in my place and allelectronics.com says it cannot ship the product to my place

 

[audioguru]

The lie detector project sometimes lies.
It measures the resistance of skin and the resistance of its connection to skin. It is supposed to use a 1mA moving coil meter so you can see the meter's needle move. A digital meter will just be a blur of changing numbers if anything changes.

 

[nightcatz]

thanks the info audioguru ......can u tell me if using the digital ammmeter can spoil the multimeter

 

[audioguru]

What will its max current of only 1.5mA do to a multimeter?

 

[hates]

It is a cool project

 

[audioguru]

It is a cool project

It measures the resistance of the connections to the victim, not if they are lieing.

 

[sheng_james]

nice topic. im gonna try this one.

 

[HiTech]

It measures the resistance of the connections to the victim, not if they are lieing.

It banks on the fact that a person may have sweaty palms while lying, creating a change in skin conductivity. A better circuit would measure pupil dilation and overall eye movement. Typically, the eyes don't lie!