Fun With a Lie Detector!

Is your friend or spouse lying to you? Build this Lie Detector and find out the truth! Great for parties too!

This is the FIRST Lie Detector project on Instructables.

Watch the video and then build it...

Step 1: Parts you will need...

I picked up all of the components at Radio Shack except for a few I list at the end.

1. Project Enclosure
2. 4.7K Resistor
3. 82K Resistor
4. 0.01uF Capacitor
5. 2N3904 Transistor
6. 2N3906 Transistor
7. 2-AA Battery holder
8. Small circuit board

Spare PC speaker, velcro, aluminum foil, wire and AA batteries in my extra parts bin.

Comments

[ETfonehome]

How do you tell if someone is lying to you? Does the speaker change tone or something like that?

[punom]

where the R2 resistor ends?

[tceferino]

Pienso, que el final de R2 es tu dedo, cuando aterrizas con R1.

[dog digger]

It goes to a finger pad. The line sticking out in between C1 and R1 goes to the other finger pad

[Kzummo]

An 82k resistor? Is that what that is? Because it doesn't at all look like it? PLEASE HELP!

[MrMistermister]

I tested this with a breadboard, and when I put on the finger pads the speaker makes noise, but the tone slowly goes up and up and it doesn't stop for, like, 30 seconds. I tried lying with it but the speaker didn't change tone :/

[MrMistermister]

Is there a switch? Or does it turn on when you put on the finger pads?

[gmarian]

What can substitute 2N3906 PNP transistor?

[toast5678]

2N3906 are NPN transistors, and no no other transistors can replace it in this particular circuit

[rhapsodium]

great tutorial. used a breadboard instead of a circuit board, worked fine. the computer speaker had too much resistance, though, so i just bought a tiny eight ohm one from radioshack.

[lovechickens]

hey could i use a breadboard instead of a circuit board to build this?

[karatekid99]

yes

[robofish723]

Same here, there isn't even a peep coming from the speaker, anyone haveany help?

[mshahzeb]

i have same problem no sound is coming from speaker.

[raza896]

Will it help to catch the 9/11 attackers??????????

[cwart81]

I tried to build this lie detector. But I'm not getting any sound from the speaker. Can any one help me figure out what I did wrong? Thanks.

[soapdude]

(Nooby question) Can the circuit be made on a breadboard? (Even more nooby and irritating question) Anyone want to either explain how to put together this schematic or point me to a webpage that explains it. I've looked everywhere, and I found useful legends of various symbols but not how to place them. Thanks in advance.

[punom]

i have made this accordingto the video says but the speaker is not even making a sound!!!!!

[dwatord26]

could you post a picture of it on the circuit board. I have yet to learn how to read these

[TheIronHobo]

well try first. this was actually the first circuit i ever built(it worked) its pretty easy when you read the key like in a map and it just takes some think work

[chisse]

Hi im bilding your Lie Detector but I cant get it to work. I added the Transistors the Resistors and the capacitor and my speker. But not sure where the batteries go. Also can the Resistors be 1W? If some one can help me out that would be cool Thanks.

[brooklynlord]

The wattage of the resistors just means the maxium power the resistor can handle before it overheats and (literally) blows up. So yes, they should be able to be replaced with 1W resistors without any problems, even though it is a quite big overkill.

The batteries are at the top of the schematic and they are hooked up to the PNP transistor's emitter and the emitter of the NPN transistor and the speaker.

[oweng4000]

Will 47k resistor work instead of the 82k?

[oweng4000]

Thx kipkay.  I will be using this as a music maker.

[microboard]

Can you solder on  an LED where the speaker would go so when someone tells a lie the LED flashes?

[doomfire]

can I use any other PNP tranzistor cuz I cant get a 2n3906 what PNP tranzistor will work??

[M4industries]

Are the Emitter and base pins connected on Q1?

[godfreyandthandi]

no they are not.

[patapon]

Finely I can prove I'm telling the truth!

[turbokids]

OK hooked it together (can send the schematic from Radio shack circuit board) but there is no sound unless I short the first Transistor across the emitter to collector. Then I hear sound from spkr.

[jasonad]

i am having the same problem. No sound unless i short the 2N3906 transistor. What am I doing wrong?

[Jodex]

I had to face that too. When I didn't have anything to do I just tried to put another capacitor there (just for fun) and it somehow started working..(?) And it worked somehow when I made somewhere a short... So keep on trying!

[iacon]

Well I think I got it working, the tone doesn't change when someone lies though. :P Might have to re-examine the circuit.

[sm7078]

it Work for me

[deepcore]

Ok, I am fairly sure i have made this circuit correct. I am, however not getting any sound at all, tried both 4 and 8 ohm pc speaker. The only sound i am getting is a faint "click" in the speaker when i touch the pads. Any ideas what might be wrong? Had slight problem understanding which way to turn the transistors, but found some info on the net stating that the side with the arrow (emitter) is on the left side if you hold the legs towards yourself and the flat side up. Can anyone confirm this?

[Hawaii00000]

Is there any way this can be modified to turn an LED on and off? What about taking out the capacitor?

[cooldog]

can we replace the 2N3904 Transistor with a 2N3906 Transistor

[oldrocker]

No the 3904 is a NPN transistor and the 3906 is a PNP transistor so it needs both transistors to work.

[acaz93]

Hey , Kip .. Remember The Comment That Says It needs a tuning pot It's actually simple , just put (instead of 82k resistor ) a 50K Potentiometer and a 33k Resistor In series , And That's It , VOILA tuning ready!

[instructa-fan]

can you get the pentiometer thingy at radio shack as well as the 33K resistor? its kind of convenient I read now cuz thats the one thing i don't have yet! lol!