Dewey Mac here, awesome 12-year-old detective from Stoney Fork. This is the lie detector that I used on my sister in the book Dog Gone Dog.
Don’t be scared by words like resistors, capacitors, and weird symbols like Ω; electronics are easy. Once you understand the basics, there’s no end to the types of cool projects you can make! The Interrogator 3000 is a great project to introduce you to electronics, and with the provided circuit diagram you don’t even have to worry about confusing circuit boards.
The lie detector works on a principle called galvanic skin response. Basically, it detects sweat, which usually happens when someone lies. Sweat increases the conductivity (ability to let electricity pass through) of your skin. Your sympathetic nervous system controls the sweat glands on your fingertips and is completely subconscious (not controlled by you). When caught in a lie, these fight-or-flight nerves are alerted. Your body will sweat a little. The sweat will cause the electricity to flow in higher amounts and produce a higher pitched sound from the Interrogator 3000.
The problem with this lie detector is that you can’t fake lies. If I said, “My name is Barack Obama,” it wouldn’t increase the pitch. This is because my nerves were not aroused, I was just joking around. The more you can get someone to take your lie detector seriously, the better.
This is based on Slater Harrison’s lie detector on sciencetoymaker.org. Check out his website for many more fun and inexpensive gadgets you can make.
Full and partial kits can be purchased the Dewey Mac Labs store
Step 1: Materials & Tools
Materials: (Can be purchased here.)
- 82KΩ resistor
- 4.7KΩ resistor
- 3906 transistor
- 3904 transistor
- .01 mfd ceramic disk capacitor
- 9 volt battery clip
- 9 volt battery
- scrap cardboard
- 8 Ω speaker (many other speakers will work too)
- 18 gauge solid wire with jacket/coating
- 2 alligator jumper cables
- a printed copy of the paper circuit board
- card stock (like from junk mail)
- 2 paper clips (metal ones, with no plastic coating)
- glue stick
Step 2: Circuit Template
- Print a copy of the circuit board. Be careful not to enlarge it. For this reason, I've included a PDF to print from.
- Cut along the outside lines.
- Fold it over on the center fold line.
- Add glue to the underside of each half of the fold sections. Glue circuit template around a piece of cardstock from some junk mail. This will make your circuit board stronger.
- Place the circuit template flat on your scrap cardboard. Carefully use a thumbtack to poke a hole on every black dot. There are 16 total.
Step 3: Circuit Board
- Add the transistors (half circles) to their correct spot on the circuit board. Make sure that the flat part of the transistors faces you.
- Add the capacitor (round) through the labeled holes, it doesn’t matter which leg goes through which hole.
- Add the resistors (long and skinny). Make sure you match the colors. As long as you are using the correct resistor, it doesn’t matter which leg goes through which hole.
- Make a jumper cable by cutting a small section of wire and stripping the ends of the plastic coating. Insert your short (only a few inches long) jumper cable where indicated at the bottom of the circuit template.
- Poke your touch cables through the circuit template where indicated. Make sure each end is stripped. These touch cables will be attached to paper clips later.
- Turn the circuit template over. Following the images, twist together the wires and components making sure that twisted wires don’t touch each other.
Step 4: Speaker, Battery, and Touch Wires
- Twist the red wire from the 9 volt battery clip where indicated. Twist the black wire from the 9 volt battery clip where indicated.
- Add the alligator clips according to the template. One should connect to the positive (+) terminal of the speaker and then to the wires by “speaker (+).” The other wire connects the speaker’s negative (-) terminal to the wires on the circuit template near “speaker (-).”
- Twist the stripped end of the touch wires around the paper clips. Make sure it is the short end of the paperclip that has one loop, not two.
Step 5: Testing
- Place the paperclips together. Connect the battery to test the
circuit. If you don’t hear a sound, quickly disconnect the battery so you don’t burn out the transistors. Check that all of your components were added in the correct spot and make sure your transistors are facing the correct way. Continue to test and check until it works.
- Put a finger from each of your subject’s hands in the two paper clips. Ask them questions. Find the truth, the ugly truth.
Now, the issue with this lie detector is that you can’t fake lies. If I said, “My name is Barack Obama,” it wouldn’t increase the pitch. This is because my nerves were not aroused, I was just joking around. The more you can get someone to take your lie detector seriously, the better.
- Use the picture above to double check all connections.
- Check that all of your components are in the correct spot, especially the transistors.
- Check the transistor numbers match. Also check that they are installed just as the picture illustrates.
- Be sure that your twisted component leads are not touching.
- Check that your alligator clips are not touching a twist or another alligator clip.
- The transistor legs can easily get caught in another twist because they are so short. Check these leads especially.
Step 6: Build the Project Enclosure
Now is the fun part, making or finding a cool enclosure for your lie detector. I mainly use dollar store pencil boxes, because I'm cheap. I've also used a Mr. Potato Head doll, a cigar box, a can of SPAM, or really anything. Be creative. Make sure it looks intimidating to get the whole truth, and nothing but the truth. For the one in the picture, I used a pencil box and then used Mod Podge to glue torn newspaper parts to the box.
If you want, wrap the touch wires around a pencil or marker to give them a cool spiral look.