Homemade Lie Detector Test

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A lie detector test, or a polygraph, actually consists of a number of tests that a person undergoes during questioning to try to determine whether or not they are telling the truth. Polygraphs measure physical reactions to stressful situations, such as pulse and perspiration; theoretically, if a person is more stressed by certain questions, he is lying. If you’re looking for an easy science project or a fun game to play with your friends, you can build your own lie detector.


Remember that building a homemade lie detector will take some basic knowledge of electronics and wiring, and also some electrical devices. Many of these parts can be found in hardware stores, home improvement stores or electronics retailers such as Radio Shack, and also through Internet retailers. The types of parts here are listed based on suggestions from the teachers who support and write for the school-based Web site www.sciencetoymaker.org.

If you are working with students or children for a science project, make sure that they understand the safety of using these electronic devices and tools such as soldering irons, or make sure you do the dangerous work yourself.

Each lie detector needs two small signal transistors, two ¼ watt resistors, a small ceramic disk capacitor, a small speaker, a nine volt battery and snaps, 18 gauge wire, an electric solder and a soldering iron, 100 percent clear silicone caulk and a clear flexible plastic sheet.


The Science Toy Web site provides a map and pattern for the lie detector project; you may want to print it out to make things easier. Cover the pattern with the clear plastic sheet. Cut the lines from the pattern out of the plastic sheet; they are eight strips, about three inches long and a quarter inch wide. Gently sand the plastic with fine grit sandpaper and squeeze silicone caulk in a wavy pattern over the plastic. The roughness the sandpaper causes on the plastic will help the silicone stick. Spread the silicone around with a flat piece of cardboard or putty knife.

Strip the insulation off of about 20 inches of the wire, and cut it into eight two-inch strips. Spread another line of silicone at each end of the pattern lines and insert the stripped wire pieces onto each of the eight pattern lines. Make sure the wire is straight, or it may cause the lie detector to short circuit.

Remove the pattern from the silicone. On the plastic side of your project, number each of the wires one through eight, starting on the left. With a small thumbtack, poke holes just beside wires two through seven; these six holes will later hold the transistors.

Insert the transistors. These pieces have three legs, and the legs need to spread out to resemble a pitchfork in order to reach all three wires they are supposed to touch. Make sure the legs of the transistor are touching the copper wire, then solder them into place. When you have secured the transistors to the wire, cut off any extra metal from the transistor legs.

The capacitor is attached the same way as the transistors, with holes allowing it to touch and be soldered to wires four and eight.

Cut two more pieces of wire, each about an inch and a half long, and strip about a third off of one end of each wire. These will connect to the speaker. Loop the stripped ends through the solder tabs and solder them into place. Again, remove any excess wire. Connect the other ends of these wires to wires four and five on your project.

Connect the nine volt battery to the battery snap. From the battery snap, the red lead will go to wire number two and the black lead to number five.

Cut two more pieces of wire and strip the ends; these are the touch wires that the subject will hold to determine if he is lying. Length can vary depending on how far a reach you want it to have. These wires should connect to the project at wires one and eight.