Classic Computer Magazine Archive ANTIC VOL. 4, NO. 11 / MARCH 1986



State-of-the-art Sherlock Holmes


Never again will you need to jump the gun when making accusations. Just round up a set of Atari paddles, some insulated wire, a phillips screwdriver a pair of needle-nosed pliers, two pieces of aluminum foil, two rubber bands, four alligator clips, any Atari computer and your favorite pathological liar. That all it takes-plus this BASIC program- to build your own biofeedback monitor lie detector. You don 't need to know how to solder and you don't have to worry about "frying" your computer (or your suspect!) if you make a mistake. However, Antic assumes no responsibility for improper or irresponsible use of this project.

It was one of them dive neighborhoods so bad, even the crooks drove around in police cars. I brought Mark "The Shark" Quark to Ace's Bail Bonds and Bar and Grill to get the bird's eye lowdown. I cased out the joint. It was so sleazy, even the guns had guns. I pulled "The Shark" into the back room.
   "Where were you on the night of November 20?" I asked.
   "Get that bright light out of my eyes," he said.
   "I have ways of making guys like you cough up the cold, hard facts," I said. "They don't call me the Lie Detective for nothing." I whipped out my secret weapon.
   "Aaaaaaggh!" he screamed. "Not an Atari computer! A Trash-80, an Adam, a Commodore Amiga, ANYTHING but..
   Gone were the days when the tools of the detective's trade consisted of little more than a trenchcoat, a notepad and a well-oiled revolver. I knew I hadn't wasted my time typin' that short listing and checkin' it with TYPO II. I wrapped the electrodes around his fat fingers. And I'll tell ya, he talked all right, he spilled his guts...

First, make two electrodes to be attached to the fingers of your willing "subject" (or suspect, if applicable). Cut two pieces of insulated electrical wire in two-foot lengths, and strip the ends of each piece. Attach alligator clips to both ends of each wire.
   Cut two 4 x 5 inch rectangular sheets of aluminum foil. Lay one sheet flat with the shiny side down, and fold it in half lengthwise. Now fold it in half crosswise, twice. You should have a shiny pad about 1 1/4 inches wide by 2 inches long. Repeat these steps on the other piece of aluminum foil. Attach one alligator clip from each wire to the short edge of each electrode. Now you're ready to hook the electrodes to the paddle controller.
   First some detective work. Figure out which paddle controller is paddle 0. Plug the paddles into joystick port 1 and sit in front of your computer. The cord coming out of the left side of the paddle plug goes to paddle 0. On an Atari XL, it's the cord nearest you. With a phillips screwdriver, remove the back cover of paddle 0.
   Inside you'll find a little switch, a red plastic button, some wires, and a circular component with three terminals on it. It's called a potentiometer, or "pot" for short. There are wires soldered to two of the pot's three terminals. Clip your alligator clips to these two terminals. Make sure the clips don't touch each other or the unused terminal. Now turn the paddle knob all the way COUNTERCLOCKWISE.
   This is a good time to type in Listing 1, LIAR.BAS, check it with TYPO II and SAVE a copy before you type RUN.

Now it's time to put your Atari lie detector to the test. After you RUN the program, you should see a light green screen with a dark horizontal line in the center. In the text window is the skin conductivity readout. This should read 000. If it reads 227, your electrodes are probably touching. Separate them. If it still reads 227, check your alligator clips. They shouldn't touch each other or the other terminals on the "pot". If you still don't get 000, make sure the paddle knob is turned all the way counterclockwise.

Wrap the aluminum foil electrodes around any two of your "guinea pig's" fingers. NOTE: If the electrodes touch each other, it will throw the readings off. You can eliminate this problem by placing them at least one finger apart.
   The electrode should face the "sweaty side" of the finger, not the fingernail side. Secure each electrode with a rubber band, but not too tightly... you don't want your subject's fingers to fall off! Most people won't feel the five volts of current that runs though the paddles. However, if your subject feels anything more than a mild tingling, disconnect the paddies IMMEDIATELY and check to be sure you assembled the project correctly.
   The graph on the monitor should now show the approximate stress level. Skin conductivity will increase if the person is nervous or physically active. Jogging in place will usually send the graph up, relative to how out of shape the subject is. As the person relaxes, skin conductivity will decrease, and the graph will fall.
   The bottom 32 lines of the graph are hidden by the text window. If you wish to see them, press [SELECT]. Press [SELECT] again to get the text window back. Press [OPTION] To move the fixed horizontal line. This will move the line to the present graph position. To pause the graph, press and hold [START].
   As listed, the graph program will display all possible paddle values. This is fine, but sometimes small changes in conductivity won't show up on the graph. If you wish to magnify these changes, you must change the MAG variable in line 10. For example, MAG = 2 will give twice the magnification and half the range.

The biofeedback monitor and the graphing program can be used together as a simple lie detector. To do this, the person being tested should look away from the screen. Ask the person a few unimportant questions, and press [OPTION] to mark their stress level. If the graph rises far above the stationary line, they may be lying. If it rises off the screen, you may well suspect that they're telling a whopper.
   This test works better if the subject knows the wording and order of all the questions in advance. However, remember that your simple "lie detector" is much more inaccurate and easier to fool than a polygraph, the expensive machine normally used to give these type of tests.

The principle behind the biofeedback monitor is simple. The "pot" in the paddle controller is a "variable resistor." In other words, its resistance to an electric current can be changed by turning the paddle knob. The Atari has circuitry in it to measure this resistance and convert it into a number, which can be read using the PADDLE(x) function of Atari BASIC.
   When you hooked your electrodes to the paddle contoller, you added another variable resistor to the circuit- the skin of the subject! When people get nervous, perspiration forms on their skin, making it more conductive (resistance decreases). As people relax, they perspire less, and their skin becomes less conductive (resistance increases).
   The Atari measures the resistance of the subject's skin the same way it reads a paddle controller. This makes it easy to use the monitor in your programs. PADDLE(0)-l is the resistance of the subject's skin at any time, 0 being the least resistance and 227 being the most.
   I'm sure you can think of more ways to use the program. You could write a stress-controlled video game or a program to teach relaxation. Who knows? You might even want to build a complete repertoire of counterspy accessories by hooking up other devices, such as light sensors and infrared detectors, to the paddle controller circuit

Most BASIC graphics modes include a four-line text window at the bottom of the screen. Adding 16 to the Graphics mode will give you the same type of screen without a text window.
   For example, a Graphics 8 instruction will create a Graphics 8 screen with a four-line text window at the bottom. A Graphics 8+16 instruction generates a full Graphics 8 screen.
   Normally, every standard GRAPHICS call from BASIC clears the screen, even if you're just adding or removing a text window. This program, however, features a seldom-used technique for adding and removing a text window without erasing the graphics.
   Line 120 contains a Graphics 8 + 32 instruction. The '8' part of the instruction tells the computer to build a Graphics 8 screen with a text window, and the "+ 32" tells the computer to preserve any graphics already on the screen. This instruction can also be written as Graphics 40.
   The Graphics 56 instruction in line 110 is equivalent to a Graphics 8+16+32 call. The "8+ 16" part of the instruction tells the computer to build a Graphics 8 + 16 screen (no text window). Again, the " + 32" tells the computer to preserve whatever graphics are on the screen.

Michael Krueger, 15, is a sophomore at Vermillion High School in Vermillion, South Dakota.

Listing 1  LIAR.BAS Download