Step 5: Let's Try Some Experiments With the 555 Timer Circuit

Build a practical joke lie detector:

Parts needed:

1 Jumper Wire 8" (White) # 6SC J3F
1 Jumper Wire 4" (Blue) # 6SC J4

The lie detector build is very simple: replace the photo resistor with the white and blue jumper wires as shown in picture.

Few educated people take polygraphy ("the practice of using a polygraph, or lie detector") seriously nowadays  http://antipolygraph.org. Polygraphy is pseudoscience like ufology or paranormal research. There are, nonetheless, people who actually believe that a lie detector can detect deception. If someone you know believes this nonsense then you can have a little fun at this person's expense.

Explain that you have built a lie detector. It works because when someone is lying they tend to perspire more than when they are not lying. An increase in perspiration will change the resistance between the electrodes which, in turn, will change the pitch of the lie detector. Switch the lie detector on and gently hold the white snap between the the thumb and index finger of your left hand and the blue snap between the the thumb and index finger of your right hand. You will hear clicking from the speakeror perhaps a low picthed tone. Have your friend ask you two questions but inform your firend that you will answer the first one truthfully but you will lie in response to the second question. Continue to gently hold the white and blue snaps while you answer the first question truthfully, but then squeeze both snaps while you answer the second question--squeezing the snap will change the pitch a lot.

By squeezing the snaps, you increase the amount of the skin between your fingers that covers the snaps. This reduces the resistance in the circuit and increases the pitch of the lie detector.

Now have your friend try. At first he or she may be holding the electrodes too tightly changing the pitch. Ask your friend, "why are you so nervous...just relax." Wait until the tone is steady then start asking your questions.

Eventually your friend will catch on that even when he or she is lying that pitch won't change until the electrodes are squeezed.

Since we have replaced the photoresistor with the jumper wires we'll now call this circuit the 555 test circuit, there are a few experiments that can be done to test resistance:

Does adding salt to water decrease it's resistance?

In this video I add Sea Salt to water to see if the resistance of ordinary tap water at room temperature changes. The electrodes from the 555 test circuit are held submerged in the beaker with ordinary clothespins. I apologize for the the loudness of my magnetic stirer/centrifuge--I built it using Erector set pieces. It might have been better to use my K'nex magnetic stirer/centrifuge which is a little quiter but I had to borrow the neodymium magnets from the K'nex model to test the Erector set model. You may be able to hear the change in pitch as I stir the Sea Salt into the 50ml beaker of water (if you change the progress bar back and forth between 10 seconds and 43 seconds it is easier to distinguish the change in pitch). The change in pitch is only a little higher, but yes, adding salt to water does decrease the resistance of water. I also noticed that a lot of the bigger chunks of the Sea Salt did not dissolve, which may account for why Sea Salt seems to taste less salty than ordinary table salt.

Use a magnet to alter the picth of the 555 timer circuit

Parts Needed:

Snap Circuits Electromagnet # 6SC M3
Snap Circuits Iron Core Rod # 6SC M3B
Neodymium magnet (I got mine from a magnetic key chain)

In this next video, I attached the jumper wires to the Snap Circuits electromagnet block and stuck my Neodymium magnet to the Iron core rod. Then I used the magnet to alter the pitch of the 555 test circuit.

Make a pencil lead (graphite) organ

For a bit of fun, in this next video I've turned the 555 test circuit into a simple musical instrument. I drew a thick bar on a piece of paper with a Number 2 pencil and taped the white jumper wire to one end of the bar and used the other jumper wire to move along the bar to change the pitch of the 555 test circuit.

Cable tester

Parts Needed:

2 Multimeter test leads with banana connectors (male)
2 Alligator clips with banana connectors (female)

In this last video I've attached some test leads from an old multimeter to the 555 test circuit. The alligator clips have banana connectors and easily connected to the end of the test leads that plugged into the multimeter. I use the test leads to test pin 1 on the first connector and find which pin it is connected to on the second connector. At the beginning of the video I also demonstrate how the "lie detector" works by squeezing the lead between my thumbs and index finger.

<p>Note that the Snap Circuits Extreme SC-750 no longer contains the 8 pin IC socket block so you will need to purchase the additional part.</p>
who will use this anyway?
Makers, people who want to learn electronics, K-12 teachers, post-secondary educators, scientists, technologists, engineers, mathematicians...

About This Instructable




Bio: You can try my projects AT YOUR OWN RISK. There's infinite ways to damage or destroy persons and property. I can't think of ... More »
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