555 Timer Hacks: Cable Testers, Magnetic Stirrers, and Lego Grabbers Oh My!

Want to learn about the 555 Timer IC? In this Instructable I'll show you how to add a 555 to your collection of Snap Circuits blocks and build circuits you can use to experiment with the following:

Build an optical Theremin

Build practical joke lie detector.

Does adding salt to water decrease it's resistance?

Use a magnet to alter the pitch of the 555 timer circuit.

Make a pencil lead (graphite) organ.

Use the 555 circuit as a cable tester.

Build a tone generator

Test Pusle Width Modulation (PWM) with combination centrifuge/magnetic stirer.

Operate a Lego grabber arm

Currently there are no Snap Circuits sets that have the 555 Timer IC. So, you will need to purchase a 555 Timer IC from Allied Electronics or your favorite electronics supplier. If you don't have the Snap Circuits Extreme SC-750 set you can purchase the Snap Circuits Eight-Pin IC Socket block from C&S Sales. Adding these two components to your set of Snap Circuits blocks will allow you to create dozens of circuits built around the 555 Timer IC.

Snap Circuits is an educational toy that teaches electronics with solderless snap-together electronic components. Each component has the schematic symbol and a label printed on its plastic case that is color coded for easy identification. They snap together with ordinary clothing snaps. The components also snap onto a 10 X 7 plastic base grid analogous to a solderless breadboard. There are several Snap Circuits kits that range from a few simple circuits to the largest kit that includes 750 electronic projects.

All the kits include manuals printed in color with easy to follow diagrams to assemble the projects. The illustrations for each project look almost exactly like what the components will look like on the base grid when finished. Because the electronic symbol is printed on each electronic component, once the project is completed, it will look almost exactly like an electronic schematic.

Step 1: Insert the 555 Timer Chip Into the Snap Circuits IC Socket Block

The 555 Timer IC was introduced by a company called Signetics (later bought out by Philips) in 1972 and was designed by Hans R. Camenzind in 1971. The 555 chip has 25 transistors, 15 resistors and 2 diodes in an 8 pin DIP (Dual In-line Package) and looks like a square bug with eight legs. It has a notch at the top and Pin 1 is in the top left corner. (See picture 1) (Source: http://en.wikipedia.org/wiki/File:Signetics_NE555N.JPG)

The Snap Circuits Eight-Pin IC Socket block is picture 2 (source: http://cs-sales.net/eiicso6u8.html)

Insert the 555 timer chip into the Snap Circuits Eight-Pin IC Socket block. Make sure that the notch in the top of the 555 timer chip is aligned with the diagram of the chip pictured on the IC socket block. (See picture 3) (Source: http://www.snapcircuits.net/learning_center/designer)

Step 2: 555 Timer IC Pins

The following are the pin outs for the 555 Timer IC (source:
http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php):

Pin 1 is ground. It is connected to the negative side of your battery or power along with any other components in your circuit connected to ground.

Pin 2 is the Trigger pin. It will be connected to ground and thus switches on pins 3 and 7.

Pin 3 is the Output pin. In this circuit it outputs a square wave signal that can be heard on a speaker.

Pin 4 is the Reset pin. It is not used in this circuit. See http://en.wikipedia.org/wiki/555_timer_IC or http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php for more info on this pin.

Pin 5 is the Control pin. It is not used in this circuit. See
http://en.wikipedia.org/wiki/555_timer_IC or http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php for more info on this pin.

Pin 6 is the Threshold pin. The 0.02uf capacitor C1 will charge up and when it reaches about 2/3 Vcc (voltage from the battery), this is detected by the Threshold pin. This will end the timing interval and send 0v to the Output pin 3 (switches it off).

Pin 7 is the Discharge pin. This pin is also switched off by the Threshold pin 6. When pin 7 is switched off it cuts the power to the 0.02uf capacitor C1 which causes it to discharge. Pin 7 also controls timing. Pin 7 is connected to the 50K ohm Variable Resistor resistor RV and the Photoresistor. As you move the slider on the Variable Resistor RV, it changes the amount of resistance in the circuit. This changes the timing of pin 7 and thus changes the pitch of the square wave heard on the speaker.

Pin 8 is connected to the positive side of your battery or power along with any other components in your circuit connected to positive.

Step 3: The Voltage Regulator

To power the 555, you can use the Snap Circuits B5 block (it is also called a 9 volt Holder and Switch). The advantage to using this block is you can connect a standard 9 volt battery to the block and it will deliver a reliable 5 volts to the 555 through the L7805 voltage regulator circuit (see circuit schematic). The B5 also has two 5 volt outputs--one to power the Theremin and one that you can use to power another device. HINT: since the output of the 555 in Astable mode is a square wave you could use the output for Pulse Width Modulation (PWM) to control a device such as a motor.

Step 4: Build an Optical Theremin

A theremin is a musical instrument that is played without actually touching the instrument. The original theremin used radio frequency interference caused by the movement of the player's hand to change the pitch of the instrument. The optical theremin depends on the intensity of light that falls on the photoresistor also controlled by the movement of the player's hand.

Parts needed

1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
1 0.02uF Capacitor # 6SC C1
1 100 ohm Resistor # 6SC R1
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
1 Photosensitive Resistor # 6SC RP
1 9 Volt Battery Holder # 6SC B5
1 Press Switch # 6SC S2
3 Single Snap Conductor # 6SC 01
8 Conductor with 2-snaps # 6SC 02
2 Conductor with 3-snaps # 6SC 03
1 Conductor with 4-snaps # 6SC 04
1 Conductor with 5-snaps # 6SC 05

Snap Circuits Parts can be ordered separately from http://cs-sales.net/sncirepa.html

Build the circuit show in the photographs.

Once you build the circuit, switch the B5 block on. You will hear a tone from the Whistle Chip. Move the slider on the Variable Resistor (RV) and you will hear the tone rise and lower in pitch depending on which way you move the slider. Next move your hand so that it casts a shadow on the Photosensitive Resistor (RP) and listen to the pitch as it changes.

As seen in the video in Step One you can replace the 3 snap conductor with the press switch (S2).

Let's see if we can make sense of what is happening. The 555 chip is in astable mode which means that Pin 3 is sending a continuous stream of pulses called a square wave signal to the piezoelectric speaker (WC) that you hear as a tone. The square wave signal is caused by the charging and discharging of the 0.02uf capacitor C1.

When you switch on the power at the B5 block:

Step 1. The 0.02uf capacitor C1 charges up.

Step 2. When the charge in the capacitor reaches 2/3 Voltage, this is detected by pin 6, the Threshold pin.

Step 3. The Threshold pin 6 switches off the Output pin 3.

Step 4. The Threshold pin 6 switches off pin 7, the Discharge pin.

Step 5. When the Discharge pin 7 is switched off this cuts the power to the 0.02uf capacitor which causes it to discharge.

Step 6. When the discharging capacitor reaches 1/3 Vcc, this is detected by the Trigger pin 2.

Step 7. The Trigger pin 2 sends 6 volts to pin 3 the Output pin.

Step 8. The Trigger pin 2 sends 6 volts to pin 7 the dischrage pin which causes the 0.02uf capacitor to charge up.

Step 9. Go back to Step 1.

This process repeats creating the square wave signal (see picture) and you hear that signal from the speaker as a tone.

When you move the slider on the Variable Resistor (RV) this changes the resistance of the circuit. Since the Variable resistor is connected to pin 7, changing the resistance controls the timing of how often the 0.02uf capacitor (C1) charges and discharges. Since the photoresistor (RP) is connected to pin 7 as well, the amount of light that falls on the photoresistor changes the resistance too, and this also controls the timing of how often the capcitor (C1) charges and discharges. You'll notice when more light that falls on the photoresistor this makes the pitch higher. Less light makes the pitch lower.

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.

Step 6: Build a Tone Genterator

Parts needed

1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
1 0.02uF Capacitor # 6SC C1
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
1 9 Volt Battery Holder # 6SC B5
4 Single Snap Conductor # 6SC 01
7 Conductor with 2-snaps # 6SC 02
2 Conductor with 3-snaps # 6SC 03
3 Conductor with 4-snaps # 6SC 04

Build the circuit shown in the photographs.

When you move the slider on the Variable Resistor (RV) this changes the resistance of the circuit. Since the Variable resistor is connected to pin 7 changing the resistance controls the timing of how often the 0.02uf capacitor (C1) charges and discharges. When you change the resistance in the circuit, you change the pitch of the the square wave tone you hear on the speaker.

Step 7: Testing Pulse Width Modulation

Parts:

1 Base Grid (11” x 7.7”) # 6SC BG
3 Battery Holder (2-AA) # 6SC B1
1 Three-Spring Socket # 6SC ?Q
1 Two-Spring Socket # 6SC ?1
1 Jumper Wire 18" (Black) # 6SC J1
1 Jumper Wire 18" (Red) # 6SC J2
1 250ml beaker (http://www.dynalon.com/)
1 Magnetic Stir Bar (http://www.dynalon.com/)
2 Centrifuge Tubes (http://www.dynalon.com/)
2 Toothbrush holders (any dollar store) to hold centrifuge tubes
2 neodymium magnets for magentic stirrer

Special parts:

Circuit board from CyberK'nex motor
Snap Circuits jumper wire to alligator clip conversion cable
Bunsen burner stand (mine is a prototype built from erector set parts in which I am still working out the bugs)
Combination centrifuge/magentic stirer (also a prototype built from erector set parts in which I am still working out the bugs)

Since the output of the 555 timer in astable mode is a square wave and by sliding the variable resistor one can change the pitch of the output signal, it occurred to me that I might be able to create a variable speed motor controller. A square wave signal is simply a pulse, or change in voltage from 0 volts to 5 volts (in the case of this circuit) and then back to zero volts, or a switch that switches on and off very quickly. The "duty cycle" of this circuit is 50% which means that the square wave is at 0 volts for 50% of the time and at 5 volts for 50% of the time. I swapped out 0.02uf capcitor with a 1uf capacitor so you could hear the square wave pulses being sent to the motor.

Next I needed an application in which to test pulse with modulation. I was tinkering around with k'nex to see if I could build a combination centrifuge and magnetic stirer. Here's a video of the k'nex model:

Sadly the CyberK'nex motor is geared for torque, not speed and therefore turned too slowly to be an effective motor for the centrifuge nor the magnetic stirrer.

Next I built a model out of erector set pieces. When powered by 9 volts, the erector set motor turns fast enough, but once any kind of load is put on the motor, it would overheat the circuit and I would watch as the thermistors in the battery blocks would slowly shut the circuit down.

Later I decided to see if I could find the pinouts for the CyberK'nex motors and found them here:

http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2011/tjm238_nw225/tjm238_nw225/index.html#hardware

Using the probes from the cable tester circuit but with the black probe connected to Pin 3 on  the 555 and the red probe connected to +5V on the battery holder block (B5), I verified the pinouts and tested the variable speed adjustment using the variable resistor (RV). I could easily control the speed of the CyberK'nex motor.

Once I was able to control the speed of the CyberK'nex motor I decided to take the circuit board out of it. I took a three wire header connector and connected the 555 timer to the circuit board pins and the next video was the result of the test:

Next I decided to try drive a Lego Technic motor and found that I did not need the Cyberk'nex circuit board. I could send PWM pulses directly to the lego motor. In this next video I built a circuit to reverse the polarity so that I could turn the motor in forward and then reverse directions. Unfortunately Snap Circuits does not have a double pole double throw switch (the easiest way to reverse polarity) so, I had to build my own double pole double throw switch (with center off) out of two singl pole single throw switches (Slide Switch S1) and a press switch (S2). I also added a green and a red LED to indicate forward and reverse direction. The Lego is connected to the 555 time circuit with a Lego to Snap Circuits convesion cable. Note too that I added two 4.5 volt battery blocks for the 9 volt needed to drive the lego motor.