Introduction: Build a Tone Generator--Improve Your Snap Circuits by Adding a 555 Timer IC
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. The first circuit that I will show you how to build, is a simple square wave tone generator. It was inspired by Forrest Mims III, and it is a very useful circuit that you can use to learn about electronic components such as resistors, and capacitors.
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.
Snap Circuits Parts:
1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
1 Speaker # 6SC SP
1 Slide Switch # 6SC S1
1 100K ohm Resistor # 6SC R5
1 0.02uF Capacitor # 6SC C1
1 470uF Capacitor # 6SC C5
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
2 Battery Holder (2-AA) # 6SC B1
1 Single Snap Conductor # 6SC 01
7 Conductor with 2-snaps # 6SC 02
3 Conductor with 3-snaps # 6SC 03
3 Conductor with 4-snaps # 6SC 04
1 Conductor with 5-snaps # 6SC 05
1 Conductor with 6-snaps # 6SC 06
1 Conductor with 7-snaps # 6SC 07
Snap Circuits Parts can be ordered separately from http://cs-sales.net/sncirepa.html (all of the above parts are included in the Snap Circuits Extreme SC-750 set)
Non Snap Circuits Part:
1 555 Timer IC # NTE955M
This part can be ordered from Allied Electronics: http://www.alliedelec.com/search/searchresults.aspx?dsNav=Ntk:Primary%7C555+timer%7C3%7C,Ny:True,Ro:0&dsDimensionSearch=D:555+timer,Dxm:All,Dxp:3&SearchType=0
Optional Snap Circuits Parts (useful for testing the circuit but not required for building the circuit):
1 0.1uf Capacitor # 6SC C2
1 10uf Capacitor # 6SC C3
Here's a video of the tone generator (note: I connected the speaker on the demonstration of the 10uf Capacitor C3 because I wasn't sure if my video camera would pick up the sound):
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 100K ohm resistor R5 and the Variable resistor. 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: Build the Circuit
Build the circuit shown. If you want to design your own Snap Circuits diagrams go to http://www.snapcircuits.net/learning_center/designer
The photographs show the steps to build the circuit.
Once you build the circuit, turn on the Slide Switch (S1). 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.
Step 4: 555 Timer Astable Mode
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 speaker 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 Slide Switch S1:
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 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.
Experiment with capacitors
This simple circuit will allow you to test a number of electronic components. For example, even if you didn’t know anything about how capacitors work you could learn something about them.
If you have two optional capcitors, move the Variable Resistor (RV) slider to center so that it lines up with the arrow and replace the 0.02uf capacitor (C1) with the 0.1uf capacitor (C2). What happened to the pitch of the tone from the Whistle Chip? Does it sound lower in pitch or higher? What might cause the pitch to be lower? If you know that the square wave tone depends on the charging and discharging of the capacitor then you might suspect that the 0.1uf capacitor is charging and discharging more slowly (recall that the resistance hasn't changed--you haven't moved the slider on the Variable Resistor--thus the timing of how often the capacitor charges and discharges hasn't changed either). Test your hypothesis by replacing the 0.1uf capacitor (C2) with the 10uf capacitor (C3). Make sure that the plus sign (+) on the block is on the Whistle Chip side and not on the 100K ohm resistor side. Was your hypothesis confirmed?
Using different combinations of resistance and capacitance allows you to control the pitch of the the square wave tone you hear on the speaker.
Step 5: Congratulations!
In this article you learned how to how to improve your collection of Snap Circuits blocks by adding a 555 Timer IC. You learned the functions the pins on the 555 chip. You learned that when the 555 is in astable mode, the output of pin 3 is a continuous stream of pulses called a square wave that can be heard on a speaker as a tone. Finally you learned how to build an astable mode circuit for the 555.
Note: You may wonder why the 470uf capacitor C5 is installed in the circuit. At first I simply connected a three snap conductor from pin 3 to the speaker, but the 555 started to get really warm really quickly. I did not want to burn up my 555 so I replaced the three snap conductor with a 100 ohm resistor, but the tone from the speaker wasn't as loud. I then replaced the speaker with the Whistle Chip. If you decide you want to use the Snap Circuits speaker instead of the Whistle you can use the capacitor. In future articles you will see the 100 ohm resistor used with the Whistle Chip so that other electronic components can be connected to pin 3. The 470uf capacitor or the 100 ohm resistor puts just enough load on pin 3 to keep the 555 timer from heating up.
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