Make weird noises with a pencil as you draw, and turn patterns into music.
BUILD TIME: 2 HOURS
DIFFICULTY RATING: BEGINNER
This circuit has been around in various forms on the internet for many years. In fact, one company even makes a commercial kit for it. All are quite similar, however. We’re revisiting an old friend, the 555 timer IC, and introducing another Integrated Circuit (IC), the LM386 audio amplifier. By connecting your finger and a pencil to the 555, we can create some sounds like those in old space films, or draw shapes on paper that you can play like music by moving your pencil across the page. All you need besides the electronics is a soft graphite pencil (many people know these as ‘lead’ pencils), 2B or softer, and some imagination. We’ll take you through the build step by step, with no soldering required. You will need a responsible adult for one step, and a pencil that you have permission to modify. This month’s circuit fits on one solderless breadboard.
If you have never used a breadboard before you can check out our Breadboarding Basics Classroom from Issue 15. While we’re on the subject of breadboards, we use the terms ‘columns’ and ‘rows’ to describe the lines of connected spring clips. In the rest of the world, columns are vertical arrangements, and rows are horizontal. This is a mathematical convention, and you see it in information tables, as well as real-world things like rows and columns of seats in a theatre or cinema, or columns holding up a roof.
We refer to the power rails as columns, and the lengths of five connections that make up most of the board as rows. Although this might be confusing, look at the numbers and letters on your breadboard: The board is actually designed to be used with its long length up and down. We use it with its long length across. This makes the board easier to use, and many projects in many places show the boards this way. However, most instructions still refer to rows and columns (if they do at all) in the way the letters and numbers show. We stayed with the trend.
So, in our builds, we use the breadboard at a right angle to its designed use, which means that rows and columns also turn through a right angle. Our columns go across, and our rows down.
|CRAFT MATERIALS & TOOLS REQUIRED:|
|Soft Graphite Pencil (eg. 2B)|
|Paper to draw your designs|
CRAFT MATERIALS & TOOLS REQUIRED:
|ELECTRONICS PARTS REQUIRED:||Jaycar||Altronics||Core Electronics|
|1 x Solderless Breadboard||PB8820||P1002||CE05102|
|2 x Plug-to-plug Jumper Wires||WC6024||P1022||PRT-12795|
|1 x Breadboard Wire Links Pack||PB8850||P1014A||CE05631|
|1 x 9V Battery Snap||PH9232||P0455||CE05205|
|1 x 9V Battery||SB2423||S4970B||CE05337|
|1m Twin-core Speaker Wire||WB1702||W2100||-|
|1 x Small Speaker||AS3000||C0610||ADA1890|
|1 x 10Ω Resistor*||RR0524||R7510||COM-05092|
|1 x 1kΩ Resistor*||RR0572||R7558||COM-05092|
|1 x 12kΩ Resistor*||RR0598||R7584||COM-05092|
|1 x 330kΩ Resistor*||RR0632||R7618||COM-05092|
|1 x 470pF Capacitor||RC5332||R2830||FIT0118|
|2 x 680pF Capacitor||RC5334||R2832||FIT0118|
|1 x 47nF Capacitor#||RM7105||R3021B||FIT0118|
|1 x 100nF Capacitor#||RM7125||R3025B||FIT0118|
|2 x 470nF Capacitor#||RM7165||R3033B||-|
|1 x 220μF Electrolytic Capacitor||RE6312||R5143||CE05149|
|1 x 555 Timer IC||ZL3555||Z2755||COM-09273|
|1 x LM386 Amplifier IC||ZL3386||Z2556||-|
*Quantity used, may be sold in packs. Values near but not exact will work.
#We used MKT capacitors, but you can use any type with the correct value.
The whole circuit, operationally divided into two halves, fits on a standard solderless breadboard. We’re going to build both the oscillator (the 555 bit) and the amplifier at the same time.
When building, take care to place things exactly where they need to go, counting rows and columns to help you along.
Place the breadboard in front of you with the outer red (+) rail furthest from you and the outer blue (-) rail closest to you. Insert wire links to join the upper and lower red rails, and blue rails.
Place the 555 and LM386 on the breadboard, counting rows from the ends. Take care of orientation as shown.
Insert these wire links, all of which start in a red (+) rail. Take note of which holes they end in.
Insert these wire links, all of which start in a blue (-) rail. Make sure the other ends go in the right holes.
Place the three wire links which connect pin 2 to pin 6 of the 555, and the two which reach to the right.
Place the two wire links around the LM386, one from pin 5 to the right, the other from pin 3 to the left.
Insert the 12kΩ resistor (brown-red-black-red-brown or brown-red-orange-gold) between the upper red (+) rail and pin 7 of the 555. Insert the 330kΩ resistor (orange-orange-black-orange-brown or orange-orange-yellow-gold) from pin 2 of the 555 to the left of the board.
Insert the 1kΩ resistor (brown-black-black-brown-brown or brown-black-red-gold) to the left of the wire link from pin 3 of the LM386. Insert the 10Ω resistor (brown-black-black-gold-brown)
Insert the 100nF capacitor near the 555, and the 47nF capacitor near the LM386. Also insert a 470nF capacitor across the supply rail in the middle of the upper rails, and another at the end of the wire link between the 555 and LM386.
Insert the three capacitors near the 555. They go between the longer wire link from pin 6 and the wire link to the upper blue (-) rail. They are two 680pF (stamped 681K) and one 470pF (stamped 471K), and can go in any order.
Insert the 220μF capacitor with its + lead (unmarked) to pin 5 of the LM386, and its - lead (marked with a stripe) to row 6 of the board.
Cut a plug-to-plug jumper wire in half, bare the ends, and twist through the speaker terminals. Tape the joins. You may have this assembled from previous projects.
Plug one speaker wire into the board in row 6 to connect to the 220μF capacitor. Plug the other wire into the upper blue (-) rail.
Cut another plug-to-plug jumper wire in half and bare the ends. Cut a length of speaker cable, and bare about 5mm of one end, and 50mm of the other. You can use separate lengths of hook-up wire instead of speaker wire.
Join the ends of the jumper wire to the shorter stripped ends of the speaker wire. Tape the ends. Wrap one of the longer ends around a drawing pin or push pin.
Push the drawing pin into the end of a 2B or softer graphite pencil. Make sure the pin contacts the graphite core, not just wood.
Tape the other long stripped end to the pencil, wrap around once or twice, and tape near the sharp end of the pencil.
Plug the jumper wires from the pencil into pin 7 of the 555, and the 330kΩ resistor. Connect the battery and hold the pencil so your finger touches the bare wire. Touch the tip with your other hand, and listen. If all is connected properly, you should hear a sound. If not, check all connections and component placement.
HOW IT WORKS
We have used the 555 timer IC is several past projects: We used it to flash lights, then to generate a siren for them. We later used it to make a different sound in our war horn, and we have used it to make clock signals to drive other ICs. It is made by many manufacturers, with many prefixes and suffixes. Some manufacturers use the original Signetics ‘NE’, while others use the ‘LM’ prefix. Texas Instruments makes both LM and NE prefixed 555s with slightly different specifications. So whatever is printed on your IC, there just needs to be a 555 in it.
We are using the 555 in ‘astable’ mode, which means it switches on and off continuously. The on and off times can be independently set by the relationship of two resistors. Ordinarily, in astable operation, one resistor is connected between Vcc and pin 7, the discharge pin. Another connects between pin 7 and pin 6, the threshold pin. The discharge resistor controls how long the capacitor inside the 555 takes to discharge, while the resistor between there and the threshold pin controls when the state changes. Pin 6 is normally connected straight to pin 2, and this junction connects to a capacitor, the other end of which is grounded.
In this case, however, we have connected pin 7 to pin 2 by a resistor, but not as you know resistors. Pin 6 still connects straight to pin 2, and there is still a capacitor between these two pins and ground. This is the three capacitors inserted in parallel, because capacitances in parallel add up. More on this later.
Pin 7 is connected to pin 2 by a very special resistor - your body. That, and the graphite pencil. Graphite conducts electricity, and has a resistance, depending on binding agents, of around 1Ω (Ohm) per 25mm when it is in the core of a pencil. One end of this graphite rod is connected via a wire to pin 2 or pin 7, depending on which end you plugged in (though it doesn't matter which is which). The other wire from the pencil connects to your finger as you grip it.
When you touch the tip of the pencil, current flows from the drawing pin to the pencil tip, up your arm, across your chest, and down your other arm to the finger touching the wire on the pencil body. Your body’s resistance is now part of the circuit. As you’ll see in the ‘using it’ section, you must touch the line you draw with the pencil. As you draw, the graphite line has a much higher resistance than in the core of the pencil, and adds to that of your body. This is what changes the frequency of the sound you hear.
It is true that any current passing through your body can be dangerous. Even from a 9V battery, limited by the 555 and the 330,000Ω (330kΩ) resistor. While in very rare cases this can be dangerous, you are statistically more likely to get struck by lightning, and a healthy, typical body needs well over 10mA of current to arrive at the heart to cause problems. Our circuit is limited to 0.027mA by the 330kΩ resistor alone! Then there is further resistance from your own body and other components.
The three capacitors were chosen rather than one so that you can easily change the sound by playing with the combination. Remove one or more, try a 680pF with the 470pF, two 680pF, or one of either value on its own. You can also try other values.
The other half of the circuit is an amplifier based on the LM386 Integrated Circuit (IC), which is made for just this purpose. It helps boost the signal from the 555 to audible levels, but we don't have room to explain it all here. It's a means to an end to enable our project to be heard. However, we have covered it in this month's Classroom article, which you can read for a deeper explanation of the IC and how to use it.
To get the best out of your musical pencil, you need to start by drawing a finger-sized circle. Really smooth paper, such as high-quality laser copy paper, is often too smooth to be effective. Something rougher, like sketchpad or drawing paper, or cheaper copy paper, engages better with the pencil, takes more graphite, and creates a more conductive line. Sometimes, you have to go over your line several times. The darker, the better.
After you have made a circle, you need to place your finger on it. Touch the pencil to the circle, which should elicit a sound from the speaker. Draw away from the circle without breaking the line. As before, you may need to go over the line a few times. As you take the pencil tip further from the circle, the sound’s frequency should decrease.
Of course, this on its own is novel, but soon you’ll be bored by drawing straight lines. Now you can challenge yourself, by drawing patterns that you can draw across to gain music. You may even be able to figure out where on a line certain real musical notes lay, and play them like a keyboard. We have some photos for reference, but above all else, explore and play!
As Albert Einstein said, “Play is the highest form of research”.
WHERE TO FROM HERE?
The varying content of moisture in your skin will change the resistance of the circuit and thus the frequency of the sound. When we are nervous, most people sweat at least slightly, and other reactions happen in the body to change how we conduct electricity. Try using masking tape to attach the wires to your forehead and have someone else ask you questions. Even little lies often change how much we sweat, and the forehead often changes first. The current will only flow across the skin, and not even though all its depth, so this is a safe experiment so long as you keep to the front of the forehead. You’ve made a very basic lie detector!
Lie detectors themselves are falling out of use (except on TV and in the movies) as research proves how inaccurate they are, but it could still be a fun experiment. Just don’t rely on it to prove your sibling stole your chocolate!