[Video Link] My daughters love to show the Friendstrument to their friends, and it never fails to elicit squeals of delight when they play with it. The Friendstrument looks like a colorful keepsake box, with one difference: on the lid sit two pennies. One girl touches a penny with her finger, and her friend touches the other penny. With their free hands, the girls tap each other on the arms and face. With each tap, the box emits a tone. You can vary the pitch with the amount of pressure you apply with your finger. The reason we call it the Friendstrument is because it turns friends into a musical instrument!
This is one of the more challenging projects in the book, because it involves electronics and soldering. If you’ve never made an electronic circuit before, don’t fear The friendstrument has a simple circuit, and you and your daughter will enjoy making it. If you’ve never soldered before, this is a great opportunity to learn. (Read Mitch Altman’s excellent comic book guide to soldering here.) I know parents who have taught their 5-year-olds how to solder.
If you aren’t ready to dive in and learn soldering, We’ll show you how to complete the circuit without soldering (except for soldering wire to a couple of pennies).
Things You Need to Make a Friendstrument
- Wooden box We went to the local craft store and bought a few wooden boxes for a dollar each. An ideal box is between 3" and 5" in diameter and about 3" tall. (You can also use a cardboard box or other container that can be drilled.)
- Paint and paint brush
- Mod Podge ($8)
- Comic books and magazines to cut images from
-  pennies
- Battery holder, 2 AA, with a built-in On-Off switch. ($5)
-  AA batteries
-  Transistors  NPN Transistor, such a 2N3904 and  PNP Transistor, such as a 2N4403. (Get a 40 pack of NPNs and PNPs for $3 on Amazon.) They look the same, but you need one of each kind to make the circuit. These are very cheap in bulk — on eBay I bought 100 PNPs for a penny apiece and 100 NPNs for five cents each — so stock up and buy a bunch because you’ll probably burn one or two out by mistake and it’s nice to have a few spares laying around. If you can’t find the models I used that’s OK — most any general purpose, low-power transistor should work.
-  capacitor, 4.7 nanofarads. (Buy an assortment of 645 capacitors with different values from Amazon for $20.)
-  Resistors,  1M Ohm Resistors,  100K Ohm Resistor, and  1K Ohm Resistor. (Get a jumbo box of 860 different kinds of 1/4-Watt resistors for $18 on Amazon.) Here's a guide that shows you how to determine the value of a resistor by looking at its color bands.
-  8 Ohm Mini Speaker ($3)
- 22AWG hookup wire. RadioShack (Model: 278-1224) sells a 75-foot set of three spools (black, green, and red) for $7.49. (Here's a 6-color pack on Amazon for $15)
- Rosin-core solder ($6)
- Scissors (Not shown)
- 30 Watt pen-type soldering iron ($7)
- Solderless prototyping breadboard ($6) This allows you to make circuits without soldering the connections. I use a large one with numbers and letters to try things out, and often use a mini solderless breadboard (10 for $14) to make a semi-permanent circuit.
1. Paint the box You can use acrylic paints, water color, color makers, or any other coloring method you’d like.
2. Find art for the box
Collect magazines, comic books, stickers, colored paper, and other materials to put on the surfaces of the box. When the paint from Step 1 is dry, use a glue stick to attach your images to the box.
3. Apply Mod Podge with a brush
Mod Podge is a decoupage sealer and finish that will protect your decorated box and make it look shiny. One coat is usually enough.
4. Allow box to dry
Find a place that’s free of dust and pet fur and let the box dry for at least a few hours before handling it. While it’s drying, you can move to the next steps: making the electronic circuit that produces the music.
Sidebar: Get to know your components, Part 1 Meet the Solderless Breadboard
A solderless prototyping breadboard allows you to quickly build a circuit without soldering components. Instead using solder to join component leads, you insert the leads of components into holes in the board. A typical solderless breadboard has 64 numbered rows and 10 columns, marked A through J. Every hole in each row from A to E is connected, and every hole in each row from F to J is connected. The four outside columns, marked with red + symbols and blue - symbols are where you’ll connect the battery case wires, and are the source for the circuit’s power.
Meet the Transistor
Transistors are electronic components that can be used both as switches (to turn power on and off to a part of a circuit) and as amplifiers (to increase the amplitude of an electrical signal, such as a musical waveform). Most common transistors have three wires, or leads, coming out of them. One lead is an emitter (E), one is a base (B), and one is a collector (C). There are two types of transistors: NPN and PNP, and it is important to use the type called for in your circuit or it won’t work. It’s easy to blow a transistor by wiring it incorrectly, but the good news is they are cheap (as little as a penny apiece). Be sure to refer to the specification sheet for the transistor you are using to see which leads are E, B, and C (they aren’t always arranged as shown here). You can Google the part number (eg., 2N3904) to obtain the specification (or “spec”) sheet.
5. Insert NPN transistor
E goes into hole 27C, B goes into 26C, and C goes into 25 C.
Note: If you have never used a solderless breadboard, it may take a little getting used to. Use firm but gentle pressure to insert the leads into the breadboard’s holes. Push them in as far as they will go. They should slide in and then stop. Don’t ever force a wire or component lead into a hole — you could damage the component by breaking a lead. With practice, will soon get comfortable using the breadboard.
6. Insert PNP transistor
Place it on the opposite side of the breadboard from the NPN transistor, like a mirror image. E: 25H, B: 26H, C 27H.
7. Insert a wire between hole A27 and into any hole in the (-) rail of the breadboard
Here, I used a short orange wire. The negative (-) rail is usually marked blue. This is also known as “ground.”
Sidebar: Get to know your components, Part 2
Meet the Resistor
The resistor is the most common electronic component. Almost every circuit has at least one resistor. The Friendstrument uses four resistors. Their purpose is to resist the flow of electricity through a circuit. They are like kinks in a hose. The higher the value of a resistor, the more of a kink they put in the flow. They are often used to protect other components in a circuit by limiting the amount of current in the circuit. Unlike a transistor, resistors don’t have polarity, so you don’t have to worry about which end is which. Resistors are measured in Ohms, and the symbol for Ohm looks like this: Ω.
8. Insert a 100k ohm resistor into holes A22 and A26
(We bent the resistor down to make it easier to see the wires.) The thing to keep in mind while you build you circuit is avoid allowing any leads from touching each other, which could cause a short circuit and possibly ruin components.
9. Insert a 1M ohm resistor into hole B26
Don’t insert the other end of the resistor into the breadboard. Just let it stick out for now. This is the end that you will solder to a penny later on.
10. Insert a 1k ohm resistor into holes D25 and G26
Note that this resistor spans the middle channel dividing the two sides of the breadboard. That’s just what we want.
11. Insert the other 1M ohm resistor into hole I25
Don’t insert the other end of the resistor into the breadboard. This is the end that will connect to the other penny.
12. Insert a wire between J25 and into any hole in the (+) rail of the breadboard
The + rail is usually marked red.
Sidebar: Get to know your components, Part 3
Meet the Capacitor
A capacitor is kind of like a battery. It stores electrons. Unlike a battery, however, a capacitor can discharge the stored electrons very quickly. The ceramic capacitor we are using here doesn’t have polarity, so you don’t have to worry about which end is which. But electrolytic capacitors do have polarity and they will fail (sometimes with a bang!) if you use them backwards. Also, large capacitors can deliver a dangerous electric shock if you touch both leads. The capacitor here is too small to cause any damage to you.
13. Insert the capacitor into B22 and G27
This is a bit of a tight fit, so make sure that none of the leads of any components are accidentally touching each other.
14. Insert the battery holder leads
Put batteries into the holder, and make sure the switch on the holder is turned “OFF.” Then, insert the red wire of the battery holder into a hole in the (+) rail and the black wire into a hole in the (-) rail.
15. Insert speaker wires
Insert one wire from the speaker into a hole in the (-) rail and the other wire into 27J.
16. Test the circuit
Make sure none of the component leads are touching other leads. Next, turn the battery holder switch to the “ON” position. Test the circuit by pinching the free end of one 1M Ohm resistors with your finger and thumb of one hand, and the other resistor with your other hand. The speaker should squeak. If it doesn’t, check your work: Are you using fresh batteries? Did you put the transistors in correctly? Did you put any components into a wrong hole? Don’t give up — you’ll eventually figure out the problem!
17. Make a permanent circuit
Now that you have verified that your circuit works, it’s time to miniaturize it so that it fits into your box. You have at least 3 different options for making a permanent circuit. You can insert the components into a smaller breadboard. You can solder the components together directly. Or you can use a piece of perforated board (known as perf board) to mount the components prior to soldering them. We made circuits using the first two options. Let’s look at the mini breadboard method first.
17a. Using a mini breadboard circuit
This is the quickest and easiest way to make a permanent circuit. The advantage is that you don’t have to solder anything, and if you make a mistake you can just pull the components out of the and start over. The disadvantage is that a breadboard circuit is not as robust as a soldered circuit, and the components might fall out of the breadboard if they are jostled. Most mini breadboards have a peel-off adhesive back so you can stick it to the inside of your box. Note that there are no (+) or (-) rails. No problem. Just pick one row for (+) and another row for (-).
17B. Using a soldered circuit
If you’ve never soldered before, read Mitch Altman’s soldering how-to comic book (see makerdad.org) and practice soldering some pieces of wire. If you know how to solder already, go ahead and solder the leads together, making sure the leads are not so long that they will accidentally touch each other and cause a short circuit. Use theis schematic as your guide.
18. Prepare the pennies
Even if you go to the mini breadboard route, you are still going to have to do a little soldering to complete this project. But don’t worry, it’s easy. Start with a couple of clean pennies. Place the pennies on a thick piece of cardboard. Cut two pieces of wire and strip off about ½" of insulation from each end.
19. Solder the pennies
Heat a penny with the soldering iron. Then, hold the solder right next to the tip of the soldering iron. It might take a minute or two but eventually the solder will begin to melt on the penny. Wait until a nice puddle forms as shown and then insert the end of a wire into the puddle. Remove the soldering iron and hold the wire still for at least 30 seconds to allow the solder to cool. Wait a couple of minutes to let the penny cool down before you touch it. Now, test your solder connection. Don’t jerk it but pull it with firm and steady pressure. If it breaks, try again! Repeat with the other penny.
20. Drill holes through project box for penny wires
Before you solder (or attach, if you are using a breadboard) the free ends of the penny wires to the rest of the circuit, drill two holes in the box you decorated. The holes should be large enough to accommodate the solder bump so that the pennies lay flat on the box. You can use glue dots or hot glue to secure the pennies to the lid.
21. Drill a hole in bottom of box for battery pack on-off switch
Determine where to drill a hole in the bottom of the box so that you can access the battery pack’s on-off switch. Then use glue dots or hot glue to attach the battery pack to the inside of the box.
22. Finishing up
Solder or attach the free ends of the penny wires to the free ends of the resistors and put the lid on the box. Now it’s time to try it out!
How to play the Friendstrument
The funnest way to play the friendstrument is with two people. First, turn it on. One person puts a finger on one penny and another person puts a finger on the other penny. Then, with your free hands touch each other on the arm, nose, ear or any other exposed body part. The speaker will emit a tone. You can change the tone by pressing harder on the penny or on each others’ skin. You can also play solo by touching the pennies with one finger from each hand. You’ll soon get a feel for how to change the sounds of friendstrument by varying pressure on the pennies.
How it Works
Your body conducts electricity. When you touch the friendstrument’s pennies, you are allowing current to flow across your skin (it’s a tiny, harmless amount) and complete the circuit. This kind of circuit is used in lie detector machines. When people are lying, they sweat more, which increases the skin’s conductivity and, therefore, the current flowing through the circuit, causing the pen on the lie detector’s graph to jump. You can use the friendstrument as a crude lie detector, but if you’re a police officer, don’t rely on it as proof of guilt or innocence.
[Video Link] Here's the first prototype of the Friendstrument circuit, which started out as a lie detector but turned out to be more fun as a way to turn your friend into a music instrument.