KIT #1: “2-Transistor Ribbon Kit”

TEACHER NOTES

BACKGROUND

This project started with our Cub Scout Den work towards a Science Pin. Yes, it seems crazy to try and get elementary school kids to wire up transistors, but we had already made motors, electromagnets and flashlights: it was time for a new challenge! We knew from experience that it would be trouble to try and have a dozen second-grade boys wielding soldering irons en masse. As a result, we developed a technique for wiring simple circuits by twisting their wires together, using a piece of cardboard as a circuit card. All of the rest of this process is just refinements on that basic idea.

The key to these projects is simplicity: very few parts, easy to install, fun to demonstrate, easy to fix. Second-graders are almost up to the challenge... how about your group?

Note on color-blindness: because the test connections are made by reference to color, check your students who do not have good color vision to see if they need assistance in identifying where to connect the wires once the circuit is ready to test.

BASIC TECHNOLOGY

All of these circuits demonstrate basic transistor operation: amplification and switching (which is just another type of amplification!). None of these parts are exotic or expensive or very delicate, most of these parts can be widely substituted for additional experimentation.

The kit demonstrates how transistors work. Builders make a variable resistor and find out how capacitors work. There are no dangerous voltages or other hazards (don't get poked with a wire!).

We've provided a couple of “theory of operation” notes, one aimed at the rank beginner and another for a more savvy crowd. Read both of them, especially the more advanced theory page since it contains more information you will need when selecting materials to use for the project.

PUTTING IT TOGETHER

You will need a set of the cards and parts. Here's the list, one for each participant:

• Business card, any type (or use our pre-printed cards)
• 2N4401 transistor, NPN general purpose, TO-92 package, Fairchild Semiconductor
• KSA642G transistor, PNP power, TO-92 package, Fairchild Semiconductor
• 0.01uF ceramic capacitor, 50VDC, 5mm lead spacing
• 39K ohm resistor, 1/4W, 5% carbon film
• A photocopy of the artwork for our business card kit card. This copy of the artwork is then cut out and glued to your cards. It provides the guide: where to punch holes and put the parts, and how to wire up the circuit.

Sources for parts: Many vendors will carry these parts, and it is not required to use the exact type of part we've specified.

One of our suppliers is Mouser (www.mouser.com). Mouser's catalog numbers for these parts is:

2N4401 Transistor: 512-2N4401BU

KSA642G Transistor: 512-KSA642GBU

0.01uF Capacitor: 140-50Z5-103M-RC

39K Ohm Resistor: 291-39K-RC

If you find that these numbers have become obsolete, call our tech support line. We'll get some good numbers for you, and we'll be thankful to you for letting us know.

If you would like to order sets of these kits from us, they are greatly discounted. Click this link for information on how to get group kits.

Changes to the specified components

In almost every case, the component values and types are not very important. We chose these because they are very inexpensive and easy to get. Unless you are making hundreds of these kits, the cheapest and easiest way to get them is to probably to buy them from us in bulk sets.

If you're going to change any of the values, go read our comments in the “theory” page about what we know works and what doesn't. Also, get some parts and experiment with a couple of kits before you order a lot of sets of different parts. Some combinations don't work very well.

This kit is not complete, it requires a rig with a speaker and battery to provide power and a way to hear the circuit in operation. In a group setting, you can provide these in small ratios, as few as one for every six participants. One advantage of building this in a group setting is the opportunity for participants to help each other, with several sets of eyes looking on each circuit under test.

We're also developing more complete sets for the group, these ready-to-go sets include handy items like jumper clips, speakers and battery holders. If you'd like to get a ready-to-go set, click this link to go to our product page to check availability of ready-to-go sets.

Ideas for power sources

The power source can be any low-voltage supply or battery cell. In fact, a single D-cell could be shared by any number of groups, each having their own speaker. 1.5V to 3V DC can be used. The optimum setup is a single D-cell in a battery holder. You can get battery holders from most suppliers (see Mouser above) or at Radio Shack.

One completely cheap solution is to hack an inexpensive flashlight into a battery holder by smashing the glass out of its bulb and soldering power wires to the smashed bulb's screw base before reassembling the flashlight. This has the advantage of also giving you an on-off switch in the bargain.

An even easier option is to get a few abandoned chargers from old Nokia cell phones and use them as a 3V power source. You can also use a 6V lantern-style battery. In that case, use a resistor (try 220 ohms) in series with the positive terminal to limit the current that the circuit will draw from the higher-than-normal voltage.

Speakers

Any speaker from 4 to 16 ohms impedance can be used, we use either a simple 8-ohm speaker from a transistor radio or we borrow a speaker from a personal computer (use the unamplified speaker!). If you want to drive the circuit output into another piece of audio equipment, clip your equipment input across the speaker terminals. If you want to do without the speaker, you'll need to replace it with a 6.8 ohm resistor and clip across the resistor to get your audio output.

Setting up your groups. Group dynamics.

The kit is designed to be built by a group. One of the big things that helps is the use of color-coded jumper clips to help the participants hook things up right. We definitely recommend this for your group: five sets of jumper wires several inches long with five different colored (red, black, green, white and yellow) alligator clips at each end. This dramatically decreases the problems your students will have in making things work right.

You can get test leads with alligator clips (a set of 10 leads, 2 each of five colors) several places on the web. They're more expensive at Radio Shack, but easy to find there.

If you do have an advanced group, you may choose to provide plain wire and use the kit as means of teaching how to read schematic diagrams. The kit is still really easy to use: punch additional holes in the circuit card for each of the five wires and strip the wires leaving enough bare copper to easily twist them around the circuit wires.

Tools

You will probably need to provide a set of pliers for every six to ten participants. It's a bit difficult to twist the wires, and some of your participants will be unable to twist the circuit together completely. Often, a moment's quick attention with a pair of needle nose pliers will get a circuit up and running with no problems.

The kit was originally designed to be twisted together by hand. We've built many dozens that way. When first doing this project with a large group, we brought pliers just for young people to use. It turned out that almost everyone had better results using pliers than any other method. Our biggest fear in using pliers was that the torque provided would rip the components apart. We discovered those worries were unfounded - but it's still a good idea to mention that excessive force will ruin the parts.

Different types of pliers work O.K., but the best ones are inexpensive small needle-nose pliers. If the end of the pliers is too big, it's hard to rotate the tip. If the pliers' handles are really big, you end up twisting the wires so much that they break. There's not much force required, and the pliers won't get much use, so the cheapest pliers will be O.K.

We've used these pliers, less than $2 each, but you should be able to scrounge better - especially given the trouble of placing an order and paying shipping & handling.
http://www.widgetsupply.com/page/WS/PROD/specials/SAC3-811L

We've also made a great little tool by drilling two small holes (a #71 and a #76 hole) in the end of a 3 ” long, 3/16” hardwood dowel rod. These holes go up into the rod at a slight angle so that the drill bit eventually emerges from the side of the dowel, making a hole about 1/2” long. The participant sticks the wires to be twisted into the end of the dowel and pushes the dowel onto the wires until they are fully inserted. A few quick turns of the dowel provides an acceptable wire twist, even for kids with the softest fingertips.

WHAT TO SEE AND DO

Follow the instructions given on the main page. Instruct the participants in each step and confirm that everyone keeps up. This is a great opportunity for people to work together to keep things going.

One of the best parts of this group activity is the creativity that comes from using different sources of resistance. Encourage creative thought in the patterns that are drawn for the conductive ribbon. Highlight the different types of patterns that people make. Find out if anyone has a song to share.

Choose different experiments from the main page to try. Your choices will depend on your group. One of the things we always do is build a number of the optional circuits in advance, as a demonstration to the group of the power and flexibility of the circuit they have just created.

Each of the kits operate slightly differently. This is because each of the parts has its own variation. Each of the kits will be built differently. All of these can affect the way the circuit operates. Take the opportunity to discuss the concept of cumulative error.

Please contact us with your experiences and comments.