Kit #1: “2-Transistor Ribbon Kit”

OTHER EXPERIMENTS

A BETTER RIBBON CONTROLLER

The easiest way to make a better ribbon controller is to make a better ribbon. Take a long piece of wood and a length of video tape. Glue the tape to the wood. Take one wire from your paper ribbon (yellow or green) and connect it to the video tape. Use the other wire to trace back and forth along the tape to make a nice continuous change in tone.

Cut the ribbon in half, lengthwise. Does the tone change? Why? What does this tell you about the resistance of a material?

Another improvement is to take a metal guitar string (.009” steel electric guitar G string for example) and suspend it 1/8” above the video tape. Connect one of the wires (yellow or green) to the guitar string, and the other wire to the video tape. This gives a very smooth change in tone from one end of the ribbon to the other.

BONGO BEATMAKER

Take two sheets of thin metal, about 6 inches (150mm) square (any size will do!). Lay them on top of each other, but between the two place a piece of black conductive foam, like the kind integrated circuits are stored on. You don't have to have a whole big piece, you can take four or five pieces and put them at a few spots: just enough to keep the two plates from touching. Put a wood block on top of the whole assembly, so it covers the metal. Hook your ribbon circuit to the two plates. Listen to the sound when you strike the wood block.

SIREN SOUND EFFECT

You can make your ribbon circuit into a sound generator that sounds like a police siren! This simple modification requires an extra 15uF electrolytic capacitor (any voltage rating, any tolerance) and a 100K ohm resistor (any power rating, any tolerance). The values of these components is not critical, you can also try whatever part may be lying around that's close to these values.

Start by setting up the circuit as before, but don't connect the paper ribbon: the GREEN and YELLOW nodes will be hanging free (for now).

Hook one side of the 100K ohm resistor to the YELLOW node. Leave the other side of the resistor unattached. We'll call this free-hanging resistor lead the NEW YELLOW node.

The electrolytic capacitor is polarized, that means, like a battery, it has a sense of direction in it. If you hook the capacitor wires up backwards, the circuit won't work. Look at the capacitor and see where there is a minus (-) sign marked. Connected this negative-marked capacitor wire to the BLACK node. The capacitor's other (positive) wire is connected to the free end of the new resistor (the NEW YELLOW node).

Now, operate the circuit by touching the GREEN node to the NEW YELLOW node. Hold those wires together for a moment and then release them. What do you hear? What happens when you change the values of the components in the circuit?

LED FLASHER

You can convert your ribbon circuit into an LED flasher! This requires an extra resistor and capacitor, and an LED which you can either scavenge from an old broken radio around the house, or pick up at Radio Shack. You also need to provide 3 volts of power, a single 1.5V battery cell isn't energetic enough to light an LED. We like to use a 3V lithium battery cell for this project, but that's because we have thousands of them in stock. Your mileage may vary.

Replace the supplied capacitor with a small electrolytic capacitor from 4.7uF to 22uF (10uF suggested), any voltage rating, any tolerance. You can experiment with any capacitor, but these values work well. The electrolytic capacitor is polarized, (see the description in the “siren” project above) so if you hook the capacitor wires up backwards, the circuit won't work. Look at the capacitor and see where there is a minus (-) sign marked. This is the wire that is connected to the “WHITE” node. The other wire is connected to the “YELLOW” node.

Connect the LED to the circuit. The LED has two wires but it is polarized (a-ha!) so you have to connect it in the correct direction. All LEDs have some kind of mark to show the negative side, this is usually a flat spot on the case, but sometimes it's just that one leg is longer than the other (the wire on the negative (cathode) side is always shorter). The negative side of the LED connects to the “RED” node. The battery positive terminal connects to the positive end of the LED.

Disconnect the speaker and connect a 8.2 ohm resistor (any power rating, any tolerance) to the “WHITE” node. Connect the loose side of the 8.2 ohm resistor to the negative (-) battery terminal.

Now, connect the test wires together. Your LED should be flashing brightly! If you put the wires on the conductive ribbon, you will find that the rate of flashing changes as you go back and forth on the card.

One great thing about this project is that this flashing takes very little power. If you connect this circuit across a couple of D cells, it will quickly flash for more than a year. If you slow down the rate of flashing, the circuit will operate for several years on the same pair of batteries. Another neat thing about this project is the way the light turns on when you're running at a slow rate. Try it and see! For another experiment, set the rate very very slow and watch the LED in a dark room. Cool!

CONDUCTIVITY TESTER

You have now built a fairly sensitive resistance indicator. The two wires that were connected to the piece of paper (yellow and green) can now be used to measure resistance between them. Use the indicator to test the resistance of different things in your environment.

There is a great website by the staff of mikroElekronika on this application.

http://www.mikroe.com/en/books/keu/12.htm

Their circuit is almost exactly the same, but substitutes a 0.05uF capacitor for the one we use. The result is a bit louder sound in the speaker. Their website gives great details on the use of this tester for exploring and repairing electronic circuits.

One of the best things about this circuit is it's excellent response to different resistances, and its ability to help your ear quickly “see” the complex impedance in a circuit under test. Not only that, the circuit sips on current so gently that a small battery can power this circuit through years of daily use.

WATER TESTER – Wrap the loose ends from the indicator's two wires to two nails. Hang the two nails on either side of a water glass. Fill the glass with tap water. Can you hear a noise? What does that tell you about the water's ability to conduct electricity? Replace the tap water with purified water. Can you tell anything about the water, based on the sound you hear? Add some sugar to the water. Does the tone change? Add some salt. Does the tone change? Can you make some conclusions about what's going on with the water when you add things to it?

PEOPLE TESTER – Take one of the wires and hold the exposed metal wire end in one hand. Take the other wire and rub the exposed metal end on the back of that hand. Can you hear a sound? If so, move the wire to different parts of your body. Where can you hear a tone and what are the differences between the place you touch the wire and the sound you hear?

Now, hold hands with a friend and have them hold the wire in their other hand. Can you hear anything now? If so, does the tone change when you squeeze your hands tightly? Does the tone change if your hands are slightly damp?

TESTING YOUR ENVIRONMENT – Taking the two wires (yellow and green), touch their exposed metal ends to different things and see how the tone changes. You can prove that most metal conducts electricity by hearing the tone that is generated as you move the wires on its surface. Some metals do not, especially painted metals or metals that are coated (such as anodized metal). Glass, wood, paper, cloth and many other substances have such high resistances that your indicator cannot tell you how high their resistance is. Is dirt conductive? How about damp earth?

ANALOG-TO-DIGITAL CONVERTER FOR MICROCONTROLLER OR COMPUTER CONTROL

The speaker output from this circuit is a signal that switches from 0 volts (ground) up to the supply voltage level (1.5V to 6V). The frequency of this switching is a function of the resistance across the test wires. Replace the speaker with a 10 ohm resistor. Replace the battery with the power supply from your microcontroller and connect the emitter from Q1 to an input pin. The rate of switching will be proportional to the resistance across this circuit.

One of the great things about this circuit is that it takes almost no power to operate it. You can hook this circuit up to a microcontroller, connect its power across the battery supply, put the microcontroller into sleep mode and walk away. This circuit draws less power than most sleeping low-power microcontroller chips, making it a perfect addition to a battery-based remote sensing system.

The same circuit can be used on a parallel port or serial port for a regular PC. Briefly, the circuit is powered by stealing current from one of the unused control lines coming from the PC. The output of the circuit is connected directly to an input pin on the interface. This basically requires a bit of moxie and requires more detailed exposition than we will provide here, but many resources are available on the web for further exploration in making this connection with a PC.

Here are a couple of easy and fun project ideas to do with the circuit hooked to a microcontroller:

WATER DETECTOR / ALARM – Use the circuit output to control an interrupt pin on the microcontroller. Place the test leads in an area that is subject to flooding. As dampness increases, the circuit will begin to switch, providing a control signal to the microcontroller that water is present.

LIGHT DETECTOR / ALARM – Connect the test leads to a photocell and place the circuit in a dark room. The rate of circuit switching will be proportional to the amount of light striking the photocell, giving the microcontroller a direct indication of a change in light from an open door, moving light source or other changing illumination.