Kit #1: 2-Transistor Ribbon Kit

SUPER SIMPLE THEORY OF OPERATION

The circuit is designed to show how basic electronics work. Although there are only a few parts, a lot is going on in the circuit. Here's a simplified picture of what's going on.

The Power Source

The circuit operates on a current of electrons (electrical current). This current passes through the wires in our circuit. You may think of the electric current in wire the same way you think about water current in a pipe. The flow of current in the circuit makes things happen. When we control the flow in special ways, we make interesting things happen. This is why we have so many different types of components, and hook them up in specific ways. The different components in the circuit each do different things to the current and it's important to connect the parts of the circuit in just the right way.

We're using a single battery cell, (AAA, AA, C or D type) to generate our electric current. The battery cell uses chemical energy to push an electrical current through our circuit. A battery cell pumps electrons through a wire similar to the way a water pump pushes water through a pipe.

You may be able to “see” this current if you take a wire, connect it to one side of the battery, and lightly tap the other end of the wire to the other side of the battery. If you see a tiny spark where you tap the wire on the battery, that's a flash of white-hot air showing you the flow of current that the battery can provide.

When you build this circuit, it will direct the current from your battery cell to make your speaker hum and sing.

The Resistor

We provide a 39k (39 kilo-ohm) resistor. The resistor is made of a material (a conductive carbon film) that does not allow the current to flow freely... carbon resists the flow of current. Kind of like the way a very narrow pipe would resist powerful water pressure.

In fact, if you hooked this resistor straight across a battery cell it would only allow (1.5V / 39kohms =) 40uA (forty MICRO amps – that's fifty millionths of an amp) of current to flow. At that rate, it would take many years to empty the energy from a battery cell. This is great proof that the large amount of power needed to run the speaker is not flowing through this resistor.

The resistor DOES provide control current for the transistor. Transistors are amplifiers, they can take a very small current and use it to control a much bigger current. In this circuit, we use currents as small as a few micro amps to control a current that's 10,000 times larger! In another experiment (the LED flasher) we will show you this same circuit being used to provide amplification of more than 1,000,000X!

The Ribbon

A piece of paper and a pencil? Yes, this is a resistor! The pencil 'lead' is graphite. Graphite is a form of carbon that can carry an electrical current. Remember that carbon is not great at carrying current.

When you use your pencil to put a layer of it on a piece of paper, it's not anywhere near as good as a piece of wire – that's why we use it as a resistor. The thickness (darkness) of the pencil trace and the width of your line will set the amount of resistance.

Your line also has to be really dark since the only thing that's allowing current to pass is the fact that the graphite dust particles are lying directly on each other. If there are not enough carbon graphite grains touching each other, there won't be any path for the current. Make that line really dark!

As you move your wire along the length of the trace, you can tell how the resistance changes by how the circuit works differently.

The Capacitor

There is a 0.01uF (1/100th of a micro Farad) capacitor in the kit. Capacitors store electric current, much like a bucket would store water current. A 0.01uF capacitor describes a very small bucket! This capacitor stores the energy of the resistor's current. The amount of stored energy slowly builds up in the capacitor until it is large enough to turn on the transistor. Another thing about capacitors is that, like a bucket, they don't leak. You can't pass current through a capacitor.

So, if you can't pass current through a capacitor, what good is it? Well as we mentioned we can store current in it, since it can't leak out the other side. The other cool thing about capacitors is that they act a lot like a see-saw or a water level: if you push down hard on one side, the other side will pop up hard. If you pull up hard on one side, the other side will jerk down just as hard. Of course, we're not talking about pushing on the wires with our hands! We are talking about using current on the wires: if we try to push current into one side of the capacitor, the capacitor will try to push current just as hard out of the other side's wire!

Later, we'll show you how this see-saw characteristic is used in this circuit to great advantage.

The Transistors

Transistors are amplifiers, and they're sort of like switches. There's nothing moving in them, they're a solid piece of specially treated silicon crystal. It's pretty amazing how well they switch. When turned on, the transistors in your kit can carry enough power to create a pretty good spark, but when they're turned off, it's very, very difficult to measure the amount of power they leak.

You can think about transistors like they were water valves, controlling the flow of electric current like a water valve controls the flow of water current. One difference is that you turn a water valve on and off with your hands. We turn a transistor on and off by pushing a tiny bit of current into the base.

The best part about a transistor is that it is really easy to turn them on and off. It takes a tiny amount of base current to turn a transistor on and off, but when you do, that little bit of base current can control a lot of current flow. That's amplification. The transistors in your kit can amplify current more than 500X!

Our circuit is designed to take this idea even further by storing a very tiny current until it gets just large enough to turn on the transistor. By this method, an extremely small current can control enough power to move a speaker coil or to light an LED.

Our transistors have three terminals: collector, base and emitter. The base is the control arm. If you make current flow from the base to the emitter, it will cause the transistor to allow a much larger amount of current to pass from the collector to the emitter. When you stop putting your tiny amount of current in the base, all of the collector current stops. The ability to use a small current to control a larger current is called amplification, or “gain”. This circuit uses a lot of gain, especially in the first transistor, the 2N4401-type device.

There is a LOT of really great information about transistor operation on the web if you want to study this topic in more detail.

The Relaxation Oscillator

The meat of this circuit is the three components: resistor, capacitor and the 2N4401 transistor. They are brought together in a circuit called a relaxation oscillator. When you put power into this circuit, it causes the capacitor to charge up, discharge and then charge up over and over again. The circuit “oscillates” between the two conditions.

The circuit allows the resistor to load up the capacitor until the capacitor has lots of energy stored up: enough energy to push current into the base of the transistor. Once that happens, the capacitor starts to discharge. That's cool, but it's not the big picture: once current is flowing, it forces the transistor to turn on.

Once our 2N4401 transistor is turned on, we use it to turn on a second transistor. Even more amplification! This second transistor is a power transistor. When we turn on the power transistor, it allows a big current to flow from the battery cell, pass through it and into the speaker.

Normally, this would seem to be a pretty gentle process for the capacitor: charge, discharge, charge again. This circuit has a nice trick that makes it operate even better.

Here we get to our demonstration of the see-saw behavior of a capacitor. Once the second transistor turns on, that transistor allows current to flow. It is connecting the battery cell V+ to the speaker wire. When that happens, the voltage on the speaker wire is pulled up. This also jerks up the capacitor leg connected to the speaker wire.

As soon as that leg of the capacitor is jerked up, this causes the voltage on the charge-storage side to instantly drop, fast. Because this side of the capacitor is connected to the first transistor, we see the potential on the base of the 2N4401 transistor instantly drop. That kills the current into the base of the 2N4401, so it shuts off.

Once the 2N4401 is shut off, the second transistor shuts off as well. Once everything is shut off, the capacitor starts to charge up again, starting the cycle all over again. Oscillation!

Of course, the rate of oscillation changes depending on how much current flows to the capacitor: that's why it's so interesting to move the wires along the ribbon: the rate (frequency) changes depending on where you are along the pencil pattern.

The Speaker

A speaker is really a nice piece of work. It's just a very thin piece of wire that's wrapped many times around a piece of paper to make a coil. This coil is put near a magnet. When the second transistor turns on, current starts to flow through the thin wire coil. This causes the coil to make a magnetic field. The coil's magnetic field pushes against the magnet's own permanent field, making the piece of paper move away. As the paper moves, air is being pushed towards your ear. This moving air is the sound you hear when the second transistor turns on and off.