Kit #1: “2-Transistor Ribbon Kit”

THEORY OF OPERATION

Our simple ribbon controller uses a circuit called a relaxation oscillator.

We make a conductive “ribbon” which charges the capacitor on the base of transistor Q1, which eventually turns on Q1. This turn-on causes the capacitor to discharge. Transistor Q1 is used to drive a simple common-emitter amplifier (transistor Q2) which powers a small speaker. When the speaker is turned on, the capacitor leg is pulled up, causing the other leg of the capacitor to snap down and the voltage on the base of Q1 to drop quickly. This “relaxation” of the Q1 base voltage turns off the amplifiers and starts the cycle all over again. The operation of this oscillator is described in a bit more detail towards the end of this document.

Comments about each element of the circuit, and applications notes about their selection and use are provided in each of the sections below.

The Power Source

This circuit is designed to operate from 1.5VDC up to 3VDC but it can be operated from 6VDC with a series supply resistor of 220 ohms or more. A wall power supply may also be used.

The Resistor

We insert a 39kohm resistor to be used in the ribbon control path. This is provided to allow the circuit to continue to operate if the probes are shorted together, but it also protects the 2N4401 base-emitter junction against over-current drive. The primary resistance in this circuit comes from the object under test, or the ribbon. The range of functional resistance in the object under test can be changed somewhat by changing the size of the 39kohm resistor but the greatest effect comes from changing the size of the capacitor in the circuit.

The Ribbon

Pencil graphite makes a great resistive path for easy experimentation. One problem with the graphite comes if you grab the paper with an alligator clip or other clamp: with handling, the graphite dust tends to work its way away from the contact points. You will find that the pattern just stops working after a bit of experimentation. If this happens, unclip the paper and make your contact point darker with another layer of pencil graphite in the same spot. When you re-clip, the contact point will be good as new.

The pencil that  we've found works best is a soft sketching pencil. Basically, the softer the lead, the better the conduction. Pencil leads are graded on a hardness scale, Pencils are made hard by mixing the conductive graphite with fine clay. The harder the lead, the less graphite and more non-conductive clay. A #2 pencil is hard, so it doesn't put down enough graphite to make a great ribbon.

As a side note, the "2" for a pencil marked "#2" stands for lead hardness, which is synonymous for grade "HB" which you also sometimes seen on writing pencils. The "H" stands for 'hard'. All drawing pencils are soft, and you find their lead hardness noted with the a number and the letter "B". A "2B" pencil is two grades softer than a "#2" or "HB" pencil and it makes a good drawn ribbon. A pencil marked "2H" is even harder than a #2 pencil, and unsuitable for this experiment.

The pencil we've used in large groups is a soft artists' "Sketching Pencil", just about as soft as they get, an "8B". This is EIGHT grades softer than a #2 pencil lead! Your local art supply will probably stock something like the "Derwent Sketching Pencil, Dark Wash, 8B". It's great, but it's maybe a little too soft: the conductive path has such a low resistance that you don't get a great change in tone without making the path pretty thin. I'd suggest trying a harder leaded drawing pencil, maybe a "4B" or "2B". These pencils that are a bit harder are more commonly called "Drawing Pencils".

Sketching/drawing pencils are expensive ($1.50 to $2 each), but you may not need more than one or two for a big class, or you could buy one and saw it into two shorter pencils. The ribbon marking goes fast and many kids may want to experiment with a regular #2 pencil anyway.

We've also enjoyed experimenting with the black plastic conductive bags that ICs are packaged in. If you try this, you may want to increase the value of the capacitance in the circuit, since many varieties of conductive black plastic are a lot less resistive than a pencil path. Also, decrease the value of the fixed resistor, to provide more frequency range in the circuit using the black plastic ribbon.

The Capacitor

We've used a 0.01uF capacitor because it's inexpensive. In fact, this capacitor is the most expensive component in the kit. If you are using this circuit as a continuity tester, 0.05uF is a much better size, it discharges a bit more slowly giving a louder sound output. You will have to decrease the size of the resistor to make a 0.05uF capacitor value work for you in the continuity tester application.

The Transistors

We selected our transistors based on cost: they are the least expensive workable transistors available in a TO-92 package. Having said that, they're no slouches. The 2N4401 is an old workhorse, but it's still a great NPN switching transistor. High gain, tough as nails. The KSA642Gis a new part for us, but it's already a favorite PNP power transistor. Very high power, very high gain, very low cost. In this circuit, transistor selection isn't so important: almost any pair of transistors will function, more or less.

The Speaker

A speaker impedance from 4 to 16 ohms is required. If you want to run the audio output into an amplifier or other equipment, replace the speaker with a 6.8 ohm resistor and connect your audio input cable across that resistor.

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 energy stored up: enough energy to push current into the base of the transistor. Once that happens, the capacitor starts to discharge. 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, providing more amplification and the correct voltage swing to help shut off the capacitor's base voltage for the 2N4401 transistor. When the second transistor turns on, that transistor tries to connect 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's other leg which is connected to the speaker wire.

As soon as that leg of the capacitor is jerked up, this causes the voltage on the base-connected side to quickly drop. This 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.