Experiment 5 : Parallel and Series LED Circuits

There are 2 circuits here ; the top one shows two LED's connected in parallel. This means that both LED's get the same voltage of the batteries, this is about 4.5V.
Did you notice that the green LED seems to shine brighter than the red ? This is because our eyes are not so sensitive to red colors.

The serial circuit puts the two LED's in series. They both get only the half of the battery voltage, so they will shine more dimly.

A small question : what do you think of the current flow in both circuits ?
In a parallel circuit, the current will be the sum of both LED's, this is about 15.2 mA
In a series circuit the current would be 7.2 mA, but since we use only 4.5 V, the current will be smaller since both led's already consume 4 V. By measuring across the build-in resistors of 330 Ohms, we find that the current is about 1.07 mA.

If you have a voltage meter, you can also measure these currents.

Later on, we will build our very own galvanometer, which is also capable to measure very small voltages.

Experiment 4 : LED ON Voltage

This circuit shows us that a led needs a certain voltage to start emitting light.
When we use 2 batteries of 1.5V in series, we get 3V.
This voltage is high enough.
With only 1 battery, the voltage is only 1.5V, and this is too low to light the LED.
Remember from previous experiment that a LED needs around 1.8V to start lighting ?

Experiment 3 : Paths for the Flow of Electricity

This experiment lets us meet the LED and how it works.
In our kit, we see that the upper connection is the 4.5V and the lower connection is the grounding.
So, when we use blocks with nothing else then wires inside, we see that the current flows from the ground through all the blocks till it reaches the LED.
Inside this LED cube there is a resistor of 330 Ohms which protects the LED from burning.

With the law of Ohm, which states : U = R x I, we can calculate how much current flows through the LED.
U = Voltage
R = Resistance
I = Current

In our circuit, we know that the Voltage across the red LED is about 1.8V to 2V ( depending of the type of LED ). Let us assume the max. value of 2V
So, when we use the formula of Ohm, we can calculate the current through the resistor of 330 Ohms ( which is the same current through the LED )
4.5V - 2V = 2.5V
2.5V / 330 Ohm = 0.0076A = 7.6mA
This is a safe value for our LED

Experiment 2 : Diode tester

The second experiment can be used to test if the 2 diodes and LED's included with the kit ( red & green ) are OK.
After switching to ON, the LED will light up.
When you reverse the diode ( arrow pointing UP ), the LED stays dark.

This happens because a diode ( in our kit we use a Germanium diode ), can conduct electricity in only one way.
The same counts for the LED. When reversing ( arrow pointing up ), the LED stays dark, even though we use the diode in both directions.

Later on, we will explain everything in more detail.

Experiment 1 : Transistor tester

How the circuit works :

The LED needs about 1.8 V to light up.

A transistor is a device that can act as a switch or as an amplifier, depending of the circuit design.
The transistor is used here as a current amplifier. 
By touching A & B, a small body current will be amplified and will slightly close the C & E gate, so the LED will get the needed Voltage.

A small voltage applied to the Base, will be greatly amplified and the path between Collector and Emitter will conduct electricity.

The body resistance is so great ( around 2 Meg Ohms ), so we need a dark area to see the LED illuminate. Our body resistance conducts electricity, needed to turn the transistor on.

This is a great transistor tester to test the amplification factor.
Later on, we will calculate the transistor voltages.

Introduction pages

Thanks to Gakken Japan, they have provided the translation of the introduction pages.
You can find them here
They were also kind to keep the original layout of the book

A little video about this system