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Electronic Projects: LED

This project introduces the LED - a modern components used in phones, dvd players, and as a replacement to a light bulb. To make them work in your own circuit, you need to buy them along with some other basic parts such as batteries and leads to join everything up. Below is a list of the parts and a brief description of what they do.

1. 9V PP3 type battery. An alkaline one works best, but any will do.

2. Crocodile test leads for joining the circuit together. About 10 leads should be enough for most experiments.

3. A Blue high quality LED. I say high quality to mean high brightness at a low current. With this type of LED you can make it light up even with a very very small amount of electricity. This is usefull for some of the projects to see what things conduct electricity that you might not think of. LEDs have to be connected the right way round. The + from the battery must go to the long lead of the LED. They only conduct electricity in one direction. LEDs can also break easily. DO NOT CONNECT AN LED DIRECTLY ACROSS THE BATTERY. YOU MUST USE FINGERS OR THE CIRCUIT BOARD AS SHOWN IN THE DIAGRAMS, OTHERWISE YOU MIGHT BREAK THE LED.

4. A 6B pencil. It needs to be a very high B number, that is very soft, to make it easy to make dark thick lines.

5. Some cardboard - about 1mm thick. Can be any type really, but we will connect the crocodile leads to it, so a thickish type is easiest to use. 50mm x 50mm square is enough.

Below is a circuit I use to kick things off in a class. With the good Blue LED, we can see it light up, maybe only dimly, but none the less light up just with current flowing through us. You must connect the LED the correct way to the battery, otherwise it may break the LED.



This circuit works and lights the LED, even if only dimily. However, if you replace the LED with an ordinary bulb, the circuit will not light the bulb. An old fashioned light bulb is not as efficient as an LED. So, although current flows around the circuit, it is not enough to light the bulb.

LED stands for Light Emitting Diode. A diode that emits light.. but what is a diode? A diode is like a valve that only lets things through in one direction. A diode in an electronic circuit onlyallows electricity to go in one direction through the circuit. Diodes are used a lot in electronic circuits. Diodes are used in very simple radio receivers to decode or demodulate the single.


Pencil PCB

You can make your own printed circuit board PCB using a pencil and some thick card. the carbon or graphite in the pencil conducts electricity. If you make a continuous pattern on some card, you can make a circuit light up an LED. BUT the lines must be thick and they must be continuous! See the examplebelow:


The carbon in pencils conducts electricity. Make sure the lines are thick.



Transistor Circuits
The leads on the transistor must be connected correctly. The picture show the transistor from the bottom where the leads come out from.



Which LED is brighter? What is the transistor doing?




Electricity and Electronics


  • Electricity is the flow of electric charges. In most cases with everyday circuits, this is the flow of ELECTRONS.
  • Electrons are super small. If you look the word up in a good dictionary, electron will be defined as :
    • A lepton with
      • a negative charge of 1.6022 x 10-19 coulomb
      • Rest mass 9.1096 x 10-31 kilogram
      • Radius 2.818 x 10-15 metre
      • Spin ½
These are unusual words with very specific meanings. Until these characteristics were well understood, electronic components could not be made.
  • There are an incredibly large number of electrons flowing through the wires in a second – about 100,000,000,000,000,000 or so..
Electronic components take advantage of the characteristics of electrons to allow them to be controlled. Therefore, electronic devices could not be made until we understood well what electrons were and how they reacted to outside effects. Thus, electronic components such as LEDs and Transistors are very recent inventions of the last 50 years or so. Improvements are consistently being made to improve the efficiency and to reduce the size of components. New materials are constantly being developed to create new components that give new effects and uses – eg the blue LED has been developed extensively in only the last ten years..
  • LED stands for Light Emitting Diode
  • Diodes only conduct electricity in one direction.
  • To connect properly, connect the positive or + side of the battery to the LONG lead of the LED.
  • Directly connecting an LED across a battery can break the LED. It can get hot, and so can the battery. The LEDs used will tolerate a few seconds of direct connection. However, the circuits have been designed to use the fingers or the carbon circuit board to limit the current flowing. To minimise the number of components, extra current reducing resistors have not been used. See the web site for alternative circuits.
  • Some modern LEDs are so sensitive and efficient that they will glow even from the small amount of electricity that can flow through a person using a 9volt battery
  • The more electrons that are flowing through the LED, the brighter it will be.
  • The transistor being used has three terminals called:
    • Base
    • Emitter
    • Collector
  • Transistors can be considered as switches or amplifiers or multipliers.
  • Multipliers are probably the easiest word for most people to understand. The number of electrons flowing through the base is amplified or multiplied by the gain of the transistor. This result is then how many electrons will flow through the collector and the emitter (almost)..
  • The gain of a transistor is a characteristic of each transistor, based on its fabrication style and parameters. The BC108 transistor being used has a gain of about 300. So for every electron that flow through the base, about 300 will flow though the collector/emitter part of the circuit.
  • In the transistor circuits, you can see that the transistor has amplified the number of electrons flowing through your fingers or PCB, because of the difference in the brightness of the two LEDs in the circuit.
Example of multiplication of electrons in the circuit – for every electron flowing through the Base, 300 or so will be flowing through the Collector/Emitter.



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