Electronics is the branch of electrical science and technology dealing with the flow and emission of electrons in and from conductors and semiconductors.
A material is described as a semiconductor if its electrical resistance is lower than a good conductor but less than an insulator’s.
Silicon, germanium and lead sulphide are examples of semiconducting materials.
The resistance of some semiconductors decreases with increasing temperature or by increasing the amounts of impurities in the material. The purposeful addition of impurities is known as doping.
An integrated circuit is a very small solid-state circuit made up of interconnected semiconductor devices like diodes, transistors and capacitors printed into a single silicon chip.
A diode is a two terminal semiconductor electronic device that only allows current to flow in one direction through it. The diode has very low resistance in one direction and in this direction it is said to be forward biased – current will flow when the diode is forward biased facing the positive pole of the battery.
The diode has very high resistance in the other direction (reverse biased). The silver band on the diode is closer to the cathode (-) electrode. The other electrode is the anode (+). Current flows through the diode from the anode to the cathode. Current will only flow if the diode is forward biased with the anode is connected to the positive terminal of the battery.
Know the symbol for a diode – a circle with a horizontal triangle which shows the direction of the current from positive to negative (from anode to cathode).
Know how to set up a direct current circuit with a forward biased diode so a lamp will light plus how to set up reversed biased diode to a lamp will not light.
Everyday Applications of Diodes
1. Converting an alternating current supply to direct current. Many electrical appliances (radio, television, computer, music systems) need direct current but alternating current is the supply from the ESB. So the a.c. supply has to be converted to d.c..
2. Protection of electrical devices which would be damaged by wrong flowing current.
Light Emitting Diode (LED) is a diode that emits light when forward biased and a small current is flowing through it. When reversed biased no light is emitted because there is no current flowing through it. LEDs were invented in the 1960s. The LED is a diode with higher resistance than normal but when current flows through it light is produced instead of heat. The symbol is the same as that for a diode with two arrows pointing away from it. The cathode electrode (-) is usually shorter than the anode (+). The cathode electrode (-) is close to the flat straight edge (Flat Cat). The lens at the top of the LED focuses the emitted light.
A LED uses much less current than a filament bulb and so a suitable resistor must be in series with it to ensure that it only receives a small current. A large current would damage the LED.
Everyday Applications of LEDs
Displays on VCRs, CD players, radios, digital clocks, calculators, car instrument panels and so on. Infrared LEDs are use in remote controls. LED ‘bulbs’ are now in use for lighting e.g. torches, traffic lights, car brake lights. LEDs last much longer than filament bulbs, light up quicker and use about 5% of the energy.
Light Dependent Resistor
A light dependent resistor (LDR) is a variable resistor in that its electrical resistance reduces as the intensity of the light (brightness) falling on it increases. In darkness the LDR has maximum resistance and no measurable current flows. As the light intensity increases, the resistance decreases and the current increases.
Know the symbol for an LDR: small resistor symbol surrounded by a circle with two arrows pointing towards the circle.
Everyday Applications of LDRs
1. Used as light controlled switches.
2. Can be used to switch on street lighting in the evening.
3. Use in light meters and in cameras.
Measuring the Resistance of a Light-Dependent Resistor (LDR) Under Varying Degrees of Brightness of Light.
1. Cover an LDR with an opaque card to block all light. 2. Connect a multimeter to the LDR.
3. Move the switch to the ohmmeter setting. 4. The ohmmeter reads a very high resistance.
5. Remove card and note the drop in resistance reading.
6. Gradually bring a bright light closer and closer to the LDR – note that the resistance continues to decrease.