Transcript Class Notes
Electronics Objectives • Describe a diode as a device that allows current to flow in one direction only and recall that a light emitting diode (LED) requires less current than a bulb • Set up simple series circuits using switches, buzzers, LEDs and resistors • Measure the resistance of a light-dependent resistor (LDR) under varying degrees of brightness of light • Identify everyday applications of the diode, including the LED, and of the LDR Objectives • To outline how diodes are positioned in simple circuits. • To define LED’s and look at their function • To outline the function of LDR’s Electronics Examples of electronic devices include iPods, radios, computers etc. We also have detectors e.g. smoke detectors, burglar alarms, carbon monoxide detectors etc. All these devices are operated by small carefully controlled electric currents. Components of a Microchip A microchip is a tiny chip usually made of silicon or germanium which can contain many complex circuits. Each circuit has many components. E.g. the cathode (negatively charged end of a diode), the Light emitting diode (LED) and the Light Dependent Resistor (LDR) Diode LED LDR The Diode A diode is a device which allows current to pass through it in one direction only. Image: symbol: The diode has a silver band at one end and this end is called the cathode (-) The opposite end is called the anode (+) The diode gets its name from the fact that it has two (di) electrodes. The Diode The triangle represents the anode (+) and the straight vertical line represents the cathode (-) Forward Biased If the cathode (-) is connected to the negative terminal of the battery and the anode (+) is connected to the positive terminal of the battery the diode will conduct a current. When connected like this the diode is said to be forward biased. When connected like this the diode conducts electricity and the bulb lights. Forward biased and reverse biased diode Bulb in circuit lights because diode is forward biased. Bulb in circuit does not light as diode is reverse biased Reverse Biased If the cathode (-) is connected to the positive end of the battery and the anode is connected to the negative end of the battery the diode is said to be reverse biased. When connected like this the diode does not conduct electricity and the bulb does not light. Uses of a Diode 1. 2. Main function of a diode is as a rectifier.(a rectifier is used to change alternating current (a.c.) into direct current (d.c.). Diodes are also used to protect appliances that use direct current. Simple series circuits using diodes • In the diagram on the right, current would normally flow in an anti-clockwise direction (from the positive end to the negative end). • In this case however current will only flow through part A, and not through part B because the diode in part B is pointing in the wrong direction. Light Emitting Diode (LED) An LED is a special diode which gives out light when current passes through it. Structure of LED A lens at the top of the LED helps to focus the emitted light. The shorter cathode lead is near the flat edge at the base of the LED LED’s LEDs come mainly in 3 colours: red yellow and green. They are very important in electronics because they use very little electricity If putting LED’s into a circuit also need to put in a resistor to reduce the current. Simple series circuits using LED’s • Simple series circuits using LEDs • Note that in both circuits the current flows in an anticlockwise direction (can you remember why?). • However the LED in the second circuit is turned the wrong way around so no current will flow in the second circuit and no light will be emitted. Important points about LEDS • An LED is a diode. Like all diodes current will only flow through it when it is forward biased. • A LED requires only a very small current. • To protect a LED a large resistor should always be used in series with it. Uses of LEDs 1. LEDs make a very useful indicator lamps on appliances e.g. red dot to show appliance is on. 2. LED can be used as a digital display e.g. on alarm clocks or cash registers Advantages of LEDs 3. LEDs are very cheap, reliable and long lasting and can be used in bicycle lamps and traffic lights Light Dependent Resister A LDR is a resistor whose resistance decreases with increasing light intensity. Image Symbol LDR’s • In dim light the Light dependent resistor has very high resistance and allows little current through. • In bright light the resistance of the LDR is low and it allows much more current through. Uses of LDR’s 1. They are used to switch street lighting on and off at appropriate times of the day. 2. They are used in light meters for cameras and burglar alarms.