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Input Components Moisture Sensors Moisture sensors consist of a flat plate with two conductors etched upon them. They can come in different shapes. The type illustrated gives maximum intimacy between conductors for a given surface area. Another type takes the form of a long strip with the conductors running parallel to themselves. When moisture bridges the gap, the resistance is lowered between the conductors. This drop in resistance can trigger atransistor circuit, which in turn could operate a relay. Uses:- Plant watering in dry weather, sensing when it rains, flood warning, sensing damp conditions in electrical equipment etc. Light DependentResistor (LDR) The resistance of these devices decreases when light falls on them. A typical example is the ORP 12 type which will vary its resistance from about 2 Megohms in the dark to 100 ohms innormal room lighting. LDRs are frequently used as sensors inlight activated switch circuits, for example, to switch on lights in the evening when it starts to get dark. Thermistor The resistance of these devicesdecreases when they are heated (negative temperature coefficient type). Typical resistance values are 1 kilohm at 25°C and 80 ohms at 100°C. Thermistors are suitable for temperature controlled switches and temperature measurement. Control Components Transistors Transistors are used as electronic switches and current amplifiers. They have three "legs" - base , collector and emitter. A small current to the base switches on the transistor allowing a much larger current to flow between collector and emitter. The ratio of the collector current to the base current is known as the "amplification" or "gain" of the transistor (hfe = Ic/Ib). They come in various different shapes and sizes and it is important to decide which type is required at the outset.Factors to be considered are voltage and power handling capacities, cost and availability. Transistors require a base voltage of at least 0.6V with respect to the emitter to "turn on". (Darlington Pair transistors require 1.2 V). A limiting resistor - typically 1K is required in the base circuit of the transistor stage to prevent overrunning and hence overheating and destruction of the transistor. Transistors come in NPN and PNP types. Itis important to use the correct type when designing and building circuits. 555 This chip is designed to function as a monostable (one stable state) multivibrator or as an astable (freerunning) multivibrator. It contains the equivalent of approximately 20 transistors, resistors, diodes etc. It is used for timing or switching circuits which give pulsed or digital outputs. Data: Pin 1 connected to the negative (-ve) supply 2 TRIGGER a voltage greater than 1/3 supply volts enables the chip. 3 OUTPUT is equal to the supply voltswhen the output is on (600mW max). 4 RESET when connected to positivesupply volts. 5 CONTROL OUTPUT applying a variable voltage to this pin varies the frequency of the output. 6 THRESHOLD a voltage higher than the threshold voltage (2/3 supply volts)turns on the output. 7 DISCHARGE when the output is turned on the pin goes to 0v allowing discharge of any connected capacitor. 8 positive supply voltage (+ve) (4.5v -16v) Uses: Oscillators, triggers, inverters, timers 741 OPERATIONAL AMPLIFIER (OP-AMP) This chip is designed to accept two inputs on pins 2 and 3. The difference between the inputs is amplified to give the output. If the voltage on pin 2 is greater than pin 3 the output is negative. If pin 3 is greater than pin 2 the output is positive. The Op-Amp gives a voltage gain as opposed to a transistor which gives a current gain. Uses: Switching, amplifiers, oscillators and analogue computing. Data: Pin 1 OFFSET NULL not normally used 2 inverting input 3 non inverting input 4 0V is connected to the zero supply 5 OFFSET NULL not normally used 6 OUTPUT 7 positive (+ve) supply voltage (+3V to+18V) OFFSET NULL is used to balance output in high gain applications. Output Components LEDs Light Emitting Diodes (LEDs) come in different colours, shapes, sizes and brightness. The more common colours being red, green and yellow. They are also available in bi and tri - colour versions Round ones come in 3, 5 and 8mm sizes. There are also rectangular, square and bar types. Flashing types are available and seven segment displays are made of a number of LEDs in a particular pattern. LEDs are easily damaged by excess current therefore it is normal to connect the LED to the supply via a series current limiting resistor. The value of this resistor is not critical but is usually within the range 100 - 330R. Polarity must be observed whenconnecting LEDs (it should be noted that LEDs are damaged by reverse biased voltages larger than about 6 volts). The Anode is connected to the positive supply via the limiting resistor. The Cathode is connected to the negative supply. Bulbs Bulbs are used as a form of illumination indication and the type commonly used has a filament coil which glows white hot when a current passes through it. The filament is in a space filled with an inert gas to aid in light output andto prevent oxidisation of thefilament.Bulbs consume more power than LEDs but give a higher light output. There are two common types of bulb usedin schools MES and LES - which are smaller in physical size than MES types. They may be bayonet or screw cap but screw types are more common. Motors A device for changing electrical energy into rotational movement. The turning power or torque of a motor can vary greatly. Generally speaking small motors have low torque whereas large motors usually have a high torque rating. Motors come in ac and dc versions, dc types of between 3 - 12 volts being most suitable for general use. The current direction controls the direction of rotation for dc motors.It is usually necessary to geardown a motor as their speed of rotation typically 2400 rpm is too great This can be achieved either mechanically or electronically. DC motors have the advantage of being relatively inexpensive and can be obtained in high torque versions which are most suitable for driving models. Solenoid A solenoid is a coil of wire wrapped around a former. The plunger is free to move and is attracted into the coil when a current flows in the coil. When the current is turned off, a spring pushes the plunger out of the coil.Solenoids are used for converting electrical energy into mechanical energy (transducer) in the form of linear motion. They are current and voltage rated and attention must be taken to prevent these limits from being exceeded. Uses include electrical door locks and model animation. Bells These are electromagnetic devices with contacts arranged so that once the coil is energized the armature is attracted to it and the circuit is broken. Once broken the coil is de-energized and the armature springs back to complete the circuit once more. Each time the armature is attracted to the coil the hammer strikes the bell. This continues in an oscillatory manner until the power switched off. Bells normally work on low voltagetypically 6 volts. Bells are used as outputs for alarm circuits. Loudspeaker The loudspeaker is an electro-mechanical device that turns alternating currents into audible sounds. When a signal of sufficient amplitude is applied to the coil it produces a magnetic field which attracts or repels the permanent magnet. This produces movement which is amplified by the diaphragm to give sound. Speakers can reproduce sounds in the audio spectrum between approximately 20 - 16,000 Hz. Small speakers handle high frequencies well, whereas large speakers are mainly suited to low frequencies. A combination of two or more speakers can reproduce frequencies over the whole audio range. Where hifi quality is not required, a small speaker will normally suffice. Buzzers Buzzers are usually of the single tone variety and are normally rated between 6 and 24 volts. They are suitable for alarm circuits, test equipment, timing and control. To acheive maximum sound ouput from the device it should be firmly mounted.