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Electronics in Engineering Technology Introduction Electronics is the art and science of getting electrons to move in the way we want to do useful work. Electrons are a component of atoms. There are about 100 different kinds of atoms. Each different kind is called an element. Some elements have structures that hold tight to their electrons so that it is very hard to make the electrons move. Materials made with these elements are called insulators. Other elements have structures that allow their electrons to move easily. Materials made with these elements are called conductors. In the following example a sensing circuit is illustrated. This circuit can be easily modified to operate as a temperature, moisture or vibration sensor. By varying both the input (or sensor) and the output (e.g. a motor, buzzer or bulb etc) this circuit can fulfill a variety of functions. Circuit 1 – Dark Sensor (Transistor). Function When light level falls below a certain value. The LED will light. The light level at which the LED will light can be set by use of a variable resistor.
Circuit diagram Components SW1 = Toggle switch R1 = 470 ohm R2 = 2200 ohm R3 = 1k ohm R4 = LDR VR1 = 47k ohm Transistor (BC108) LED = 5mm standard
Operation This circuit uses a transistor (BC108) as an electronic switch. Transistor action. When the voltage on the base of the transistor is less than 0.6V the transistor is switched off. No current can flow through the transistor from collector to emitter so no current flows through R1 or the LED; thus the LED is unlit. If more than 0.6 V is applied to the base, the transistor switches on. Now current can pass easily from collector to emitter so current can flow from the top rail, through R1, the LED and the transistor back to the battery. The LED is now lit. Consider the variable resistor (VR1) and the LDR (R4) in series (Ignore R3 which is small in comparison to VR1). They form what is known as a voltage divider circuit.
Operation of the voltage divider circuit Consider the value of the variable resistor set so as its resistance is very high. Consider the LDR placed in bright sunlight i.e. its resistance is very low (in the region of 400ohms). With the large resistance (VR1 = 47k ohms) and resistance of the LDR (400ohms) connected in series, a large voltage drop will occur across VR1, so the voltage at the connection of VR1 and the LDR will be very small, (less than 0.6 volts). As this is the voltage, which appears at the base of the transistor, so in effect the transistor is switched off, the LED is unlit. In darker conditions the resistance of the LDR increases, as a result less voltage is dropped across VR1 and the voltage at connection of VR1 and the LDR will increase (i.e. more than 0.6V). This voltage applied to the base will switch on the transistor and thus the LED is lit. Thus by adjusting the value of VR1 the transistor can be made to switch on at a particular light level.
Calculation of the value of R1 It is most important an LED has no more than 2 volts dropped across it, also the current flowing through the LED should not exceed 20 mA. When the transistor switches on, the voltage at the collector is approximately 0 V, between the top rail and the collector is 9 V, so 7V must be dropped across R1. With 7 V dropped across it and the maximum permissible current of 20 mA flowing through the LED. The size of resistor can be calculated using Ohms law:
Note 1 In electronic circuits it is not good practice to have components operating at their upper limits so a resistor value of 390 or 470 ohms should be used for R1 Remember the larger the value of R1 the less current is drawn from the battery. So battery will last longer at the expense of the LED being a little dimmer.
Note 2 By adjusting VR1 so as its resistance is small the voltage on the base of the transistor could be very high. This high voltage could damage the transistor, for this reason a base resistor (R2) should always be inserted (value 2200 ohms). R3 is also placed in circuit as a safety precaution.
Further information may be found at the following sites. http://www.design-technology.info/electronics/ http://www.williamson-labs.com/ http://library.thinkquest.org/10784/atut1.html (An introduction to electronics.) http://elchem.kaist.ac.kr/vt/chem-ed/electron/devices/devices.htm |
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