1 The advantages and limitations of electronic systems Electronic system... 3

Transcription

1 1 The advantages and limitations of electronic systems Electronic system... 3 (a) Input sub-system... 3 (i) Switches... 3 (ii) Light sensor... 4 (iii) Temperature sensor... 4 (iv) Pulse generators... 4 (b) Process sub-system... 5 (i) Transistor switch... 5 (ii) Logic gate... 7 (iii) Binary adder... 9 (iv) Flip-flop (v) Amplifier (vi) Comparator (vii) Analogue to digital converter (viii) Integrated circuits (c) Output sub-system (i) Light bulb (ii) Light emitting diode (iii) Solenoid (iv) Buzzer (v) Loudspeaker (vi) Motor (vii) Relay Electronic control (a) General principle (b) Electronic circuit symbols (c) Electronic control circuit design (i) A practical example (ii) Safety Measures Interactive information.23 Exercise

2 1 The advantages and limitations of electronic systems Electronic systems have many advantages, such as: (a) Small in size and light in weight, e.g. mobile phones and electronic notebooks. (b) Information can be transmitted to distant destinations easily and quickly using electric wires, e.g. fax machines. (c) Electronic signals can be converted into radio waves easily and transmitted to the surroundings, e.g. radios. (d) Fast operation and high accuracy, e.g. electronic calculators, electronic game machines, hard disk drives, etc (Fig. 1b). (e) Lower energy consumption allows batteries to be used for a longer time. Fig. 1 (a) Fax machine (b) hard disk drive However, electronic systems have various limitations, such as: (a) The transmission of electronic signals is subject to interference from electric fields and magnetic fields. (b) The system may not work properly under unfavourable conditions (for example, extremely cold, hot or humid environment). (c) The cost of development is very high. Much investment is needed for production lines to go into operation. (d) Electronic parts and circuit boards are relatively fragile and may be broken easily upon impact. (e) The efficiency of electronic systems is low unless they are used together with mechanical systems. (f) It is often difficult to repair electronic components and circuit boards. Therefore, they must be replaced when they are broken. 2

3 2 Electronic system Electronic systems can be divided into the input sub-system, the process sub-system and the output sub-system (a) Input sub-system Input sub-systems include various input components and electronic circuits, such as switches, light sensors, temperature sensors, pulse generators, etc. (i) Switches The main function of switches is to enable or interrupt the flow of electric current. They are used in electric circuits. Switches come in many forms, such as button, control rod, slide, rotary and sensor (Fig. 2). When designing an electronic system, one should always look at the situation and choose the most suitable switch. Fig. 3 shows the circuit symbols of some common switches. (a) Button switch (b) Slide switch (c) Rotary switch Fig. 2 Different kinds of switches (d) Sensor switch (a) General Fig. 3 Circuit symbols of switches (b) Button switch 3

4 (ii) Light sensor A light sensor is in fact a variable resistor. Its resistance varies with the intensity of the light it receives (Fig. 4). The resistance will increase when the sensor is exposed to weak light, and decrease when exposed to strong light. Therefore, a light sensor can convert differences in light intensity (light signals) into differences in current and voltage (electric signals). Light sensors are mainly used in automatic control, infrared detection, etc. Fig. 4 (a) Light sensor (b) Circuit symbol of a light sensor (iii) Temperature sensor The resistance of a temperature sensor varies with temperature. They are commonly used in switching circuits. The resistance of a common temperature sensor increases under low temperature and decreases under high temperature. Temperature sensors are responsible for temperature detection in fire alarm systems, and body temperature measurement in electronic thermometers (Fig. 5). Fig. 5 (a) Electronic thermometer (b) A thermistor (c) Circuit symbol of a thermistor (iv) Pulse generators The term pulse is used to describe a sharp difference in electric voltage or current that takes place within a very short time. If we express the change of pulse voltage (vertical axis) with time (horizontal axis) in the form of waves, we can see the variation of the pulse voltage. Examples of pulse waveforms include square waves, triangular waves, saw-tooth waves, drum waves, etc (Fig. 7). Fig. 6 Pulse generator 4

5 (a) Triangular waves (b) Saw-tooth waves (c) Drum waves Fig. 7 Square wave is the most common form of pulse waves, because its shape can show clearly the difference between high and low voltage, which allows the representation of the two digital values 1 and 0 (Fig. 8a). Therefore, square waves are commonly used as the input and output signals of digital systems. Ideal square waves are supposed to have only either high voltage or low voltage and no other voltages in between, but in reality such ideal square waves are very difficult to produce. Usually square waves look like those in Fig. 8b. Fig. 8 (a) Ideal square waves (b) Real square waves A pulse generator can be applied to a high voltage pulse circuit and used to change the strength of pulses. Pulse generators are often used in camera flashlights, because they can supply high voltages within a very short time. (a) Process sub-system The process system within an electronic system changes the input electronic signals according to preset instructions. The processed electronic signals form the output of the system. There are numerous components and systems that can process electronic signals, examples include transistor switches, logic gates, binary adders, flip-flops, amplifiers, comparators, integrated circuits, etc. (i) Transistor switch A transistor is an electronic component that can be used to switch on/off electronic circuits, or to amplify current or voltage. A transistor is formed by three p-type and n-type semi-conductors, arranged in p-n-p or n-p-n form (Fig. 9). A transistor has three poles: base (B), collector (C) and emitter (E). The two types of transistors are represented by different symbols. The symbol of a PNP transistor contains an arrow pointing at its base, while that for an NPN transistor contains an arrow pointing away from its base. As NPN transistors can be mass-produced more easily, we will use it for the following discussions. 5

6 Fig. 9 (a) Transistor (b) PNP-type transistor (c) NPN-type transistor A transistor circuit can be connected in many different ways. As shown in Fig. 10, if we connect the emitter of an NPN-type transistor to both its base and collector, the resulting circuit is called a common emitter circuit. In a common emitter circuit, the base-emitter voltage forms the input voltage V in, while the collector-emitter voltage forms the output voltage V out.. For the transistor to work properly, V in has to be larger than the cut-off voltage V BE (ON), which is about 0.7V. Otherwise, the transistor will be in a cut-off state and will cease to be conducting. The currents flowing in the base, collector and emitter are called base current I B, collector current I C and emitter current I E respectively. As the current flowing into and out of the transistor must be the same, it can be deduced that I E = I B + I C. Fig. 10 Common emitter circuit Fig. 11 Input / Output voltage properties curve The curve in Fig. 11 shows the different height of two plateaus of the output from the transistor. They can be applied to switches and the components of logical calculation. Heat sensitive switch Fig. 12 shows a simple heat sensitive switch circuit that involves the use of a transistor. As the thermistor R T has a higher resistance under low temperature, the potential difference V in across variable resistor R is smaller, and if V in is smaller than 0.7V, the transistor will remain in a cut-off state, the output voltage V out would be 6V, and I C would be zero. Therefore, the indication light L will not glow. 6

7 Fig. 12 (a) Heat sensitive switch circuit (b) A pictorial diagram of the heat sensitive switch circuit When the temperature rises, R T will decrease, causing the potential difference across R V in to increase and the output voltage V out to decrease close to zero. I C will increase to a maximum (saturation), causing the indication light L to glow. This circuit can be used in a fire alarm system. R can be used to set the temperature that will set off the alarm. (ii) Logic gate A transistor circuit can be used to make one or more logic gates. A logic gate can process only binary signals, such as high/low voltage, true/false, conducting/non-conducting, etc. All these signals can be represented by the two logic values 0 and 1. The relations between the input and output of a logic gate can be expressed in a truth table. Examples of logic gates include NOT gate, AND gate, OR gate, NAND gate, NOR gate, etc. NOT Gate If you look at the transistor input/output voltage properties curve in Fig. 11, you can see that the values of V in and V out are exactly by opposite. Therefore, the transistor is called a NOT gate, which is designed to produce an output opposite to the input. If we input the logic value A, the NOT gate will generate a result that is not A, or A. Table 1 shows the truth table of a NOT gate. Fig. 13 shows the circuit symbol of a NOT gate. Input A Output F A Table 1 Truth table of a NOT gate Fig. 13 Circuit symbol of a NOT gate 7

8 AND Gate An AND gate is a logic gate that generates the output 1 only if its inputs are also 1. Its output is called A and B, represented by A B. Table 2 shows the truth table of an AND gate. Fig. 14 shows the circuit symbol of an AND gate. Input A Output B Output F A B Table 2 Truth table of an AND gate Fig. 14 Circuit symbol of an AND gate NAND Gate The output of a NAND gate is the exact opposite of that of an AND gate. Table 3 shows the truth table of a NAND gate. Fig. 15 shows the circuit symbol of a NAND gate. The output of a NAND gate is called not (A and B), represented by A B. Input A Output B Output F A B Table 3 Truth table of a NAND gate Fig. 15 Circuit symbol of a NAND gate OR Gate In an OR gate, whenever one of the two inputs A and B equals 1, the output would be 1. The output of an OR gate is called A or B, represented by A + B. Table 4 shows the truth table of an OR gate. Fig. 16 shows the circuit symbol of an OR gate. Input A Output B Output F A B Table 4 Truth table of an OR gate Fig. 16 Circuit symbol of an OR gate 8

9 NOR Gate The output of a NOR gate is the exact opposite of that of an OR gate. Table 5 shows the truth table of a NOR gate. The output of a NOR gate is called not (A or B), represented by A B. Fig. 17 shows the circuit symbol of a NOR gate. Input A Output B Output F A B Table 5 Truth table of a NOR gate Fig. 17 Circuit symbol of a NOR gate (iii) Binary adder Decimal number Binary number Table 6 Decimal and binary numbers Many electronic systems need calculation to finish their jobs. One example would be the circuit of an electronic calculator. Calculation in electronic systems are mainly done in binary numbers, that is, numbers formed by the two digits 0 and 1. Table 6 shows the corresponding decimal and binary forms of a set of numbers. Fig. 18 shows how binary numbers can be converted into decimal numbers. Value Binary = = = Fig. 18 When an electronic system wants to add two numbers together, it has to use a binary adder, which is formed by a half adder and a full adder. Both half adders and full adders can be made from integrated circuits. Half adder A half adder adds two binary digits together and outputs the result. Fig. 19 shows the four possibilities when adding two binary numbers A and B together. After they are added together, the adder will return the sum S and the carry C. 9

10 Fig. 19 Fig. 20 Fig. 20 shows the circuit symbol of a binary half adder. The adder has two inputs A and B, and two outputs C and S. Full Adder A half adder can only process the summation result of two one-digit binary numbers, while a full adder can process the summation result of two multiple-digit binary numbers. Fig. 21 (a) (b) A full adder can be made from two half adders and an OR gate. Fig. 22 shows its electronic circuit diagram. To perform four digit summation, we need a combination of three full adders and one half adder, as shown in Fig. 23. Such arrangement forms the basic mode of a binary adder in an electronic system. Fig. 22 Fig

11 (iv) Flip-flop Electronic systems often have to store numbers for further use in calculations. The component responsible for this task is called a memory. A flip-flop is a simple kind of memory. Flip-flops come in many models, such as S-R flip-flop, S-R NOR gate flip-flop, D type flip-flop, J-K flip-flop, etc. Fig. 24 shows the circuit symbols of different flip-flops. (a) S-R flip-flop (b) J-K flip-flop (c) A flip-flop device Fig. 24 A flip-flop can be formed by logic gates. The S-R flip-flop in Fig. 24a is formed by two transistors. Although the operation of a flip-flop is rather complicated, it still plays an important role in an electronic system because it is needed to make memories, which are very useful. Examples of memory include the RAM found in computers. (v) Amplifier The function of an amplifier is to strengthen weak signals, one example would be the amplifiers used by radios to strengthen the weak signals received by its antenna. Fig. 25a shows the circuit of a simple amplifier that employs transistors to amplify the input voltage. When the input voltage V in falls within the linear amplification area, a linear relation will form between V in and the output voltage V out. When used with a suitable base resistor R B and load resistor R L, a transistor can amplify the input voltage of alternate current. The result is shown in Fig. 25b. Fig. 25 (a) (b) An amplifier can amplify not only the voltage, but also the power. The extra energy is supplied by a second electric energy source in the output circuit. Fig. 26a shows two common symbols of amplifiers. Fig. 26b shows the circuit of an amplifier. 11

12 Fig. 26 (a) Symbols of amplifiers (b) Circuit of an amplifier (vi) Comparator A comparator (Fig. 27) is commonly used to compare the input voltage with a preset value. If the input voltage is smaller than the preset value, the comparator will output a lower voltage. On the contrary, when the input voltage is larger than or equal to the preset value, the comparator will output a higher voltage. The output allows other systems to respond correspondingly. For example, in a fire alarm system, a comparator is used to compare the preset temperature with the signal it receives from the temperature sensor. When there is a fire, the voltage signal generated by the temperature sensor will become larger than the preset voltage signal, the comparator will then activate the alarm. (vii) Analogue to digital converter Fig. 27 A comparator Electronic signals in an electronic system can be transmitted in two forms: analogue signals and digital signals. An analogue signal, such as the sinusoidal waves in Fig. 28a, carries with it a continuous stream of varying signals. A digital signal, such as the square waves in Fig. 28b, can transmit only two values. Fig. 28 (a) Example of analogue signals (b) Example of digital signals Most electronic sensors receive analogue signals. Due to the resistance in the circuit, these signals may be weakened greatly after transmission and repeated calculation, resulting in errors. As digital signals can carry with them only one of the two values, the values stored in them cannot be changed easily. Therefore, digital signals are more suitable for use in repeated calculation and transmission. 12

13 Fig. 29 Circuit of a square wave generator Analogue signals can be converted into digital signals. Fig. 29 shows a transistor circuit that can convert the sinusoidal wave voltage V in generated by the low voltage power source into square wave voltage V out (Fig. 30). Fig. 30 Sinusoidal wave is changed to square wave An analogue to digital converter (ADC) can convert the analogue signals it receives from the electronic sensor into digital signals, so as to allow further processing and transmission. Its principle of operation is shown in Fig. 31. Similarly, a digital to analogue converter (DAC) can convert digital signals into analogue signals. Fig. 31 The principle of operation of an analogue to digital converter Fig. 32 shows an example of an analogue to digital converter. The electronic scale seen in the picture contains a microprocessor that can execute certain orders repeatedly. Analogue signals, which are the weight of the food, are sent to the strain gauge, where an analogue to digital converter converts them into 1 and 0 digits, which are in turn converted into decimal numbers for display on the screen. 13

14 (viii) Integrated circuits Fig. 32 Operational flow diagram of an electronic scale As technology advances, electronic systems also become more and more sophisticated. The numbers and brands of electronic components also increase sharply. Examples include transistors, diodes, resistors, capacitors, connecting wires, etc. However, when a large number of electronic components are put together, the size of the circuit will become very large. Much energy will be wasted, and a large amount of heat will be produced. Integrated circuits (IC) are formed by a large number of electronic components combined together on a tiny microchip (usually silicon made) with the help of advanced technology (e.g. photo chemical machining). The components are then sealed in a plastic shell, exposing only a few legs for connection. Integrated circuits reduce not only the size of circuits, but also their energy consumption. Integrated circuits are often used to produce microprocessors, read-only memories (ROM) and random access memories (RAM) (Fig. 33) Fig. 33 Example of integrated circuits 14

15 (c) Output sub-system The output sub-system of an electronic system is responsible for the display and execution of the output. Examples include light emitting diodes, solenoids, buzzers, loudspeakers, motors, relays, etc. (i) Light bulb A light bulb usually consists of a hollow glass ball that contains metal filaments. The metal filaments, made of tungsten, will glow when heated. To prevent the filaments from oxidizing too quickly, light bulbs are usually filled with inert gas such as nitrogen and argon (Fig. 34a). Halogen bulbs contain gases like iodine, which allows the filament to be heated to a higher temperature so the bulb will become brighter. The temperature of the filament is very high. In a 100 W light bulb, the temperature of the filament can be as high as 3300 o C. Fig. 34 (a) Structure of a light bulb (b) Light bulbs are used as decorations on a bridge A light bulb can be used as the output device of an electronic system. For example, the colourful light bulbs found on buildings are often controlled by electronic systems (Fig. 34b). Another example would be the light bulbs found in traffic lights. (ii) Light emitting diode A light emitting diode (LED) is an output component that emits light and allows current to pass through in only one direction (Fig. 35). If we connect the longer leg of an LED to the positive terminal of the power source, the LED will allow current to pass through and glow. On the contrary, if we connect the shorter leg to the positive terminal, the LED will not allow current to pass through and so will not glow. LED are very small in size, so they are often used as indication lights in electrical appliances. The other advantages of LED include low energy consumption, low heat emission and they are safe to use. Fig. 35 (a) Light emitting diode (LED) (b) Circuit symbol of an LED 15

16 (iii) Solenoid A solenoid consists of a series of metal coils arranged together. When current passes through the solenoid, it will generate a magnetic field like that of a magnet (Fig. 36). If we add a soft metal core to the solenoid, the solenoid will be strengthened to form an electromagnet. As the magnetic field generated is very strong, the solenoid will attract nearby metal objects. However, if the current is cut off, the solenoid will cease being magnetic. Electromagnets are often used in cars, motors, lifters, etc. Fig. 36 (a) A solenoid (b) When the current passes through the solenoid, a magnetic field is generated (iv) Buzzer A buzzer is an output component that emits a buzzing sound when current passes through it (Fig. 37). As buzzers can only emit one kind of sound, they are mainly used in alarms and signal lights. Fig. 37 (a) Buzzer (b) Circuit symbol of a buzzer (v) Loudspeaker A loudspeaker converts electric energy into sound energy. A loudspeaker is formed by a paper or plastic cone and a movable coil. At the end of the coil lies a permanent magnet (Fig. 38). When an electronic signal induced by the input sound passes through the coil, the current will react with the magnetic field of the permanent magnet, producing a force that moves the cone. As the cone vibrates and vibration is transmitted to nearby air, sound will be emitted. In this manner, the cone has amplified the vibration and thus copied and boosted the input sound. 16

17 Fig. 38 (a) Loudspeaker (b) Structure (c) Circuit symbol of a loudspeaker (vi) Motor A motor converts electrical energy into magnetic energy and kinetic energy. Examples include electric fans, air conditioners, washing machines, electric dryers, etc. Motors are formed by a metal coil suspended in a magnetic field generated by a permanent magnet or electromagnet. A magnetic field will be produced when current passes through the metal coil. Due to the attraction and repulsion of the two magnetic fields, the coil rotates and moves the axis. A rotating coil is called an electric main (Fig. 39). Fig. 39 (a) Electric motor (b) Principle of motor (c) Circuit symbol of a motor Electric motors can be divided into two classes: direct current motors and alternate current motors. In both models, a force is generated to drive the machines when current passes through the metal coil. Direct current motors are driven by direct current, while alternate current motors are driven by alternate current. (vii) Relay A relay is a device that uses a relatively small current to connect or cut off a larger current. As high-power motors usually employ large current, it would be dangerous for users to switch on directly. Thus a relay can allow the user to start or stop the machine safely. A relay is a combination of components such as electromagnet, armature, spring, contact, etc (Fig. 40). When the switch is closed, current passes through the solenoid. The magnetic core is magnetised, attracting the armature and closing the contact, resulting in a closed circuit. When the switch is open, the current 17

18 in the coil is cut and the magnetic core is demagnetised. The armature is restored to its original position by the spring, cutting off the contact. The result is an open circuit. In this manner, a relay can be used to control the switches of motors, electrical appliances, etc. Common symbol A.C. relay Fig. 40 (a) Relay (b) Circuit symbol of a relay 3 Electronic control (a) General principle Electronic Control means the process of using electronic devices to perform designated tasks. Examples include thermostat control, complex aviation control, automatic door control, etc. Fig. 41 shows the block diagram of a typical electronic control circuit: Fig. 41 Block diagram of a typical electronic control circuit External signals provide operational orders for electronic control devices. These signals can be in the form of light, heat, electric current, pressure, speed, sound, etc. A signal receiver collects these signals and sends them to the signal converter. A signal converter is a device that converts physical signals into electronic signals. It is an important part of electronic control systems. All electronic devices work with electronic signals (for example, electric current, voltage, resistance, etc) and not physical signals. The electronic signals generated by the signal converter are usually very weak. Therefore, an electronic amplifier is required to amplify them so they can be used to drive other components. Typical electronic amplifiers are usually formed by components such as resistors, transistors and other semi-conductors. 18

19 (b) Electronic circuit symbols In order to simplify the representation of electronic circuits, engineers employ different symbols to represent different circuit components. With the help of these symbols, one can easily draw an electronic circuit diagram and describe the relations between each electronic component. Fig. 42 shows some common circuit symbols. Fig. 42 Common circuit symbols (c) Electronic control circuit design Before designing an electronic control circuit, one should first understand the properties of each electronic control component. After that, one can follow certain basic steps and combine the components to form the correct circuit. (i) A practical example Scenario We shall use a simple streetlight system to show the basic steps in designing an electronic control circuit. The system uses time to control the streetlight, so it can turn on at night and off in the daytime. One drawback of this system is that the light will remain off on cloudy days or in the winter, when the streets are quite dark. Fig. 43 The lighting decoration in the street 19

20 General concept In order to avoid the problem mentioned above, we should improve the system so that its operation depends on the brightness of the streets. Design details Fig. 44a shows the block diagram of the original streetlight system. Fig. 44b shows the block diagram of the improved system. The improved system uses a light sensor (such as a light sensitive resistor) to detect the brightness of the streets. The signals from the sensor are fed back to the system to control the streetlight. (a) (b) Fig. 44 Block diagram of the original streetlight system Data collection As this electronic control system involves the use of light sensitive resistors and switches of high voltage streetlight, refer to related texts on light sensitive resistors, transistor switches, relays and control circuits. Solution Fig. 45 shows a light sensitive switch circuit that uses a transistor as the switch. The system uses a relay to operate the streetlight. The variable resistor is set at 200 k. 20

21 (a) A pictorial diagram of a light sensitive switch circuit Fig. 45 (b) A light sensitive switch circuit Situation 1 When the light sensitive resistor is exposed to light, its resistance will be reduced to approximately 9k. The potential difference V LDR across the light sensitive resistor is (refer to the Fig. 45): 9 V LDR = 6 = V As a result, the potential difference across it will be smaller than the cut-off voltage of the transistor at about 1 V, causing the transistor to go into a cut-off state. As no current will pass through the base of the transistor, the output voltage V out will be 6V. The circuit is an open circuit. The streetlight circuit will not be activated. 21

22 Fig. 46 The variable resistor and the light sensitive resistor serve as a potential divider in the circuit Situation 2 When the light sensitive resistor is put in a relatively dark place, its resistance will rise to about 600k. The potential difference across it will exceed the cut-off voltage of the transistor, causing a current to flow through the transistor (refer to Fig. 46): 600 V LDR = 6 = 4.5 V The output voltage V out will fall close to zero. The relay will then switch on the streetlight circuit as well as the streetlight itself. A diode is connected to the relay to prevent the transistor from damage by the eddy current generated when the relay is turned on or off. By adjusting the variable resistor R, the system can be set to activate at a lower or higher level of brightness. Realisation, Testing and Evaluation After finding the solution, we can test the above system by connecting a light bulb to it. We can also learn to make fine adjustments to the system, such as adjustments to the resistor R. Next, we can evaluate the performance of the system and find ways to improve it. But remember, when connecting the circuit, one must pay attention to safety. (ii) Safety Measures For safety reasons, one should pay attention to the following: 1. When assembling circuits, do not deviate from the design plans. 2. To ensure safety, choose the appropriate voltage. 3. Positive poles should be distinguished from negative ones. 4. Handle electronic components with care so as to avoid percussion and damage to the components. 5. Use a relay to control high power / voltage electronic components. 6. Understand the functions and output signals of electronic components so as to avoid faulty connection, which may cause overheating and electric leakage. 22

23 Exercise 1. Draw the circuit symbols of the following electronic components: (a) Thermistor (b) Light sensor (c) Light emitting diode (d) PNP type transistor (e) NPN type transistor (f) Relay 2. State the working principles of a transistor and show how to distinguish different types of the transistors. 3. Draw the circuit symbols of the following logic gates, and write down the truth tables of AND gate and NOT gate. (a) AND gate (b) OR gate (c) NOT gate (d) NAND gate (e) NOR gate 4. What is a logic circuit? 5. In what mode of signal does an input exist in a logic circuit? 6. What is an amplifier? 7. What are analogue and digital converters? 8. Give three examples of using analogue and digital converters in daily life. 9. Design an electronic control circuit to fit the following processing requirements: Patients are sent to the Intensive Care Unit (ICU) after operations in a hospital. They need medical staff to take special care of them. Some kind of help devices are needed to be installed beneath the beds, so that the patients can press the alarm buttons to ask for help in the day time (when light is available). At night, the alarm system may cause a nuisance to other patients. Thus the system will be switched off and replaced by other helping systems. (a) (i) What types of input devices should be used? (ii) Which logic gates should be used for the system? logic gate. (iii) What types of output devices should be used? Write down the truth table of that 23

24 (b) By using the following decision module, show how to connect the circuit to establish the help system for the patients. 10. Design an electronic control circuit to fit the following processing requirements: Some careless parents always forget to lock the brake of a baby cart. When they do not hold the cart tight, the baby cart will slide down the slope. Accidents may happen and the baby will get hurt. Design an alarm device for the baby cart. The alarm will show a siren when the parents forget to lock the brake and their hands do not grasp the handle of the cart. (Note: When the sliding switch is ON, the braking system of the baby cart will be locked.) (a) (i) Which two types of input devices should be used? (ii) Which logic gates should be used for the system? (iii) What types of output devices should be used? (b) By using the following decision module, show how to connect the circuit to establish the electronic safety system for the baby cart. 24