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GCSE PHYSICS : ELECTRONICS M. Manser Electronics and electronic systems are quickly becoming indispensable to everyday living. They are being used more and more in our homes, schools and places of work. Semiconductor devices and their use in integrated circuits (“chips”) have become invaluable for entertainment (video games), convenience (automatic banking machines) and in medicine (pacemakers). Mass production allows for these tiny devices to be made quite cheaply. The systems (of which they are a part) are usually small, reliable, use small amounts of energy and respond very quickly in comparison to most mechanical systems. ELECTRONIC SYSTEMS Input Sensor Processor Detects changes in the environment, and converts one type of energy into an electrical signal : LDRs, thermistors, microphones, switches. AND, OR, NOT gates Output Transducer* Takes the electrical energy from the processor and converts it to another form like heat or light or sound: lamps, LEDs, loudspeakers, motors, heaters, relays. * A transducer is a device for converting one type of energy into electrical energy, or vice versa. INPUT SENSORS Some property of these devices must be affected by the environment. For example, a thermistor’s electrical resistance is affected by the surrounding temperature. When included in a circuit, the current and voltage distribution in a circuit is automatically and instantly affected by changing temperature. A potential divider is usually used to provide a required voltage. Often an input sensor and a variable resistor are used. The total voltage supplied (Vin) is then shared across the two resistors depending on the environment (ie temperature or light intensity for example) and also on the value of the variable resistor. If either resistance is increased, the share of the voltage across it also increases. Vin The potential divider equation : Vout = Vin x R2 , (R1 + R2) R1 Vout is then fed to the processor. Vout Changing the setting of R1, allows the circuit to respond at different temperatures. R2 0V 0V Whatever device is connected across Vout will have the same voltage across it as R2 since it is in parallel with it. Consider the variable resistor to be set at a certain value. Then R1 alone will vary depending on the temperature. If it gets hotter, then R1 will get smaller, so there will be a smaller portion of Vin across it. This means that there will be a higher voltage across R2 , i.e. Vout will increase as the temperature increases. The way that this circuit is used can be altered by taking Vout across R1 instead. In this case, as the temperature increases, and the restistance of the thermistor drops, the voltage across it (Vout) drops too. Vout will be in the form of an (electrical) analogue signal, i.e. it will have many different values as the environment changes. PROCESSORS Logic gates are tiny electronic devices that are manufactured from many transistors (semiconductors) and produce a digital output from an analogue input (a voltage). They can be manufactured or set to work above certain voltages. If we consider a particular logic gate designed to “work” with 3V, then any input below this can be thought of as “LOW” or “0”, then if the voltage is 3V or above, the input is considered to be “HIGH” or “1”. Logic gates are used in a variety of combinations for different jobs. The NOT gate. The OR gate A (inverter) The AND gate O A O A B O B Truth table Input Output A B O 0 0 0 1 0 1 0 1 1 1 1 1 Truth table Input Output (A) (O) 0 1 1 0 Truth table Input Output A B O 0 0 0 1 0 0 0 1 0 1 1 1 Using logic gates Example 1 : A washing machine designer wants to ensure that the motor will not start until the on/off switch is ON and the door is safely CLOSED. Very simple circuits can be set up so that when the switch is on, a HIGH voltage sent to the processor (ie, one input = 1) , and if the door to the washing machine is shut, the second input is HIGH (or 1) as well. (A)Power on/off switch on = 1 off = 0 MOTOR (O) (B) Door switch 1= motor on 0 = motor off shut = 1 open = 0 A: 0 1 0 1 Truth table Inputs B: 0 0 1 1 Output O: 0 0 0 1 Example 2 : This circuit is a safety feature. A buzzer sounds to remind occupants to fasten their seatbelts. If the ignition switch is on, and the seatbelt is fastened, then the engine will turn ON and the buzzer will be OFF. If the ignition switch is on and the seatbelt is not fastened, then the buzzer will sound (ON) and the motor in the engine will stay OFF. A 1 1 0 0 B 1 0 1 0 Output 1 1 0 0 0 Output 2 0 1 0 0 (A) ignition switch (B)seat-belt switch on =1 off = 1 Engine : O 1 engine on = 1 engine off = 0 Buzzer : O 2 buzzer : on =1 off = 0 belt fastened = 1 belt unfastened = 0