3Boardworks GCSE Separate Sciences 2009 Logic Gates Control systemsBoardworks GCSE Separate Sciences 2009Logic Gates
4Boardworks GCSE Separate Sciences 2009 Logic Gates What are logic gates?Boardworks GCSE Separate Sciences 2009Logic GatesLogic gates are electronic switches that process information.They are called gates because they open to produce a high output signal only when they receive the correct combination of input signals.This chip contains four NOT gates.Most logic gates have multiple inputs, which are used to determine a single output.A large number of logic gates can be incorporated in one electronic chip.
6Boardworks GCSE Separate Sciences 2009 Logic Gates Types of logic gateBoardworks GCSE Separate Sciences 2009Logic GatesTeacher notesStudents can be asked to work out the truth tables for the logic gates themselves, using the buttons in the tables. Alternatively, by clicking the solve button for each logic gate, the truth table can be quickly completed and explained to the class.
7Boardworks GCSE Separate Sciences 2009 Logic Gates Logic gates summaryBoardworks GCSE Separate Sciences 2009Logic Gates
9Using sensors in potential dividers Boardworks GCSE Separate Sciences 2009Logic GatesA potential divider uses two series resistors to reduce a high input voltage (VIN) to a lower output voltage (VOUT).VINBy replacing either resistor with an input device, the potential divider can be used as an input sensor for a control system.R1VOUTReplacing R2 with a switch will produce a simple pressure sensor, while LDRs and thermistors can be used to produce light and temperature sensors respectively.R20 V0 V
10Boardworks GCSE Separate Sciences 2009 Logic Gates Potential dividersBoardworks GCSE Separate Sciences 2009Logic Gates
11Boardworks GCSE Separate Sciences 2009 Logic Gates Light sensorsBoardworks GCSE Separate Sciences 2009Logic GatesTeacher notesStudents could be asked the following questions:Q. What happens to LDR resistance as the light intensity increases?A. The resistance decreases.Q. What happens to output voltage as the light intensity increases?A. The output voltage starts high, resulting in a logic 1 signal. It decreases with light intensity, producing a logic 0 signal in bright light.Q. What effect will changing the fixed resistor have?A. This will change the ‘threshold’ or ‘switching’ light intensity, namely the light level at which the logic state of the sensor changes. This point is dealt with in more detail on the following slides.
13Boardworks GCSE Separate Sciences 2009 Logic Gates ThermostatBoardworks GCSE Separate Sciences 2009Logic GatesTeacher notesStudents could record the results of this experiment in a table, to highlight the variation in the threshold temperature of the thermostat. This example uses a heater as an intuitive manner of varying the temperature; however it could be pointed out that the heater would normally be the output device in a heating system, with the control system activating it as ambient temperature drops below a threshold value.
14Boardworks GCSE Separate Sciences 2009 Logic Gates Sensors summaryBoardworks GCSE Separate Sciences 2009Logic Gates
16Truth tables for three inputs Boardworks GCSE Separate Sciences 2009Logic GatesTeacher notesThere are multiple pause points in stages three and four. These allow each combination of inputs to be studied individually if desired.
17Three input truth tables question Boardworks GCSE Separate Sciences 2009Logic GatesTeacher notesThis activity is highly flexible, and can be used to demonstrate logic circuits to a class, or test their understanding. In this activity, the interactive logic circuit allows the students to easily visualize the interactions between the gates. The circuit could be used to work out the values for the truth table, to check the students predictions, or to clear up any problems the students have if the activity is used in a plenary role.
18Truth tables for four inputs Boardworks GCSE Separate Sciences 2009Logic Gates
19Boardworks GCSE Separate Sciences 2009 Logic Gates A burglar alarmBoardworks GCSE Separate Sciences 2009Logic GatesA burglar alarm is a great example of a control system.Can you identify the input sensor, the processor and the output?input = movement sensorprocessor = alarm unitoutput = siren and lightsTeacher notesThis could provoke discussion in the class, as students try to solve the problem using their knowledge of logic gates. The answer to the question is addressed in the following slides.The alarm keeps sounding until the owner enters the code into the control panel, regardless of the input it receives.How does this happen?
20Boardworks GCSE Separate Sciences 2009 Logic Gates The bistable latchBoardworks GCSE Separate Sciences 2009Logic GatesThe alarm contains a bistable latch, or ‘flip-flop’ circuit.This is an arrangement of two NOR gates. The output of one gate forms one input of the other.This arrangement is capable of storing a logic state; as such ‘flip-flops’ are used extensively in computer memory.input Ainput Ainput Boutput1Teacher notesStudents could be asked to work out some of the different states for this circuit. What do they think is meant by the term ‘stable state’?11Output11input Bbistable latchNOR gate truth table
21The bistable latch – how does it work? Boardworks GCSE Separate Sciences 2009Logic GatesTeacher notesStudents should understand that there are two possible stable states for the latch when both its inputs are at logic 0. A short change in voltage at the appropriate input can cause a switch to the other of these stable states. Students could be asked the following questions through the animation sequence, predicting the logic state in each part of the circuit.Stage 3 – Q: What will happen to the system if the movement sensor is triggered? Why?Stage 4 – Q: What will happen to the system when the sensor stops detecting movement? Why?Stage 5 – Q: What will happen to the system when the alarm code is entered? Why?Stage 6 – Q: What will happen to the system after the code is entered? Why?After viewing all of the stages, the students should be able to explain how the bistable latch got its name – it has two stable states, which lock its output value.
22Build your own control system Boardworks GCSE Separate Sciences 2009Logic GatesTeacher notesThis activity gives the students a chance to design their own working control system. It should be noted that the position of top left corner of the components determines the square into which they drop.
24Making use of logic output Boardworks GCSE Separate Sciences 2009Logic GatesThere are many different output devices for an electronic system. These include motors, buzzers and lamps.In a logic system both current and voltage are very small to prevent damaging the gates.This limits the range of output devices which can be run on a logic system.In order to make full use of logic systems, we must find a way to control a high voltage component using a low voltage logic system: a relay allows this.
26Boardworks GCSE Separate Sciences 2009 Logic Gates How does a relay work?Boardworks GCSE Separate Sciences 2009Logic GatesA relay uses an electromagnet to operate a switch.As electricity flows in the input circuit, the coil becomes an electromagnet.The coil attracts the steel switch in the output circuit, closing the switch.coilsteel switchElectricity flows in the output circuit driving the motor.inputoutputcircuit symbol for a relay
31Boardworks GCSE Separate Sciences 2009 Logic Gates GlossaryBoardworks GCSE Separate Sciences 2009Logic GatesGlossaryAND gate – A logic gate that gives a high output when both of its inputs are high.bistable latch – A simple memory circuit, consisting of two NOR gates.flip-flop – An alternative name for a bistable latch.indicator – An output component used to show the output logic state of a control system.input sensor – The part of a control system that detects changes, to which the system responds.logic gates – Electronic switches that produce an output signal based on the correct combination of input signals.NAND gate – A logic gate formed by combining an AND gate and a NOT gate. Its output is the opposite of AND.NOR gate – A logic gate formed by combining an OR gate and a NOT gate. Its output is the opposite of OR.NOT gate – A logic gate that gives an output opposite to its single input. It is also called an inverter.OR gate – A logic gate that gives a high output when one or both inputs are high.output device – The part of a control system that is controlled by the processor.potential divider – A device used to convert a change in resistance in the input device, to a change in voltage.processor – The part of a control system that converts information from an input device, to a response in the output device.relay – A switch, operated by an electromagnet, that allows one circuit to control another.truth table – A table which shows all possible combinations of input voltage, with their corresponding outputs, for a specific logic gate.
32Boardworks GCSE Separate Sciences 2009 Logic Gates AnagramsBoardworks GCSE Separate Sciences 2009Logic Gates
33Boardworks GCSE Separate Sciences 2009 Logic Gates Multiple-choice quizBoardworks GCSE Separate Sciences 2009Logic GatesTeacher notesThis multiple-choice quiz could be used as a plenary activity to assess students’ understanding of logic gates. The questions can be skipped through without answering by clicking “next”. Students could be asked to complete the questions in their books and the activity could be concluded by completion on the IWB.