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Boardworks GCSE Separate Sciences 2009 Logic Gates

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Presentation on theme: "Boardworks GCSE Separate Sciences 2009 Logic Gates"— Presentation transcript:

1 Boardworks GCSE Separate Sciences 2009 Logic Gates

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3 Boardworks GCSE Separate Sciences 2009 Logic Gates
Control systems Boardworks GCSE Separate Sciences 2009 Logic Gates

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What are logic gates? Boardworks GCSE Separate Sciences 2009 Logic Gates Logic 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.

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Logic gate voltage Boardworks GCSE Separate Sciences 2009 Logic Gates Digital systems have two states, ON and OFF. These simple electronic states are represented in binary code: ON is called logic 1; OFF is called logic 0. Microchips contain logic gates, which use this binary code to send and store information. Microchips can be easily damaged by high voltages. Due to this the voltages for the two logic states are standardized. Photo credit: © 2009 Shutterstock, Chris Hellyar Logic 1 is 5 V, while logic 0 is 0 V. Logic gates receive multiple signals in these two states. Different combinations of signals lead to different outputs.

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Types of logic gate Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes Students 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.

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Logic gates summary Boardworks GCSE Separate Sciences 2009 Logic Gates

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9 Using sensors in potential dividers
Boardworks GCSE Separate Sciences 2009 Logic Gates A potential divider uses two series resistors to reduce a high input voltage (VIN) to a lower output voltage (VOUT). VIN By replacing either resistor with an input device, the potential divider can be used as an input sensor for a control system. R1 VOUT Replacing R2 with a switch will produce a simple pressure sensor, while LDRs and thermistors can be used to produce light and temperature sensors respectively. R2 0 V 0 V

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Potential dividers Boardworks GCSE Separate Sciences 2009 Logic Gates

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Light sensors Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes Students 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.

12 Adjusting the threshold
Boardworks GCSE Separate Sciences 2009 Logic Gates In the previous example, there was a set light intensity at which the logic state flipped from 1 to 0. This threshold value is determined by the value of R2. Having a fixed threshold is fine if the system operated by the sensor should always activate at the same point. However in many cases it may be beneficial to vary this threshold value. A thermostat controls the temperature of a house, activating the heating system when the temperature drops below a set level. Photo credit: © 2009 Shutterstock, Daniel Gonsalves This threshold temperature can be varied, allowing the house to be kept at varying temperatures.

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Thermostat Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes Students 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.

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Sensors summary Boardworks GCSE Separate Sciences 2009 Logic Gates

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16 Truth tables for three inputs
Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes There are multiple pause points in stages three and four. These allow each combination of inputs to be studied individually if desired.

17 Three input truth tables question
Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes This 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.

18 Truth tables for four inputs
Boardworks GCSE Separate Sciences 2009 Logic Gates

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A burglar alarm Boardworks GCSE Separate Sciences 2009 Logic Gates A burglar alarm is a great example of a control system. Can you identify the input sensor, the processor and the output? input = movement sensor processor = alarm unit output = siren and lights Teacher notes This 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?

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The bistable latch Boardworks GCSE Separate Sciences 2009 Logic Gates The 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 A input A input B output 1 Teacher notes Students 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’? 1 1 Output 1 1 input B bistable latch NOR gate truth table

21 The bistable latch – how does it work?
Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes Students 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.

22 Build your own control system
Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes This 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.

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24 Making use of logic output
Boardworks GCSE Separate Sciences 2009 Logic Gates There 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.

25 Introduction to relays
Boardworks GCSE Separate Sciences 2009 Logic Gates A relay is an electromagnetic switch which allows one circuit to control another circuit by turning it on and off. A relay is used to isolate a high voltage circuit, controlling it with a low voltage counterpart. There are a number or possible reasons for this: To allow the use of a smaller, more practical switch. To isolate the user from a dangerous high voltage. To allow a logic system to control high voltage components. Photo credit: © 2009 Shutterstock, Igumnova Irina The starter motor in a car uses a current of several hundred amps, and is controlled by a relay, making it safe and easy to operate.

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How does a relay work? Boardworks GCSE Separate Sciences 2009 Logic Gates A 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. coil steel switch Electricity flows in the output circuit driving the motor. input output circuit symbol for a relay

27 Separating mains and logic voltage
Boardworks GCSE Separate Sciences 2009 Logic Gates Relays allow us to separate the output from an electronic control system, of either 0 V or 5 V, from the mains voltage of 230 V. This allows electronic control systems to turn on mains appliances, which they would otherwise be unable to run. It also protects the user from the dangerously high mains voltage. Image credit: © 2009 Shutterstock, hkann Some electronic control systems do have their own output device. This serves as an indicator, clearly showing the user the logic state of the circuit.

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LEDs as indicators Boardworks GCSE Separate Sciences 2009 Logic Gates LEDs are ideal indicators for a logic circuit – why is this? They require only a low current and voltage to emit light. Their two states, on or off, match the binary states of logic 0 and 1. 0-5 V In real circuits, we must use a protective resistor in series with the LED. Photo credit: © 2009 Shutterstock, Kevin H Knuth This ensures a voltage of around 0.7 V across the LED, preventing the current from becoming damaging. 0 V

29 Outputs and relays summary
Boardworks GCSE Separate Sciences 2009 Logic Gates

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Glossary Boardworks GCSE Separate Sciences 2009 Logic Gates Glossary AND 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.

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Anagrams Boardworks GCSE Separate Sciences 2009 Logic Gates

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Multiple-choice quiz Boardworks GCSE Separate Sciences 2009 Logic Gates Teacher notes This 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.


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