LOGIC GATES A logic gate is an elementary building block of a digital circuit.digital circuit Most logic gates have two inputs and one output terminals. At any given moment, each terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels binary

AND, OR and NOT gates There are three main types of logic gate AND NOT OR.

Two additional types In year 11 you also need to know about NAND – which is an AND + a NOT NOR – which is an OR + a NOT

Logic gates Logic gates are at the heart of digital electronics Video recorders, security lamps, alarm systems, and washing machines are just some of the things controlled by electronic switches called logic gates A logic gate contains a circuit with transistor switches, but you do not need to know anything about the circuit inside All you need to know is how the logic gate will respond when it gets a signal – that is, what will its output be.

Web sites (copy to a word document to link to) gic/and.htm gic/and.htm gic/index.htm gic/index.htm ontrollogicrev2.shtml ontrollogicrev2.shtml

1. Switches in Series If two on-off switches are connected in series they must all be closed (on) to complete the circuit and light the lamp. The ideas behind logic gates

if either A or B is OFF (open), the bulb is off We say the output of the gate is 0 If both A and B are ON (closed), the output is 1

The AND gate symbol

One input (1 and 0) – no output so lamp is off (0) (in the diagram, red means a signal)

Two inputs (1 and 1) – lamp is lit (output 1)

The truth table gives the results of all the possible switch settings. It uses two logic numbers: 0 for OFF and 1 for ON Input A Input B Output Q

The OR gate If two switches are connected in parallel then only one needs to be closed (on) to complete the circuit.

One input – there is an output so the lamp is on

the other input is on, so there is an output

Two inputs – the lamp is on

No input – so no output and the lamp is off

Truth table for the OR gate Input A Input B Output Q

The NOT gate This is also called an INVERTER, because whatever signal you put in, you get the opposite signal out. ‘signals’ mean a voltage, such as: 5v to mean 1 (on) no voltage to mean 0 (off)

The NOT gate

No input, but there is an output

An input, but there is no output

Combining gates – An inverter used to reverse a an output from a gate

The AND gate is giving an output, but the inverter is not

Nothing from the AND gate, yet the inverter gives a signal

A new truth table for this would be ABCQ ABCQ

Tape recorder logic gate The following diagram shows one use for a logic gate. The cassette recorder will only start recording if the ‘record’ and ‘play’ buttons are pressed together.

Alarm system The next diagram shows how sensors and logic gates can be used to control a security lamp. The sensors and gates have been connected so that if it is dark and someone approaches, the lamp comes on automatically. The last gate cannot provide enough power for the lamp, so it switches on a relay instead. This switches on a separate circuit with the lamp in it.

Logic gates working together to make an alarm system

Checking the logic To check that the combination behaves as intended, you can write a truth table for it. In this case, the light sensor’s output is LOW (0) in the dark the body heat sensor’s output is HIGH (1) if someone approaches the final output Q must be HIGH for the lamp to come on.

Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed together in a single device.

Two tasks for you to do Design a block diagram for a bank safe that needs two keys to activate the lock. The key sensors will send an output when the correct key is used. Design a logic system for street lights that come on when it gets dark but only if there are cars on the road. For each one, write the truth table

Arrays of logic gates are found in digital integrated circuits (ICs) used in computers and all sorts of electronic devices As technology advances, the required physical volume for each individual logic gate decreases and digital devices become capable of performing ever-more-complicated operations at ever- increasing speeds.

Sensors the input devices for logic gates The role of sensors in control Sensors are used to measure physical quantities such as temperature, light, pressure, sound, and humidity. They send signals to the processor. For example: Sensorsprocessor A security alarm system may have an infra-red sensor which sends a signal when the beam is broken. A heat sensitive sensor in the corner of a room may detect the presence of a person. Temperature sensors could be used to control the heating in a large building. Pressure sensors in the road could detect traffic.

Project task for this unit Using the web sites and other sources, find out more about sensors and logic gates. Compile the information into a brief by useful report. Direct downloads of material is not accepted. The report must show that you have organised the information and presented is as a report.