Tutorial 5 - Output Devices

Learning Objectives

• describe the use of electromagnetic relays, solenoids, buzzers, motors, and seven-segment displays in a system and understand and explain circuit protection provided by a diode in parallel with a relay, solenoid or motor;

• understand and use COM, NO and NC notation.

Electromagnetic relay

The relay is a switch operated by an electromagnet.  It allows the small voltage and small current produced by an electronic circuit to turn on a big current at a high voltage.  The circuit symbol for a relay is:

Notice that there are two different kinds:

• NO – normally open.  The contacts are open until the coil of the relay is energised, whereupon they are closed to complete the outside circuit.

• NC - normally closed.  The contacts are closed until the coil of the relay is energised, whereupon they are opened to break the outside circuit, switching it off.

• Many relays have multiple contacts, half of which are NO and half NC.

Where there are multiple contacts, there are common terminals, marked COM.  The switching arrangement is shown below.

Relays are electromagnetic devices which have a certain amount of inductance.  When they are turned off, the collapse of the magnetic field can produce a momentary “spike” of high reverse voltage that can wreck a transistor or integrated circuit.  Therefore a reverse biased diode is placed in parallel to short out the voltage spike, thereby protecting the circuit.

Solenoids

Solenoids are at their simplest an elongated coil of wire along a tubular former.  If we put a piece of soft iron (which loses its magnetism quickly) into the middle of the tube, we find that the iron core is attracted rapidly into the tube.

Solenoids have a number of uses, especially where a short straight-line movement is called for.  Uses include:

• Door chimes (the ding-dong type)

• Remote controlled door locking

• Point motors on a model train layout

• Motors to open valves

• Electric staple guns.

Like a relay, it is a good idea to have a reverse-biased diode in parallel to protect the electronic circuitry.

Electric Motors

Electronic circuits are used to drive electric motors, which convert electrical energy into rotational movement.  Examples include:

• Motors in washing machines,

• Motors in tape decks or CD drives.

• Model train motors.

• Traction motors in railway locomotives.

The picture shows a universal motor used in a variety of mains appliances, e.g. a vacuum cleaner.

There are several different kinds of motor:

• Permanent magnet motor

• Universal motor

• Mains induction motor

• Synchronous motors.

• Stepper motor where very precise movements are needed.  They are controlled using pulses.

Loudspeaker

The loudspeaker is a device that uses the motor effect.  A light coil moves forwards and backwards in a radial magnetic field in response to the variations of the signals that are fed in.  The cone moves the air forwards and backwards  to make vibrations that we can hear.

Loudspeakers are low voltage and low resistance devices, typically 4 – 8 W, so care has to be taken with load matching in the design of audio circuits.

If you want to know how the electric motor effect works, on the button.

Buzzers

Buzzers make an audible signal in response to a direct current signal.  They take a small current, in the order of a few milliamps and can be driven by a small signal transistor.

Buzzers use the piezoelectric effect, in which a crystal of quartz moves when a voltage is applied.  The noise they make is penetrating and irritating.

 Question 1 Do the interactive matching exercise.

Seven Segment Displays

These are output devices that are often use LED technology.

The circuit diagram looks like this.

The LEDs are arranged in a common cathode arrangement.  While we normally associate LEDs with round shapes, it is possible to make LEDs of any shape we like, including the flattened hexagons we see here.  Some displays have an extra LED display to show a decimal point.

The number 4 would require segments B, C, F, and G to be on.  A, D, and E are off.

Complete this table to show which LEDs would be on and off.  (On = 1; Off = 0)  One has been done as an example.
 Number A B C D E F G 1 2 3 4 0 1 1 0 0 1 1 5 6 7 8 9 0

You will also see 7 segment displays at garage forecourts to display the price per litre of petrol or diesel.  Instead of LEDs, motorised flaps are used.  One side is black; the other is white or a fluorescent colour.

The 7 segment display can also display letters such as lower case 'h', or upper case 'E'.  These can be ambiguous, e.g. the number 1, or the letter 'I'.

To increase readability, 9 segment displays can be used, or better still, 14 segment displays.  Even better is a large array of LEDs as a Dot Matrix Display.  These are often found in shops or doctors' surgeries showing messages passing across, along with simple helicopter, tank, or "muncher" characters.

More recent devices use liquid crystal displays.  Electronically they are diodes.  They work by twisting light through polarisers by changing the shape of large complex molecules with electric fields.

Modern computer screens use LCD screens.  The characters are made up using picture elements (pixels), which are tiny square dots.  In normal resolution the pixels are small enough to blend together to make curved lines.  This enables us to see different text fonts.

 Summary Electromagnetic relay It allows the small voltage and small current produced by an electronic circuit to turn on a big current at a high voltage. Needs reverse-biased diode protection for the circuit. Solenoids Move an iron core inwards. Electric Motors Electric motors which convert electrical energy into rotational movement. Loudspeakers Use the motor effect to move a diaphragm forwards and backwards. Buzzers Make a noise.

 Links How electric motors work How relays work How loudspeakers work Liquid crystal displays