Output+Devices

toc Output devices are chosen to provide a specific output signal, whether this is sound, light or movement. The output devices you will be asked about are as follows.
 * 1.8 Output Devices**

media type="custom" key="2690045" media type="custom" key="2690041"

= Buzzer/Siren = This simple device produces an audible tone when switched on. The buzzer comes in many different shapes and sizes, from small low power units to high power alarm siren’s.

Low Power Buzzers suitable for projects: High Power siren used for alarm and security applications: Every buzzer will specify the operating voltage and current required for the correct operation of the buzzer, e.g. 12V, 100mA. It is important that the output circuit matches these requirements for correct operation. = Lamps = A lamp is a simple device for providing a visual indication that an output has switched on. Just like buzzers there is an enormous range of lamps that can be used as an output indicator. Here are a collection of just a few : Once again the choice of which lamp to use will fall to the circuit designer to choose the most appropriate type for the application. As with buzzers, every lamp has a maximum voltage and current, e.g. 6V 0.06A, which not be exceeded otherwise the lamp can be permanently damaged. It is your responsibility as the designer of the circuits to ensure that these maximum values are not exceeded. Most lamps produce light from a very hotwire called a filament. This method of light production is not very efficient, and lamps burn out quite quickly, after about 1000 hours (~46 days) of continuous use

= Light Emitting Diodes (LED **)**= The LED has revolutionised electronic circuits, because it is a device that produces light in a variety of colours, without the heating effect of the filament lamp. It is therefore much more efficient than the filament lamp and it lasts much longer than a filament lamp, typically about 100,000 hours (>11 years continuous use). The LED is different from the other components we have met so far because they must be connected correctly, as it only allows current to flow through it in one direction. A closer examination of the symbol will help you to correctly insert the LED into a circuit. The anode must be connected to the positive part of the circuit. Current then flows through the LED in the direction of the arrow.

LEDs can operate on a very small current, typically 10 to 20mA, which means that they are very energy efficient. The only downside is that they have a relatively low voltage rating, ~2.0V and always have to protect the LED in a circuit to ensure that this maximum voltage is not exceeded.

LEDs are available in a huge variety of different package styles and colours. As well as being available in single devices as shown above, groups of LEDs can now be packaged together and they can be seen all around you, replacing filament lamps in a variety of everyday objects.

An LED typically requires around 20 mA of current. A series resistor is needed to limit excessive currents. The value of this resistor can be calculated using Ohm’s Law However, this must take into account that the LED needs about 2 V across it to work. For the circuit in Diagram B, the value of the current limiting resistor can therefore be calculated as follows: Normal practice would be to choose the nearest preferred value of ﬁxed resistor which is higher than the calculated need, such as a 360 Ω resistor or a 470 Ω resistor if a 360 Ω is not available. (see resistor values and series)
 * Calculation of the value of the series resistor**


 * Traffic Lights**

= Motors = Motors are used where movement is required as the output, e.g. to open a gate, or turn the drum of a washing machine. Motors again are available in many different shapes and sizes. Motors generally require quite large operating currents (>500mA) in order to perform their role correctly. Their use in this course is likely to be limited as schools do not usually carry large supplies of motors for experimental procedures, but may form part of a theoretical examination question so they should not be discounted as a usable output device.

= Speakers =

A **loudspeaker** (or "speaker") is an electroacoustic [|transducer] that converts an [|electrical] [|signal] into [|sound]. The speaker moves in accordance with the variations of an electrical signal and causes waves to propagate through a medium such as air or water. For more information, go to this link:
 * **@http://en.wikipedia.org/wiki/Loudspeaker**

= Seven Segment Displays = This requires just seven LEDs (an eighth one for the decimal point, if that is needed). A common technique is to use a shaped piece of translucent plastic to operate as a specialized optical fiber, to distribute the light from the LED evenly over a fixed bar shape. The seven bars are laid out as a squared-off figure "8". The result is known as a seven-segment LED. Use the following link to look at 7 Segment Displays - try connecting one using Yenka
 * **http://www.play-hookey.com/digital/experiments/seven_seg_led.htm**

= Solenoid = The solenoid is a specialised output device which relies on the electromagnetic effect of an electric current. The solenoid is used to operate bolts in security doors, and for engaging the starter motor of the car to the engine when the car is first started. It has a pulling action when activated, and a spring mechanism, or gravity usually returns the object to its original position. The following picture shows a solenoid lock with the bolt retracted on the left and released on the right. 

The lock would be installed in a door, and the bolt would engage with the floor to secure the door. As with motors, solenoids usually need quite a large current (>1A) to generate a strong enough magnetic field to pull the bolt into the device, against a spring or the pull of gravity, and therefore their use in the electronics laboratory is very limited. They are useful devices in the industrial and automotive world however, so once again could be used as part of a theoretical examination question.

= Recap = With the exception of the LED and low power buzzers, all of the output devices considered have relatively large output current requirements. This means that they cannot be connected directly to the sensing systems we used last lesson. As we found out in our first look at systems, an output driver will be required to provide this additional current for these higher powered output devices. Complete the following worksheet, there are two worked examples to start off with: