What are output transducers An output transducer will convert electrical signals passed to it by the process into another form of energy. ACTIVITY Can you find the symbols of the output components listed and draw them on this page? 64
What is an LED (Light Emitting Diode) An LED is a digital output device it is either on or off. The LED will emit light only when it is forward biased. The Anode of the LED is marked + or on the picture is the LONG LEG and the short leg is the Cathode. Symbol for an LED + or ANODE - or CATHODE An LED needs 2 Volts to be forward biased and can conduct a maximum of 20mA, any more and the LED will fail. With this in mind an LED must have a LIMITING resistor to protect it from conducting too much current. Calculating the protective resistor for an LED +6V R Ω 0.02A 2V 65
How can I Interface the LED Using a Transistor? From INPUT circuit or from the output of a PIC chip 330 Interfacing an LED directly to a PIC chip A PIC chip can drive one LED off each output directly. However if you need more than one a Transducer Driver circuit will need to be added. 1 2 3 4 PICAXE08 8 7 6 5 66
What is a Buzzer? A buzzer is a digital output device it is either on or off. Buzzers can produce high-pitched frequency sounds as well as the traditional buzzzzzz! Symbol for a buzzer +V Buzzzzz High Pitched Buzzer How can I Interface a Buzzer? From INPUT circuit or from the output of a PIC chip A buzzer will need a capacitor in parallel with it when controlled by a Transistor. What is a Piezo Sounder? A piezo sounder is an analogue output device, and can be made to oscillate by a driver circuit at specific frequencies. It can therefore play a tune when directly connected to a PIC chip. You can get piezo sounders to solder directly onto a PCB board, or to fit inside a device, however you will need to design the product to have a sound baffle to mechanically amplify the sound. Symbol for piezo sounder PCB mounted 67
How can I Interface a piezo sounder to a PIC chip. 1 2 3 PICAXE08 8 7 6 NB The program will use the SOUND command to drive the sounder. 4 5 +Volts lead What is a DC Motor The DC Motor is a digital device it can be switched on or off. However it cannot be switched directly from a PIC chip, and needs a BCX38B transistor to operate it directly from an INPUT circuit. Interfacing the DC motor is critically important when designing a system using such a device. Symbol M DC Motors require typically 200mA to 500mA to drive them compared to an LED that uses 20mA. However as the load on the motor drive shaft is increased the motor will draw more current, up to 1000mA or 1Amp! Because DC motors draw such large currents they are often interfaced to a controlling circuit via a RELAY or transistor. In using a relay the motor can be controlled using a different power supply to that operating the controlling circuitry. NOTE that when using a Transistor or a RELAY to interface a motor with large SPIKES can be produced and these can damage the driving circuit component. This large spike or EMF (Electro Motive Force) can be dissipated using an ordinary DIODE. 68
How can I interface a Motor You can see an ordinary diode used here to dissipate the back emf of the motor. If the diode were not in this position the transistor would be damaged by this back emf. M This transistor MUST be designed to handle up to 1000mA. Interfacing the Motor using a RELAY 9V M RELAY 3V DC Motors are operated using 1.5 to 4.5 Volts batteries. Here the relay can be used to connect a separate voltage to the motor. Again a FLYWHEEL diode is used to dissipate the back emf of the RELAY. 69
What is a RELAY Relays are digital output devices (or a really clever switch); they are either on or off. They are used as current control devices that allow low current components to operate much larger current components through the action of the relay. A relay is a coil that produces a magnetic field. This magnetic field then pulls in the armature of a switch and closes a normally open switch. The contacts on a relay will be referred to as Normally Open (NO) or Normally Closed (NC) and Common (Com). A relay can have more than one switch inside it hence 1/1 refers to Relay 1 Switch 1. If the relay has a DPDT switch inside you will see 1/1, 1/2 Relay 1 Switch 1 and Relay 1 Switch 2. A PIC chip cannot operate a relay; it will need a transducer driver transistor circuit as shown below. Symbol 1/1 Relays are described in terms of: The operating voltage of the coil e.g. 6V Maximum current load across the switch contacts e.g 5A The maximum voltage the contacts can handle e.g. 110AC The type of switch inside the relay e.g. DPDT How can I interface the Relay 9V Voice Recorder Module From INPUT circuit or from the output of a PIC chip 6V 70
What is a Solenoid? A solenoid is very similar to a relay. It is a wire would component used to provide LINEAR movement pulling forces. In industrial applications these devices are used to open valves, or to provide a controlled movement for locks. However it must be noted that solenoids provide a very small pulling force measured in Newtons and move only a few millimetres. Interfacing the solenoid see interfacing the relay. Solenoid Symbol What is a Loudspeaker? A loudspeaker is an analogue device. It will only make a sound if it is vibrated at specific frequencies. It cannot be interfaced to the PICAXE system. It can be connected to a transistor that is receiving a sound frequency signal. Avoid the use of loudspeakers at GCSE. The basic structure of a loudspeaker consists of a coil of wire suspended in the gap of a cylindrical magnet, and a paper cone attached to the coil. The electrical signal (an alternating current) from the amplifier is transmitted through the coil of wire. This electrical signal causes a magnetic field to be set up around the magnet which varies with the polarity and amplitude of the electric signal. The magnetic field in turn acts on the coil of wire and causes it to move back and forth as the magnetic field varies. The paper cone vibrates as the coil moves and causes the volume of air contained in the paper cone to vibrate, thereby setting up sound waves. A big and heavy cone would set up low frequency vibrations, causing bass notes to sound, whereas a small, light cone would cause treble note to sound. If you want to see this in action check out the net at http://electronics.howstuffworks.com/speaker6.htm. Loudspeaker Symbol 71
SAQ Make a list of Analogue and Digital output devices Analogue Digital Interfacing Complete this circuit diagram of an interface circuit to switch on 3 LEDs off one PICAXE output. Remember a PIC chip can only sink or source 10mA. +6Volts Pic Chip 0V 72
Specialist output devices Motors are very popular choices for systems and control products but converting a 3 Volt 3600rpm no load speed, low torque motor, to use with a specific task is not easy. It is better to buy in a kit to perform a particular job. The Technology Enhancement Programme provide such kits at www.tep.org.uk Linear actuator provides a 50mm linear motion controlled by a reversing relay circuit. Servo Motor provides a 270 degree rotary motion Gearbox that can reduce 3600rpm to 10 rpm using a series of compound gears. 73
Stepper motor can be controlled from 4 outputs off a PIC chip and provides rotary motion in 7.5 degrees steps. Another linear actuator that provides a push pull action for any thing connected to the end brackets. Brackets and gearbox secured with 4mm screws. 74