Applied Science A5 Communication technicians notes
Supplementary information for technicians
All through these supplementary notes, remember that you must carry out your own risk assessment and take suitable precautions.
Prototype boards
On the basis of value for money, I would go for Rapid's board Protobloc 1. Sometimes its positive and negative rails (horizontal connections across top & bottom of the board) are useful. But do keep an eye on them during lessons: the sticky black foam which keeps the contacts in place is vulnerable to vandalism.
Connector links
It is much cheaper to get a reel of 0.6 mm connecting wire than to buy ready-made connector links. Students can prepare their own connectors, and at the end of a lesson you simply throw away any that are too short to recycle.
Digital audio broadcasting
Stelar is an educational charity providing education and training about wireless communications to educators. See the downloads Stelar offers on their Resources page. STELAR
Activity AA5.12 'Building a baby monitor'
Connecting the circuit as shown in the textbook (pages 48 & 49) just makes it oscillate, causing the speaker to buzz. The technician in a C21 school has found a solution to this problem - connect an additional 100 microfarad capacitor between pin 6 and ground.
Pin numbers: A semi-circular indent in the surface at one end of the chip indicates where to find pin 1 (see diagram at the bottom of textbook page 48). Follow round anti-clockwise until you reach the pin number you want. 'Ground' is another word for the 0 volt side of power supply.
Do not replace the 6V battery with a low voltage power pack. School power packs generally produce a pulsed DC output, giving a 100 Hz ripple on the supply rails.
AA5.16 'A short history of pictures: small ac motor'
A low-voltage AC motor (1.5-3V) does not seem to be available commercially. We suggest that you make up an AC motor, using a Westminster kit, with a picture on each side of the armature. For further information see Practical Physics website. The AC version of this motor has a commutator at each end, in continuous contact with the power supply.
AA5.18 'Radio links'
It is no longer legal to transmit at 1 GHz, even with a very weak signal. You can make up your own wave kit using the 433 MHz kit available from Rapid.
AA5.19 'Tuning in: Integrated circuit M484'
For the kit needed for this see Rapidonline
AA5.26 'Signal conversion with PICs'
For the PICAXE interface board, software and battery pack – see the PICAXE-08 Starter Pack at Rev-ed
Activity AA5.27 'Serial transfer with PICs'
Three issues have been reported.
1) There are two typos in the PIC program instructions.
Activity 1 - In step a, the 3rd line of program should read:
serout 0, T300,(b0) (with a space between 'serout' and digit 0).
Activity 2 - In step i, the 2nd line of the program should read:
a1: serin 3,T300,b0 (with a space between ‘serin’ and 3).
2) The circuit diagram for Activity 1 is correct, but a connection shown on the prototype board is wrong. Connect A to pin 6 of the chip (not pin 5). Otherwise pin 5 is left floating so that its state is erratic. You may also find it helps to connect pin 2 of the chip to O V.
3) Seeing the PIC signal on an oscilloscope: Every 50 ms the PIC sends out a series of 9 bits, starting with a 0, which last for 30 ms. If the timebase is set at 10 ms/division, and the oscilloscope set to trigger on a falling edge, the patterns of 1s and 0s should fit into the left-hand part of the screen. But it all depends on how many divisions the oscilloscope has across its screen. If your students don't get a static image which they can view, try different timebase settings.
