'A small business wishes to sell a range of electronic games to help alleviate boredom for young children and also be of educational value. You have been asked to design and make a suitable prototype'.
Design Brief
For my electronic products GCSE project I have chosen to produce the following project from the NEAB approved list.
'A small business wishes to sell a range of electronic games to help alleviate boredom for young children and also be of educational value. You have been asked to design and make a suitable prototype'
I have decided to design, research and make a portable infrared target game that is fun to use and good for improving reactions. After choosing this idea a number of guidelines instantly presented themselves to me:
> Appearance
> Safety
> Expense
> Guidelines set by NEAB
For this project I am going to extensively research all of the major areas of the product such as the four listed above and other areas such as chips and casing. I must try and consider every option of fulfilling the product guidelines to the greatest amount of customer satisfaction.
Design Specification
This section is going to cover the rough spec of the product I am going to produce. I have compiled a list of points to do this:
> The final product must cost less than £10
> The final product must be aesthetically pleasing
> The laser emitter must be cased ergonomically
> The product must be enjoyable for children
> The product must have an ON/OFF switch
> The product must have a battery power indicator LED
> Mains electricity cannot be used
> Battery must be easily accessible
> The product must be safe for children to use
> The electronic equipment must be housed in the casing
> The PCB must be secure in the casing
Research
This section of my coursework is going to involve researching casing materials and ways of joining, electronic components and existing products..
Wood
I am only going to research man-made boards because they are cheaper to buy so, are more suitable for my project.
Plywood
Plywood is made from layers (or plies) of wood glued together with each grain at right-angles to the last. Plywood is always made with an odd amount of plies so that the top and the bottom grains run in the same direction. Some plywood is veneered with a more expensive hardwood to improve appearance and make it more durable.
Chipboard
Chipboard is created by compressing small chips of wood and glue with heat. Most chipboard is graded meaning that it has smaller chips on the outside surfaces of it which adds to the smoothness and overall quality of the wood. Chipboard veneered with melamine or hardwoods are widely used for work surfaces and shelves.
Meranti
Meranti is a hardwood with very similar properties and looks to Mahogany, durable and strong. It is dark and much cheaper than mahogany and in plentiful supply. It is medium weight and finishes fairly well. Quite difficult to work and doesn't polish as well as mahogany
Teak
Originates in Burma and India and is coloured a rich, golden brown. It is hard, strong and naturally durable . It has a very attractive straight grain. It is difficult to work with because it blunts tools and doesn't glue very well because all of it's natural oils. It is also very expensive.
Softwoods such as pine
Softwoods come from very fast growing trees and so are less expensive to produce and buy. They are generally very easy to cut and work with but must be finished to a high standard to have a viable amount of durability.
Whitewood
Whitewood is a softwood that originates in Northern Europe and Canada. It is a strong, creamy white wood that is fairly strong and easy to work. It is not very durable and contains small, hard knots and resin pockets. It is generally used for inside work.
MDF ...
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Softwoods such as pine
Softwoods come from very fast growing trees and so are less expensive to produce and buy. They are generally very easy to cut and work with but must be finished to a high standard to have a viable amount of durability.
Whitewood
Whitewood is a softwood that originates in Northern Europe and Canada. It is a strong, creamy white wood that is fairly strong and easy to work. It is not very durable and contains small, hard knots and resin pockets. It is generally used for inside work.
MDF (Medium Density Fibreboard)
MDF is a manufactured board that finishes extremely well. It can be primed and finished easily. It is not very durable outdoors because it goes soggy. It can be used for making vacuum forming moulds. Tools are very easily blunted by MDF.
Wood Finishes
There is a variety of ways of finishing wood to improve its aesthetic impact.
Oil
Oil produces a good finish on oily timbers like Teak. Oil requires regular re-coating to keep the finish good. Examples of oil products include linseed oil, occasionally used coating cricket bats and teak oil. Olive oil is used on wood that will come into contact with food. Oil finish should be re-coated about once every week.
Wax
Wax gives a dull gloss shine that looks very attractive and shows the natural grain of the wood. To use the grain of the wood must first be sealed by using French polish (see below) then the wax must be rubbed into a smooth surface. The shine can be buffed using a soft cloth.
Plastic finishes like polyurethane varnish
Plastics produce a very good finish and also show the natural colour of the wood. They are heat-proof, immune to liquid spillages and very durable. They are usually applied with a brush, a spray or a cloth and rubbed down lightly between coats. They can be diluted with a 'turps' substitute to help them soak into the grain to achieve a natural look.
Varnish
Varnish is a solution of resins in oil or spirit. It gives a hard and durable, waterproof and glossy, transparent finish. It is used as per plastic finishes but using a brush only.
Paint
There are so many types of paint that there is paint for every job imaginable. Paint should be applied in thin layer and sanded between coats to give a good even finish. This also applies to spray applied paints. To attain a good finish the paint must be undercoated, all knots in the wood sealed with knotting and then can be varnished to give a gloss, hardwearing finish.
Wood jointing techniques
My project is going to consist of a boxed receiver unit and an ergonomically designed laser sender unit. This is why this portion of my research will consist mainly of carcase jointing techniques.
Butt Joints
Butt joints are the simplest and easiest to make, but don't support much weight and do not look very attractive. They can be reinforced with dovetail nailing and by using metal or wooden gussets.
Finger or comb joints
Finger joints are simple to make because there are no dovetail (see below) angles. They are strong because they have a large gluing area.
Dovetail Joints
Dovetail Joints are the strongest carcase joints because they can only be pulled apart in one direction and they have a large glueing area. If they are well made they can also be an attractive feature.
For my project I have decided to use dovetail joints becase they are very strong and aesthetically pleasing.
Woodworking Tools
This section of research will involve looking at the tools needed to work wood correctly and also their uses.
Try-Square
A try-squre is used to test that one surface plane is at right-angles (90°) to another and can be used to mark lines perpendicular to an edge.
Warrington Pattern Hammer
The warrington pattern hammer is used for light nailing and cabinet making. The shaft of the hammer fits into a shaped socket and is expanded by a wedge so that it cannot fly off.
Small Band Saw
The small band saw runs from the mains and is very useful for cutting detailed work easily. The band runs at a high speed to that all the operator has to do is feed the wood through.
Coping saw
The coping saw has a thin blade whose teeth usually point towards the handle so that the saw cuts as it is pulled. Rotating the handle usually tensions the blade.
Electronic Component Research
This section of my research is going to involve explaining the components that I need to complete my project and the ways in which they are used.
Capacitors
Capacitors consist of two metal plates separated by an insulator. In larger capacitors these are in the form of long ribbons wound into a cylinder. Capacitance is measured in Farads (F), but because a Farad is such a large measurement capacitors are usually measured in microfarads (µF) or sometimes even smaller quantities. A capacitor also has a working voltage that cannot be exceeded. Non-electrolytic capacitors can be connected either way around in a circuit and typically have a small capacitance of less than (1µF). Electrolytic capacitors can only be connected the correct way around in a circuit.
This graph shows how the capacitor charges (shown by the steep, positive slope on the graph) and then when it has reached supply voltage it discharges gradually, shown by the negative slope of the graph.
TBA 2800 Infrared Preamplifier
The amplifier contains four main parts: the gain-controlled amplifier, the amplifier, the pulse-separating amplifier, and the inverter. The Chip is reasonably resistant to interference in bright light, fluorescent light or in areas with high background infrared levels.
The second amplifier amplifies the signal further. The third amplifier separates the pulse-intelligence from noise. The inverter inverts the output signal at pin7 to a negative pulse and so gives a positive output at pin8. If an additional resistor is added between pin6 and GND then the unit is more immune to noise but less sensitive. Pin10 is a test pin and should not be connected to anything.
PIN CONNECTIONS
Input's Ground
2 Capacitor Pin Amplifier I
3 Supply Voltage
4 Input Amplifier III
5 Output Amplifier II
6 Pin for adjusting the Separation Threshold
7 Negative Pulse Output
8 Positive Pulse Output
9 Output's Ground
0 Test Pin
1 Input Amplifier
4017B Decade Counter
The 4017 series of decade counters can be used to drive a series of LEDs as a ten event counter if connected correctly. The counter advances one count on each low to high pulse provided to pin14 (clock input) and light the LEDs in turn. The counter is reset by taking pin15 high momentarily, and stopped by taking pin13 high. Pin12 is high for the first five input pulses and low for the next five.
An Explanation of the Circuit (Transmitter) - Block Diagram
The transmitter, in the shape of a gun, has a push switch, the gun's trigger that, when it is pressed, will trigger the encoder module. The output from the encoder is then amplified by the Darlington driver and then delivered through a high-powered Infra-red transmitting LED. The encoder configuration must be the same on the transmitter and the receiver in order for the circuits to work correctly. A single Infra-red LED is preferred because it makes the transmitter more directional.
An Explanation of the Circuit (Transmitter) - Circuit Diagram
I have produced a full diagram of the transmitter circuit on squared-paper. The encoding circuit is based on the UM3750. Switches S1 through to S12 set the code that is required. In my project I will probably use wire links as opposed to switches because in practice they do not need to be altered. The capacitor, C1, and the resistor, R1, are there to set the frequency of the encoder signal. Capacitor C2 removes any spikes in the voltage from the supply rails. The output from the encoder, via pin 17, is then fed through the resistor, R2, into the base of the Darlington driver, TR1. The amplified signal controls the high-powered infra-red LED, D2. Resistors R3 and R4 limit the current through the LED. When the push-to-make switch, S13, is pressed the charge on capacitor C4, provides a surge of energy to the circuit for a short time. This makes it harder for you to cheat by pressing and holding the push-to-make switch and sweeping it across the target. The diode, D1, and the capacitor, C3, ensure that there is a reasonable supply voltage across the UM3750 even though the voltage collapses when S13 is pressed.
Components
The UM3750 Encoder/Decoder chip includes both the encoder and decoder for a coded system. They are often used in alarm control systems, security systems, garage door openers and remote controllers. The system can be used with RF, ultrasonic or infra-red modulators and demodulators. The only place that I could find this chip was in the 1994 Maplin catalogue but there is probably a modern equivalent. The order code is UK77J and the cost would be £2.50 for two.
In the March 2004 Rapid catalogue the LEDs cost £0.09 each. I will probably use red and green LEDs. The order codes are 56-0255 for red LEDs and 56-0255 for green LEDs. 81-0172
The Darlington Transistor type TIP122 is in the 1999 rapid catalogue and it costs 32p.
I will probably need more than one infra-red emitter but this will be tested when I have built a bread-board of my design. The High Power infra-red emitters in the 2004 Rapid catalogue cost 29p and they have a serial number of 58-0112. The infra-red sensors cost 68p and have a serial number of 58-0115.
I have found the TBA2800 in the Maplin Electronics catalogue its order code is JU39P and the unit rice is £3.25
I also need a decade counter that we already have in school.
The 4001B quad 2-input NOR gate can be found in the Maplin Electronics catalogue. They cost 28p and the order code is QX01B.
An Explanation of the Circuit (Receiver) - Block Diagram
The signal from the infra-red sensor is first amplified and then decoded, before being used to trigger a monostable. The monostable is then used to trigger a buzzer that signals a successful infra-red 'hit' and 'jam' the astable which stops the rotating LED display. The astable usually oscillates when the monostable is unaffected and this drives the set of chaser LEDs.
An Explanation of the Circuit (Receiver) - Circuit Diagram
I have shown the full circuit diagram on a separate sheet. The inverted output from pin8 of the 74HC4017 is fed to the pin16 of UM3750. The second IC decodes the signal according to the switches on the input A1 to A12. The switches must be in the same position on the transmitter as on the receiver. When the code is not being received then the output from pin17 of the second IC is high. The effect of the transistor TR1 is to invert the output from pin17. When pin17 is high the output from the collector of TR1 is low and so IC3 pin1 is low. When the right code is received pin17 of IC2 changes from high to low. So then TR1 is switched off and its collector voltage changes from 0V to positive, this then triggers the monostable made up of NOR gates IC3a and IC3b. The values of resistor R5 and the capacitor C6 define the time that the monostable stays high. The output from the monostable is routed through resistor R6 to the base of the transistor TR2, this then drives the buzzer. The monostable output is also routed through to IC3d pin13. Both of the NOR gates IC3c and IC3d form the astable which, when there is no signal, oscillates normally. The speed of the oscillation is determined by R7 and C7. When the monostable output goes high the astable is 'jammed'. The output from the astable, IC3d pin 11, is delivered to the clock input, pin14, of IC4. The circuit around IC4 is the decade counter. Six counter outputs are used to drive the LEDs with the seventh being used to reset the IC so that the LEDs sequence with no gaps. Only six LEDs are shown on diagram but many more can be used as long as there are a multiple of six. A 4.5V battery must be used rather than a 9V battery. This is because IC1 cannot be used with a Voltage of more than 5.5V.