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A List to Aid in Writing Specifications |
Give your project a name
 | Your project name
could be descriptive: Sonar1 - Underwater buddy-line sonar |
| Your project name
could be just a tag: Chicago - a famous operating system |
| Your project name
could be an acronym: RIO - an MP3 player or a Remote Input/Output unit |
| Your project name
could be catchy: The Silent Salesman - enough said! |
You'll probably be using this name a lot in
the future - If you keep your project name short, it'll save you time! If it's 7
letters long, and unique enough to do a text-search on, you've got a real
winner.
Describe the project (Opening Statement)
| Keep the first
description short |
| The first sentence
should summarize the whole function of the project |
| Describe or name
equipment, devices or interfaces that the project connects to
 | We'll fill in all
the details of the projects specification later. |
|
Describe the market
| Your project may sell
to a particular group of people. |
| Your project may sell
to an established client base. |
| Your project may
satisfy your own requirements. |
| Your project probably
has a particular market position. |
Estimate the production volume
| Estimate a budgetary
sales price |
| Estimate the market
volume in units per year |
| Estimate the likely
market share at the budgetary sales price |
| Estimate the lifetime
of the market |
| Estimated production
volume = (market volume) multiplied by (market share) for (lifetime) years |
The types of specification points or clauses
The way to arrange a specification is as a
list of bullet points, or clauses. Bullet points have the advantage of being
clear and concise. A specification should describe all the required attributes
of the project - make a separate, if attached, wish list. When writing a
specification it is important to distinguish between required and desired. Think
carefully before you state a desire as a requirement - if you specify that the
project is to be no larger than a cassette case, be sure that the market really
warrants that feature. A specification reads like a list of project features,
describing the unit, and will usually include:
Inputs
| Describing the project
as a "Black Box" it has inputs, a function, and outputs. The inputs are
physical electronic connections that are activated by, for instance, an
operator pressing a button, or a temperature dropping too low. |
Controls
| Controls are often an
abstraction of inputs - a potentiometer input may be used to control the
speed of a motor, or a keyboard switch may be used to ramp the motor speed
down to zero. |
Outputs
| Outputs can sometimes
be directly related to functions - for instance an UP relay and a DOWN relay |
Indicators
| Indicators usually
show the status or condition of the device, for instance Alarms, Faults, and
Modes. |
Functions
| Describe functions
simply and generally. Describe special cases separately |
Modes of operation
| Modes of operation
affect functions, for instance Powering up, manual operation, automatic
operation |
Power Supply
| Choices are usually
between Plug packs, AC power, and battery. |
Protection
| Fail safes and
replaceable parts |
Connectors
| Types |
| Placement |
Physical
| Format and size |
| Operating environments |
Review the specification to ensure it
describes the project clearly and succinctly. The specification should describe
the project, as you need it produced. The specification can be followed by a
wish list - desirable features or possible extensions. Possible extension might
include a connector for I/O expansion.
An example of the distinction between the
required and the desirable: The size of an electronic project is frequently
described as a requirement when it is more often just desirable that it be as
small as possible. The specification is a terse point-by-point document with a
brief opening description. It often helps to attach a plain written description
of the project, which is more general, and describes the way the project works
more from the perspective of how it is used.
Inputs
Keyboard and Switch inputs
| A Keyboard is usually
used in association with a display, for instance LCD, LED, |
| At the concept stage,
the exact function of every key is often not critical - it can be set in
software later |
| Key switch types..
| Existing keyboards
- e.g. IBM PC keyboards: reliable, replaceable, cheap, reasonably easy
to interface |
| Membrane keyboards
- custom manufactured flexible plastic laminate keyboards |
| Flexible rubber
keyboards - shaped raised keys, need large production volumes to be
viable |
| Individual Tactile
or pushbutton keys - A keyboard made up of individual switches in a
custom arrangement |
| 6 x 6mm tactile
keys |
| 12 x 12mm tactile
keys |
| Several other
types, including lighted pushbuttons |
|
| DIP switches - PCB
mounted switches used for setting configuration option |
| Jumpers - not strictly
a switch, but used as one - three posts with a moveable two pin conducting
sleeve |
| Slide switches -
usually used for on/off functions or mode/option setting |
| Thumbwheel switches -
switches with an up/down button or wheel to dial a number with digits 0-9 or
0-F |
| Rotary switches -
older fashioned panel mount switches with a knob that turns in steps |
| Lock switches - barrel
type switches operated with a key, to provide secured access to an operation |
Other switch and relay contact inputs
| Microswitches - Tough
level/cam switches used to measure mechanical position or as limit switches |
| Relay contacts - Used
for isolation of circuits, more especially in industrial plant environments |
Opto-isolated inputs - Used to isolate the
input circuit from the main electronics
| Opto-interrupters -
Used to measure position - a slotted disk or a metal tab interrupts an
infra-red light beam to send the signal |
| Analog inputs -
voltage and current - Characterized by the number of bits of resolution
| 8 bits - 256 steps
of measurement |
| 10 bits - 1024
steps of measurement |
| 12 bits - 4096
steps of measurement |
| 16 bits - 65536
steps of measurement |
|
| Potentiometers; Rotary
controls, resistance varies according to angle of shaft rotation |
Temperature sensors
| Silicon temperature
sensors - medium accuracy 0-100 degree sensors, often easily interfaced
|
| Diode or transistor -
The cheapest, nastiest, temperature sensor, usually very cheap to implement |
| Thermocouples -
accurate, wide range temperature measurement requiring low level front end
electronics |
| Thermistor types -
readily available, cheaper temperature measurement devices requiring
calibration, linearization |
| Platinum resistance
sensors (PTD's) - highly accurate sensors, require linearization but
generally no calibration |
Light Sensors
| Photodiodes -
accurate, fast response sensors, need some front end circuitry |
| Phototransistors -
basically amplified photodiodes, usually very easy to apply for basic light
sensing |
| LDR - light dependent
resistor |
| OPIC - optical IC, a
whole range of optical sensors with integrated electronics are available for
specialist applications |
Magnetic Field sensors
| Hall effect devices -
generally small 3 pin devices activated by a strong magnet within close
proximity |
| Reed switches - simple
2 pin switches activated by a strong magnet within close proximity |
| Strain gauges - Used
for measuring weight, by strain of a support. Configured as a bridge, with
sophisticated front-end circuitry. |
Controls
| Controls are often an
abstraction of inputs - a potentiometer input may be used to control the
speed of a motor, or a keyboard switch may be used to ramp the motor speed
down to zero. |
Outputs
Parallel outputs, TTL level
| Standard type of
output from single chip microprocessors |
| "TTL level" has become
nomenclature for a type of interface, rather than the use of old fashioned
TTL logic |
| TTL level signals may
be protected against extraneous voltages through the use of clamp diode
circuitry |
| Typical sink ratings
vary from a "MOS drive" level of 1.6ma, to "CMOS" at 6ma, to "Buffered
Outputs" at 24ma |
| TTL level signals are
the typical levels used to drive LED's and opto-couplers |
Open collector outputs
| Open collector outputs
sink current to ground, but in the "hi" state are essentially open circuit
|
| "Open Collector" has
become nomenclature for an output topology rather than the use of TTL/Bipolar
devices |
| Open collector outputs
allow reliable connection between two units with separate, independent power
supplies |
High current outputs
| Typically implemented
with Transistor driver arrays such as the octal device ULN2803A or UDN2981
|
| High Current outputs
are more robust and better able to handle extraneous voltages than TTL level
outputs |
| Most High current
outputs are also open collector types |
| Typical ratings are
100mA - 800mA per output, with a maximum rating for the driver package as a
whole |
| High current outputs
are typically used to drive relays |
Analog outputs
| Typically implemented
using a Digital-to-Analog (D-to-A or D/A) converter, but sometimes Pulse
Width Modulation (PWM) |
| Analog outputs provide
a variable output voltage or current |
Relays
| Relay outputs provide
electrical isolation and are generally very robust |
| Relay outputs allow
reasonably heavy loads to be driven: 0.5A - 15A for PCB mount devices |
| In most Industrial
applications, the PCB mount relay will be used to drive an external relay or
contactor that then runs the load to be switched. |
Solid State Relays (SSR's)
| SSR's are invariably
optically coupled, and usually switch 110VAC/240VAC |
| SSR's use
triac/optocoupler/snubber circuitry integrated into one encapsulated package |
| Optically isolated
outputs; Optically isolated outputs use opto-couplers to provide electrical
isolation |
Servos
| Servos are usually
driven using TTL level signals with a 20ms period pulse width modulation
|
| Servos require driver
firmware, but are otherwise easily interfaced to single chip microprocessors |
Solenoids
| Solenoids provide
quick stroke axial motion |
| Solenoids are usually
used as a mechanical actuating mechanism - e.g. Solenoid controlled valves
for liquids Indicators |
Light Emitting Diodes (LED's)
| Color: Red, Green,
Yellow, Orange (only marginally different from Red/Yellow), Blue (expensive) |
| Size: 3mm and 5mm are
standard, miniature surface mount, 2mm, 8mm, 10mm available |
| Diffuse: Available
clear, color tinted and diffuse, with correspondingly wider viewing angle |
| Intensity: Available
in standard, high efficiency/low current, high brightness, super bright |
| Shape: Standard dome,
also available in rectangular, special types: flat top, square, arrowhead |
| Circuitry: Some
special types with integral current limiting or flashing circuits are
available |
| Full Color types:
LED's with Red-Green-Blue elements are now available, if somewhat expensive |
LED displays - LED bar
| Arrays of Rectangular
LED's encapsulated in DIP package block |
| Standard size is 10
indicators in one bar, can be stacked for longer lengths of display |
| Normally Red, but
available in other colors, even mixed in one package (Green--Orange--Red) |
Lamps - Incandescent
| Available in sizes
from 2mm up |
| Brightness is
inversely proportional to operating life |
| High current
requirements |
| Standard
Yellowish-White Available with colored filter caps, including color adjust
to bright white |
Audible Indicators
| Peizo
sounders/speakers - output volume low to medium, may be driven to make many
types of sound |
| Peizo buzzers - fixed
frequency, output volume medium to unbearably high |
| Speakers -
conventional magnetic speakers are available in many shapes, including
substantially flat |
Indicators: Displays
LED displays - 7 segment with decimal point
| Standard Red and Green |
| Standard sizes 0.3
inch, 0.5 inch, 0.8 inch |
| While highly visible
for indoor applications, LED's are hopeless in sunlight |
| LED displays -
alphanumeric, 14/16 segment |
| LED displays - 5x7
matrix |
Liquid Crystal Displays (LCD) - 1,2 or 4
lines of characters
| Standard type: 2 lines
by 12 characters to 4 lines of 40 characters (Typical: 2 lines by 16
characters) |
| Available with
backlight |
| Available in extended
temperature range |
| Onboard controller -
display is accessed by parallel bus and appropriate firmware drivers |
LCD: Displays - Graphic type
| Typically higher cost
than the standard character modules |
| Available in 64 x 64
pixels to 256 x 128 pixels as standard |
| Available with
controller (more easily interfaced) or without (cheaper, more costly
interfacing) |
| Available with
backlight (However LED backlights often consume excessive amounts of power) |
LCD: Displays - color Graphic type
| Available in volume |
| If a product needs a
color LCD Graphic display and keyboard, the product might more economically
be manufactured by writing suitable software for a laptop PC |
Functions
| Describe functions
simply and generally. Describe special cases separately |
Modes of operation
| Modes of operation
affect functions, for instance Powering up, manual operation, automatic
operation |
Power
Power Supplies
| Power supplies
approval according to country: |
| US - Underwriters
Laboratories (UL) approval |
| Canada - Canadian
Standards Association (CSA) |
| Australia - Individual
state energy authority approval |
| New Zealand -
Compliance to NZ or Australian equivalent standard |
| UK - British Standards
approval (BS) |
| European - Germany,
France, Swiss, Italy (VDE) |
Plug Pack, or wall adaptor - reasonably
priced for power up to 10-15W
| DC Type - standard.
Unregulated, standard nominal voltages are 6v, 9v, 12v, currents 150ma-1Amp |
| AC Type - used for
high wattages, transformer fills plug pack body. Standard 12v, 16v, 1-1.5
Amp |
| DC Regulated Type -
available but not standard. |
| Switchmode, Internal
with IEC connector - similar to PC power supply, reasonably priced for
15-65W |
Battery
| Non-rechargeable |
| Silver oxide (Watch
batteries) - very small, suitable only for low power devices |
| Carbon Zinc, Standard
batteries |
| Alkaline, Higher
capacity replacements for standard batteries |
| Rechargeable
| Nickel Cadmium -
standard rechargeable, suffer from memory effect |
| Hi Temperature
Nickel Cadmium |
| Nickel Metal
Hydride - improved standard rechargeable, less memory effect, poor
standby performance |
| Lead Acid battery
- car battery type |
| Gel Cell - Lead
Acid battery with gelled electrolyte, much less leakage, available in
smaller sizes |
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Communications
| Describe the medium
(e.g. RS232), the data (e.g. Remote control of unit), and the protocol (e.g.
ASCII). Mechanical & Environmental |
| Describe the
mechanical package, maximum footprint of the unit, or standard packaging. |
| Describe what user
access will be, e.g. will the customer need to be able to have easy access
to the internal jumpers or adjustments. |
| Then describe the
operating environment such as does the unit need to be air-cooled, water
tight, daylight readable. |
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