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A List to Aid in Writing Specifications

Give your project a name

bulletYour project name could be descriptive: Sonar1 - Underwater buddy-line sonar
bulletYour project name could be just a tag: Chicago - a famous operating system
bulletYour project name could be an acronym: RIO - an MP3 player or a Remote Input/Output unit
bulletYour 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)

bulletKeep the first description short
bulletThe first sentence should summarize the whole function of the project
bulletDescribe or name equipment, devices or interfaces that the project connects to
bulletWe'll fill in all the details of the projects specification later.

Describe the market

bulletYour project may sell to a particular group of people.
bulletYour project may sell to an established client base.
bulletYour project may satisfy your own requirements.
bulletYour project probably has a particular market position.

Estimate the production volume

bulletEstimate a budgetary sales price
bulletEstimate the market volume in units per year
bulletEstimate the likely market share at the budgetary sales price
bulletEstimate the lifetime of the market
bulletEstimated 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:


bulletDescribing 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.


bulletControls 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.


bulletOutputs can sometimes be directly related to functions - for instance an UP relay and a DOWN relay


bulletIndicators usually show the status or condition of the device, for instance Alarms, Faults, and Modes.


bulletDescribe functions simply and generally. Describe special cases separately

Modes of operation

bulletModes of operation affect functions, for instance Powering up, manual operation, automatic operation

Power Supply

bulletChoices are usually between Plug packs, AC power, and battery.


bulletFail safes and replaceable parts




bulletFormat and size
bulletOperating 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.


Keyboard and Switch inputs

bulletA Keyboard is usually used in association with a display, for instance LCD, LED,
bulletAt the concept stage, the exact function of every key is often not critical - it can be set in software later
bulletKey switch types..
bulletExisting keyboards - e.g. IBM PC keyboards: reliable, replaceable, cheap, reasonably easy to interface
bulletMembrane keyboards - custom manufactured flexible plastic laminate keyboards
bulletFlexible rubber keyboards - shaped raised keys, need large production volumes to be viable
bulletIndividual Tactile or pushbutton keys - A keyboard made up of individual switches in a custom arrangement
bullet6 x 6mm tactile keys
bullet12 x 12mm tactile keys
bulletSeveral other types, including lighted pushbuttons
bulletDIP switches - PCB mounted switches used for setting configuration option
bulletJumpers - not strictly a switch, but used as one - three posts with a moveable two pin conducting sleeve
bulletSlide switches - usually used for on/off functions or mode/option setting
bulletThumbwheel switches - switches with an up/down button or wheel to dial a number with digits 0-9 or 0-F
bulletRotary switches - older fashioned panel mount switches with a knob that turns in steps
bulletLock switches - barrel type switches operated with a key, to provide secured access to an operation

Other switch and relay contact inputs

bulletMicroswitches - Tough level/cam switches used to measure mechanical position or as limit switches
bulletRelay 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

bulletOpto-interrupters - Used to measure position - a slotted disk or a metal tab interrupts an infra-red light beam to send the signal
bulletAnalog inputs - voltage and current - Characterized by the number of bits of resolution
bullet8 bits - 256 steps of measurement
bullet10 bits - 1024 steps of measurement
bullet12 bits - 4096 steps of measurement
bullet16 bits - 65536 steps of measurement
bulletPotentiometers; Rotary controls, resistance varies according to angle of shaft rotation

Temperature sensors

bulletSilicon temperature sensors - medium accuracy 0-100 degree sensors, often easily interfaced
bulletDiode or transistor - The cheapest, nastiest, temperature sensor, usually very cheap to implement
bulletThermocouples - accurate, wide range temperature measurement requiring low level front end electronics
bulletThermistor types - readily available, cheaper temperature measurement devices requiring calibration, linearization
bulletPlatinum resistance sensors (PTD's) - highly accurate sensors, require linearization but generally no calibration

Light Sensors

bulletPhotodiodes - accurate, fast response sensors, need some front end circuitry
bulletPhototransistors - basically amplified photodiodes, usually very easy to apply for basic light sensing
bulletLDR - light dependent resistor
bulletOPIC - optical IC, a whole range of optical sensors with integrated electronics are available for specialist applications

Magnetic Field sensors

bulletHall effect devices - generally small 3 pin devices activated by a strong magnet within close proximity
bulletReed switches - simple 2 pin switches activated by a strong magnet within close proximity
bulletStrain gauges - Used for measuring weight, by strain of a support. Configured as a bridge, with sophisticated front-end circuitry.


bulletControls 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.


Parallel outputs, TTL level

bulletStandard type of output from single chip microprocessors
bullet"TTL level" has become nomenclature for a type of interface, rather than the use of old fashioned TTL logic
bulletTTL level signals may be protected against extraneous voltages through the use of clamp diode circuitry
bulletTypical sink ratings vary from a "MOS drive" level of 1.6ma, to "CMOS" at 6ma, to "Buffered Outputs" at 24ma
bulletTTL level signals are the typical levels used to drive LED's and opto-couplers

Open collector outputs

bulletOpen collector outputs sink current to ground, but in the "hi" state are essentially open circuit
bullet"Open Collector" has become nomenclature for an output topology rather than the use of TTL/Bipolar devices
bulletOpen collector outputs allow reliable connection between two units with separate, independent power supplies

High current outputs

bulletTypically implemented with Transistor driver arrays such as the octal device ULN2803A or UDN2981
bulletHigh Current outputs are more robust and better able to handle extraneous voltages than TTL level outputs
bulletMost High current outputs are also open collector types
bulletTypical ratings are 100mA - 800mA per output, with a maximum rating for the driver package as a whole
bulletHigh current outputs are typically used to drive relays

Analog outputs

bulletTypically implemented using a Digital-to-Analog (D-to-A or D/A) converter, but sometimes Pulse Width Modulation (PWM)
bulletAnalog outputs provide a variable output voltage or current


bulletRelay outputs provide electrical isolation and are generally very robust
bulletRelay outputs allow reasonably heavy loads to be driven: 0.5A - 15A for PCB mount devices
bulletIn 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)

bulletSSR's are invariably optically coupled, and usually switch 110VAC/240VAC
bulletSSR's use triac/optocoupler/snubber circuitry integrated into one encapsulated package
bulletOptically isolated outputs; Optically isolated outputs use opto-couplers to provide electrical isolation


bulletServos are usually driven using TTL level signals with a 20ms period pulse width modulation
bulletServos require driver firmware, but are otherwise easily interfaced to single chip microprocessors


bulletSolenoids provide quick stroke axial motion
bulletSolenoids are usually used as a mechanical actuating mechanism - e.g. Solenoid controlled valves for liquids Indicators

Light Emitting Diodes (LED's)

bulletColor: Red, Green, Yellow, Orange (only marginally different from Red/Yellow), Blue (expensive)
bulletSize: 3mm and 5mm are standard, miniature surface mount, 2mm, 8mm, 10mm available
bulletDiffuse: Available clear, color tinted and diffuse, with correspondingly wider viewing angle
bulletIntensity: Available in standard, high efficiency/low current, high brightness, super bright
bulletShape: Standard dome, also available in rectangular, special types: flat top, square, arrowhead
bulletCircuitry: Some special types with integral current limiting or flashing circuits are available
bulletFull Color types: LED's with Red-Green-Blue elements are now available, if somewhat expensive

LED displays - LED bar

bulletArrays of Rectangular LED's encapsulated in DIP package block
bulletStandard size is 10 indicators in one bar, can be stacked for longer lengths of display
bulletNormally Red, but available in other colors, even mixed in one package (Green--Orange--Red)

Lamps - Incandescent

bulletAvailable in sizes from 2mm up
bulletBrightness is inversely proportional to operating life
bulletHigh current requirements
bulletStandard Yellowish-White Available with colored filter caps, including color adjust to bright white

Audible Indicators

bulletPeizo sounders/speakers - output volume low to medium, may be driven to make many types of sound
bulletPeizo buzzers - fixed frequency, output volume medium to unbearably high
bulletSpeakers - conventional magnetic speakers are available in many shapes, including substantially flat

Indicators: Displays

LED displays - 7 segment with decimal point

bulletStandard Red and Green
bulletStandard sizes 0.3 inch, 0.5 inch, 0.8 inch
bulletWhile highly visible for indoor applications, LED's are hopeless in sunlight
bulletLED displays - alphanumeric, 14/16 segment
bulletLED displays - 5x7 matrix

Liquid Crystal Displays (LCD) - 1,2 or 4 lines of characters

bulletStandard type: 2 lines by 12 characters to 4 lines of 40 characters (Typical: 2 lines by 16 characters)
bulletAvailable with backlight
bulletAvailable in extended temperature range
bulletOnboard controller - display is accessed by parallel bus and appropriate firmware drivers

LCD: Displays - Graphic type

bulletTypically higher cost than the standard character modules
bulletAvailable in 64 x 64 pixels to 256 x 128 pixels as standard
bulletAvailable with controller (more easily interfaced) or without (cheaper, more costly interfacing)
bulletAvailable with backlight (However LED backlights often consume excessive amounts of power)

LCD: Displays - color Graphic type

bulletAvailable in volume
bulletIf 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


bulletDescribe functions simply and generally. Describe special cases separately

Modes of operation

bulletModes of operation affect functions, for instance Powering up, manual operation, automatic operation


Power Supplies

bulletPower supplies approval according to country:
bulletUS - Underwriters Laboratories (UL) approval
bulletCanada - Canadian Standards Association (CSA)
bulletAustralia - Individual state energy authority approval
bulletNew Zealand - Compliance to NZ or Australian equivalent standard
bulletUK - British Standards approval (BS)
bulletEuropean - Germany, France, Swiss, Italy (VDE)

Plug Pack, or wall adaptor - reasonably priced for power up to 10-15W

bulletDC Type - standard. Unregulated, standard nominal voltages are 6v, 9v, 12v, currents 150ma-1Amp
bulletAC Type - used for high wattages, transformer fills plug pack body. Standard 12v, 16v, 1-1.5 Amp
bulletDC Regulated Type - available but not standard.
bulletSwitchmode, Internal with IEC connector - similar to PC power supply, reasonably priced for 15-65W


bulletSilver oxide (Watch batteries) - very small, suitable only for low power devices
bulletCarbon Zinc, Standard batteries
bulletAlkaline, Higher capacity replacements for standard batteries
bulletNickel Cadmium - standard rechargeable, suffer from memory effect
bulletHi Temperature Nickel Cadmium
bulletNickel Metal Hydride - improved standard rechargeable, less memory effect, poor standby performance
bulletLead Acid battery - car battery type
bulletGel Cell - Lead Acid battery with gelled electrolyte, much less leakage, available in smaller sizes


bulletDescribe the medium (e.g. RS232), the data (e.g. Remote control of unit), and the protocol (e.g. ASCII). Mechanical & Environmental
bulletDescribe the mechanical package, maximum footprint of the unit, or standard packaging.
bulletDescribe what user access will be, e.g. will the customer need to be able to have easy access to the internal jumpers or adjustments.
bulletThen describe the operating environment such as does the unit need to be air-cooled, water tight, daylight readable.

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Last modified: March 27, 2016