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Guide for Writing Specifications

Getting Started

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 with
    1. 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:


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


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


  • 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.


  • Fail safes and replaceable parts


  • Types
  • Placement


  • 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.


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..
    1. Existing keyboards - e.g. IBM PC keyboards: reliable, replaceable, cheap, reasonably easy to interface
    2. Membrane keyboards - custom manufactured flexible plastic laminate keyboards
    3. Flexible rubber keyboards - shaped raised keys, need large production volumes to be viable
    4. Individual Tactile or pushbutton keys - A keyboard made up of individual switches in a custom arrangement
    5. 6 x 6mm tactile keys
    6. 12 x 12mm tactile keys
    7. 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
    1. 8 bits - 256 steps of measurement
    2. 10 bits - 1024 steps of measurement
    3. 12 bits - 4096 steps of measurement
    4. 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 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

  • 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


  • 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 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 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


  • 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 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


  • 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
    1. Nickel Cadmium - standard rechargeable, suffer from memory effect
    2. Hi Temperature Nickel Cadmium
    3. Nickel Metal Hydride - improved standard rechargeable, less memory effect, poor standby performance
    4. Lead Acid battery - car battery type
    5. Gel Cell - Lead Acid battery with gelled electrolyte, much less leakage, available in smaller sizes
  • 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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