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The Specification: Tech ingredients

A specification reads like a list of project features, describing the unit, and will usually include:
  • Inputs
  • Controls
  • Outputs
  • Indicators
  • Functions
  • Modes of operation
  • Power Supply
  • Communications
  • Protection, Fail safes and replaceable parts
  • Connector types
  • Physical format and size

The Big List of Bullet points

A list of technical terms to help you (we hope!) with your specification


  • 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
    • Keyswitch 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
    • DIP switches - PCB mounted switches used for setting configuration options
    • 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
    Characterised 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
  • Rotary encoders
    Use gray code or quadrature encoding to represent rotation and direction as binary outputs (click for example board)
  • 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, linearisation (click for example board)
    • Platinum resistance sensors (PTD's) - highly accurate sensors, require linearisation 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 (click for example board)
  • 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 LEDs and optocouplers
  • 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 (SSRs)
    • SSR's are invariably optically coupled, and usually switch 110VAC/240VAC
    • SSRs use triac/optocoupler/snubber circuitry integrated into one encapsulated package (click for example board)
  • Optically isolated outputs
    Optically isolated outputs use optocouplers 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


  • Light Emitting Diodes (LED's)
    • Colour: 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 waterclear, colour 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 Colour types: LEDs with Red-Green-Blue elements are now available, if somewhat expensive
  • LED displays - LED bar
    • Arrays of Rectangular LEDs 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 colours, 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 coloured filter caps, including colour adjust to bright white
  • Audible Indicators
    • Piezo sounders/speakers - output volume low to medium, may be driven to make many types of sound
    • Piezo 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, LEDs 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 - colour Graphic type
    • Available in volume
    • If a product needs a colour 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 supply

Power supplies to be connected to mains need 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)
  • 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 plugpack 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 replacement for standard batteries
    • Rechargeable
      • Nickel Cadmium - standard rechargeables, suffer from memory effect
      • High Temperature Nickel Cadmium
      • Nickel Metal Hydride - improved standard rechargeables, 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


Describe the medium (e.g. RS232) the data (e.g. Aluminium thickness measurements) and the protocol.

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