The Specification: Tech ingredients

• 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

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
  • 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
      • 6mm square tactile keys
      • 12mm square tactile keys (click for example board)
      • Several other types, including lit pushbuttons
  • 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

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 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

Indicators

• 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

Functions

Modes of operation

Power supply

• 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

Communications

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