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Taking Cave Rescue Telephones Forward. Stuart France. WHY? Single-wire field telephones remain the most reliable and straightforward comms tool for cave rescue in Wales. The 1988 version. Second-hand BT “4T” earpieces as mic/spkr DPDT toggle switch for PTT function

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WHY?Single-wire field telephones remain the most reliable and straightforward comms tool for cave rescue in Wales


The 1988 version
The 1988 version

  • Second-hand BT “4T” earpieces as mic/spkr

  • DPDT toggle switch for PTT function

  • STDT toggle switch for on/off

  • 9 volt PP3 battery

  • No call tone facility

  • PCB potted but switches vulnerable

  • Diecast box, no gasket, earth connection

  • Line connection as flying lead with croc clip



What happened in practice
What happened in practice?

  • Turned on by accident  flat battery

  • No LED  turned off by accident

  • Toggle switches snapped

  • Stored damp or with water inside box

  • Corrosion of wires / switches

  • No way to get the attention of called party

  • Speech level fades as battery drains

  • PP3’s not easy to get in a hurry


Nigel lovell version
Nigel Lovell version

  • Better box and construction standard

  • Higher line impedance

  • LED flashes once when turned on/off

  • Other people have added 4000 or 74 logic chips for tones etc, but basically they are all much the same as “1988”


Has anything happened in the past 20 years
Has anything happened in the past 20 years?

  • Microprocessors costing 50p

  • Software seen as an asset not a risk

  • DC-DC power supply chips

  • Battery technology advances

  • The iPod generation grow up


Updating the phone spec
Updating the phone spec

  • Single SPST push-button interface

  • 3xAA alkaline to 12V DC-DC step-up

  • Microprocessor for:

    • Analysing push-button gestures

    • Power on/off function, low battery warning

    • LED signalling

    • PTT function

    • Tone generation

    • Upgradeable software


Ruggedization
Ruggedization

  • Brand new military spec “4T” earpieces

  • Diecast box with a gasket

  • IP68 push-button not toggle switches

  • Screw terminal post for line connection

    • Croc clip lead as an accessory

  • Circuit and switch wiring potted

  • 3xAA Battery holder replaceable



Push button
Push Button

  • 3-second push to power it up

  • Once running:

    • Push and hold to talk, as per normal

    • Double-click to send a call tone

      • Click or timeout to end call tone

    • Quad-click to power off


Tone generation
Tone Generation

  • Power on/off sound

  • Warbling call tone

  • Roger bleep after a transmission

  • Short bleeps for low battery warning

    • Once a minute?


Led functions
LED functions

  • Intermittent flash while turned on

    • Every 10 secs?


2009 prototype
2009 Prototype

  • PIC12F629 with 6 I/O lines in a DIL8 device

    • Good for 2.5 to 5.0 volts, in-circuit programmable

  • MAX761 converts 3xAAs to 12 volts

    • Starts up at 2.5V, low battery logic output to uP

  • One push-button switch

  • Relay instead of the DPDT toggle switch

  • Diecast box with gasket

  • NATO specification tropicalized 4T earpiece

  • Audio circuit as per Nigel Lovell

  • Screw terminal post for line connection


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