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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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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 • 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? • 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 • 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? • 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 • 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 • 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 • 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 • Power on/off sound • Warbling call tone • Roger bleep after a transmission • Short bleeps for low battery warning • Once a minute?
LED functions • Intermittent flash while turned on • Every 10 secs?
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