Text of Taking Cave Rescue Telephones Forward Stuart France
Taking Cave Rescue Telephones ForwardStuart France
Single-wire field telephones remain the most reliable and straightforward comms tool for cave rescue in Wales
The 1988 versionSecond-hand BT 4T earpieces as mic/spkrDPDT toggle switch for PTT functionSTDT toggle switch for on/off9 volt PP3 batteryNo call tone facilityPCB potted but switches vulnerableDiecast box, no gasket, earth connectionLine connection as flying lead with croc clip
The 1988 version (sort of)
What happened in practice?Turned on by accident flat batteryNo LED turned off by accidentToggle switches snappedStored damp or with water inside boxCorrosion of wires / switchesNo way to get the attention of called partySpeech level fades as battery drainsPP3s not easy to get in a hurry
Nigel Lovell versionBetter box and construction standardHigher line impedanceLED 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 50pSoftware seen as an asset not a riskDC-DC power supply chipsBattery technology advancesThe iPod generation grow up
RuggedizationBrand new military spec 4T earpiecesDiecast box with a gasketIP68 push-button not toggle switchesScrew terminal post for line connectionCroc clip lead as an accessoryCircuit and switch wiring potted3xAA Battery holder replaceable
The development version
Push Button3-second push to power it upOnce running:Push and hold to talk, as per normalDouble-click to send a call toneClick or timeout to end call toneQuad-click to power off
Tone GenerationPower on/off soundWarbling call toneRoger bleep after a transmissionShort bleeps for low battery warningOnce a minute?
LED functionsIntermittent flash while turned onEvery 10 secs?
2009 PrototypePIC12F629 with 6 I/O lines in a DIL8 deviceGood for 2.5 to 5.0 volts, in-circuit programmableMAX761 converts 3xAAs to 12 voltsStarts up at 2.5V, low battery logic output to uPOne push-button switchRelay instead of the DPDT toggle switchDiecast box with gasketNATO specification tropicalized 4T earpieceAudio circuit as per Nigel LovellScrew terminal post for line connection