By Brian Pease
Bill Stone uses single wire voice telephones (Michiephones) to coordinate between Base Camp and deep underground camps during his Mexican Cheve Expeditions. Each camp has one or more Android tablets used for surveying and sketching with TopoDroid software. On the last Expedition a successful experiment was run to send messages and survey data from Camp 1 to Basecamp using direct TTL [transistor-transistor logic] baseband signaling with UARTs [universal asynchronous receiver/transmitters]. Bill asked me for ways to improve the system to serve remote camps as far as 15 km away.
I created an approximate model of the single wire and the grounds used in the cave. The model will pass 2,400 baud data, likely less in the real world. Poor connections, poor grounding, and especially leakage to ground from bare spots may pull the 5V [volt] TTL signal below the 1.4V switching threshold. I looked into boosting the signal to ~12V with automatic transmit/receive switching.
Next I looked at creating my own modem using Exar FSK [frequency-shift keying] transmit and receive ICs [integrated circuits], which use sine waves on two audio frequencies and can be driven directly from the UARTs. This approach is fairly complex with initial analog adjustments and the need for an L-C [inductor-capacitor] bandpass filter to improve noise immunity.