Side project In build

Seabed dolphin listener

A €50 tube that sits on the sand off Marbella for a week, records everything it hears, then lets go of its bag of bricks and floats up on its own. No winch, no radio, nothing left on the seabed.

34 seconds: the drop, a week on the bottom, the burn wire parting, the rise on the lanyard, the surface, the bricks coming up. Rendered in Blender from the actual part dimensions.

How it works

Drop it

Four bricks in a mesh bag, the float a metre above on a short tether, 25 m of lanyard wound round its body like thread on a bobbin.

Listen for a week

A piezo disc glued inside the top cap hears through the PVC. An ESP32 records 24 kHz audio to a 64 GB card on three 18650 cells.

Let go

On day seven the ESP32 puts 11 V on a 0.4 mm stainless loop and seawater eats it in minutes. A galvanic link below it is the no-power backup.

Float up

The lanyard pays off the end of the coil, nothing turns, nothing can jam. The float waits at the surface on its line and you pull the bricks up by hand.

Inside the tube

DN75 pressure pipe, a glued cap at the hydrophone end and a screw-off PVC union at the other. Everything labelled, in motion.

Labelled cutaway of the lander housing — The same cutaway as a still. Piezo disc, preamp, clock, SD card, ESP32, buck, burn-wire MOSFET, cells, hatch, tether eyebolt, Schrader valve for the bike-pump leak test.

The release, five ways

Five release mechanisms rendered on the bottom cap — The hard problem with a sealed box is letting something go outside it. Burn wire through a potted hole, burn wire on bolt terminals, a galvanic link with no wires at all, a magnet latch with a servo inside, and the pair I would build: burn wire for the schedule, galvanic link as the backup.

The numbers

Depth	10 to 30 m. 1 to 3 bar on the housing, nothing for an O-ring, a lot for a stitched seam.
Ballast	Four bricks, 10 kg dry, 4.8 kg wet. Bricks lose half their weight in water. Holds a 1 m/s current briefly.
Lanyard	2 mm Dyneema, 1.3 × depth plus 5 m. About 400 kgf breaking for a 20 kg retrieval pull.
Power	About 55 mA average recording. Three 18650s give a week with margin.
Storage	24 kHz 16-bit mono is 29 GB a week. A 64 GB card.
Cost	Under €50 in new parts on top of a homemade hydrophone and a bare ESP32.

What it would hear off Marbella

Bottlenose dolphin whistles out to about 500 m, clicks a few hundred metres
Brown meagre drumming at dusk, cusk-eels at night, damselfish in summer
Snapping shrimp around the clock, the crackle under everything
Sea urchins grazing at dawn and dusk
Boats and jet skis for kilometres, rain as broadband hiss
Sperm whale clicks carried in from the deep Alborán on quiet nights

Nobody sells the "comes back down" part at hobby scale. Pop-up pot buoys only go up and are rewound by hand on deck, which is why this one is a one-shot too. The research instruments that winch themselves back cost tens of thousands.

What already exists

Nine products and builds, one per sub-problem — One product per sub-problem, from the Fiobuoy spool float and crab-pot pop-ups to sealed underwater servos and the professionals' hydrophone moorings. The design borrows the fixed-spool pay-off and the dual release from them.