meteo · the temperature probe

Thermal Probe Wiring

A waterproof DS18B20 on a three-wire lead has to get from the sea — or from the ground — into a sealed hull, and end in a connector the carrier board accepts. Three wires in, three pins out, and one of the two penetrations on the whole probe in between.

Companion to buoy-parts-book.html and battery-pack-wiring.html. Pin order from the rev. 3 order spec's keying decision; the gland from the hull spec §7 row 14a.

⚠ HULL-09 — no hull is printed for deployment until this joint is tested

The probe's lead reaches the dry cavity through a PG7 cable gland in the sensor-pocket roof. The bore is already cut in every hull — it is unconditional in sphere200g_p123 — so the penetration exists whether or not the gland is fitted. The gland is chosen and not fitted, and the joint has never been pressure-tested.

That is one of two blocking rows on the hull, and it is the reason this page ends on a gate rather than on a build. Everything below is buildable on the bench today; none of it goes in the water until HULL-09 closes.

Three wires, one hull, three pins

Flooded — sea, or the ground an L-shaped pocket at azimuth 270, open to the sea through the hull’s bottom. A dent, not a penetration — the 3.0 mm wall is the barrier along its whole height DS18B20 tip Ø6.3 × 50 mm stainless ±0.5 °C absolute — used for gradients, not for absolutes one 3-wire lead hull 3.0 mm ASA PG7 cable gland thread Ø12.5 × 1.27, 8 mm seals on 3–6.5 mm cable ⚠ NOT FITTED — HULL-09 Dry cavity everything right of the wall is inside the sealed hull the lead is the only thing that crosses it the housing is crimped on in here — it cannot pass back through the gland afterwards (3 positions at 2.5 mm pitch is 7.5 mm; the seal shuts on 6.5) JST-XH, 3-position 1 2 3 J8 — the order is keying 1 DATA — yellow 2 GND — black 3 3V3 — red DATA and 3V3 on the OUTER pins, deliberately — see below. Do not “tidy” this into 3V3-DATA-GND. meteo-carrier OW_DATA IO4 R3 4.7 kΩ to 3V3 the pull-up is what makes 1-Wire work: the bus idles high, and every device only ever pulls it down 1-Wire: one line carries clock and data together
Left of the wall is wet, right of it is dry, and the only thing that crosses is a 3–6.5 mm cable through a gland that has never been pressure-tested. The connector is crimped on the dry side because it is wider than the hole it would have to come back through.

Why three wires and not two

A DS18B20 can run on two wires — parasitic power, where the sensor steals charge from the data line between transactions and stores it in an internal capacitor. It is a real mode and it halves the conductors through a gland, which is exactly the constraint here.

This design uses three anyway, on its own 3V3 feed. Parasitic mode needs a strong pull-up switched onto the bus during temperature conversion, is fussier about cable length and capacitance, and turns a sealed, unrecoverable probe's only sea-temperature reading into a mode with more ways to go marginal. The third wire buys margin on the one measurement that cannot be repaired at sea, at the cost of one more conductor through a gland already rated for the cable.

The pin order is keying, not convenience

J8 is the only fully-populated 3-position port on the board — the thermistor and leak ports both leave their centre pin vacant, which is what makes a wrong plug harmless there. J8 cannot do that, because it needs all three. So it is keyed a different way: by pin order.

With DATA and 3V3 on the outer pins, an NTC harness — which is loaded on its outer positions — plugged into J8 lands across DATA and 3V3, and sees 3.3 V at both ends through R3: zero static current, nothing damaged. A leak harness in the same position puts ~0 V across the hull pads: no electrolysis, no standing drain. Reorder those three pins for tidiness and both of those become live faults.

⚠ J8 is the one port a solar cable can still damage

Every other cross-plug on the board is either impossible or harmless. This one is not: a 2-position solar housing pushed into J8 lands both contacts, and puts ~6.7 V on IO4 (rated 3.6 V) or straight onto the 3V3 rail.

Nothing about pin order fixes that, because the port has no dead pin to absorb an offset. The mitigations are procedural and they are all you have: a white housing on this harness and a red one on the solar line, silk labels, and both ends of the solar cable committed. Keep them.

Build order

Step 2 is the one that is expensive to get wrong: crimp before threading and the housing will not pass the gland, and the only way back is to cut the crimps off.

  1. Identify the three wires with a meter, before anything else — see the warning below. Do not trust the colours.
  2. Thread the bare cable through the gland body first — nut, seal, then body — and only then crimp. Three positions at 2.5 mm pitch is 7.5 mm of housing against a seal that shuts on 6.5 mm of cable.
  3. Fit the gland into the pocket roof, into the female PG7 thread cut in 3.0 mm of ASA, and leave enough slack inside the cavity that the lead is not the thing holding the probe.
  4. Crimp the three contacts and load the housing yellow → 1, black → 2, red → 3 — DATA, GND, 3V3 — matching J8. Use a white housing.
  5. Bring-up check: a 1-Wire search on IO4 returns a ROM ID and a plausible temperature. A search that finds nothing usually means DATA and 3V3 are swapped; a search that finds a device reading −85 °C usually means it is not powered.

⚠ The colour-to-function map is not recorded anywhere in this project

The probe arrives with three stripped wires — red, yellow and black — and the near-universal convention for these is red = VDD, yellow = DATA, black = GND. This tree has never written that down, and cheap waterproof DS18B20s are known to ship with the mapping shuffled.

Reversing red and black destroys the sensor, and there is no spare per probe once all three deploy. Ring it out with a meter against the sensor's own datasheet pinout before the first power-up, and write what you find on the harness. Everything in the drawing above assumes the convention holds.

What each wire does

WireNetJ8 pinWhat it does
yellowOW_DATA1The whole bus. 1-Wire carries clock and data on one line; the sensor never drives it high, only pulls it low against R3
blackGND2Return. On the centre pin, which is the position an offset-inserted 2-pin housing is most likely to land on
red3V33Supply, from the always-on rail — the reason there are three wires rather than two

R3, the 4.7 kΩ pull-up, lives on the carrier and not in the harness — so the harness is three plain wires and nothing else, and a spare can be made anywhere.

Open, and not to be guessed at the bench

HULL-09
The gland is not fitted and the joint has never been pressure-tested. The bore is live in every printed hull. Blocking: no hull is printed for deployment until this closes.
not tracked
Which colour is which. See the warning above. The convention is red/yellow/black = VDD/DATA/GND, and nothing in this project has confirmed it against the probes actually owned.
not tracked
Whether the probe's cable fits the gland. The PG7 seals on 3–6.5 mm; the DS18B20's lead diameter is not recorded. Measure it before ordering glands, not after.
PROC
Two more probes to buy. Three are owned and all three probes deploy one, which leaves no spare and no calibration unit.