Hardware teardown

The original Gemini box, wiring decoded

A complete reverse-engineering of a salvaged Gemini Shale tank-volume appliance — every board, every rail, and what each wire actually does.

Platform · Raspberry Pi 3B+ Sensing · ADS1115 ADC Power · 3× CUI DC-DC Display · 800×480 HDMI

What this box is

Before it became a candidate for a Clark Farming Company field station, this enclosure spent its life as a Gemini Shale tank-volume / truck-load meter. Inside is a Raspberry Pi 3B+ that read two analog instruments through an ADS1115 analog-to-digital converter, computed the liquid volume of a tank from its geometry, showed the answer in gallons and barrels on a small kiosk screen, and broadcast the figure over Bluetooth to a phone app. A power tree of three isolated DC-DC converters let the whole thing run from a single, dirty field DC supply, and a Juice4Halt module gave it a clean shutdown when that supply dropped.

Everything below was reconstructed from 13 internal photographs of the open box, cross-checked against the device's own firmware recovered from its SD card. Where the photo and the code agree, confidence is high; where only the code knows (because the field sensors were not attached on the bench), that is called out explicitly.

Sheet 1 — power distribution

How one rough field supply becomes three clean, isolated rails.

The chain, step by step

Field power arrives through a cable gland as a single pair — red positive, black return — at somewhere between 9 and 36 volts DC, the wide range that lets the same box run from a 12 V battery, a 24 V industrial supply, or a solar/battery bank without rewiring. The positive leg passes first through an inline blade-fuse holder, the single point of overcurrent protection for everything downstream.

From the fuse, power lands on a pair of barrier terminal strips, TS1 and TS2, jumpered into two continuous rails: TS1 is the positive bus, TS2 is the return. This bus is simply a distribution manifold — every converter taps the same raw voltage from it in parallel. There is no regulation here; the converters do that individually.

Three CUI Inc. DC-DC converters hang off the bus, each isolated and each making its own clean output:

• PS5V — a PYB20-Q24-S5 (9–36 V in, 5 V out at up to 4 A). This is the heart rail: it powers the Raspberry Pi, and through the Pi the stacked ADC HAT and the LCD.
• PS24V-A and PS24V-B — two identical PDQE6W-Q24-S24-T modules (9–36 V in, 24 V out at 250 mA). These exist to power the field instruments: a 24 V loop is the standard supply for a guided-wave radar transmitter and many analog sensors.

One important honesty note, visible in every photo: the 24 V outputs land on bare green terminals. On the bench, with no tank, no radar and no inclinometer attached, those rails go nowhere — which is exactly what you would expect for a box pulled from storage. The diagram draws those loads dashed for that reason.

Power wiring — every conductor

Wire	From → to	Carries	Colour
Feed +	Gland → fuse → TS1 bus	Unregulated +9–36 VDC	red
Feed −	Gland → TS2 bus	Supply return / 0 V	black
Bus → PS5V	TS1/TS2 → PS5V Vin/GND	Raw +9–36 V in	red / black
Bus → PS24V-A	TS1/TS2 → PS24V-A Vin/GND	Raw +9–36 V in	red / black
Bus → PS24V-B	TS1/TS2 → PS24V-B Vin/GND	Raw +9–36 V in	red / black
PS5V out	PS5V +Vo/0V → Pi 5 V / GND	Regulated +5 V (Pi + HAT + LCD)	red / black
PS24V-A out	PS24V-A +Vo/−Vo → green terminal	+24 V loop supply A — bare on bench	red / black
PS24V-B out	PS24V-B +Vo/−Vo → green terminal	+24 V loop supply B — bare on bench	red / black

Sheet 2 — signal, control & I/O

What the Pi reads, how it shuts down safely, and how it talks to the world.

The analog signal path

The Pi has no analog inputs of its own, so it reads the instruments through a stacked ADS1115 — a 4-channel, 16-bit analog-to-digital converter, sold here as a red HAT silk-screened "4 CH 16bit ADC with PGA for RPi". It connects to the Pi over the I²C bus (two data lines, SDA and SCL, plus power and ground) through the 40-pin header. The green screw terminals visible on the red board are not the Pi connection — they are where the field signals land.

The firmware tells us exactly how the channels are mapped:

• Channel P0 — the pitch (X) voltage from a DAS-10A-type inclinometer. 2.5 V means dead level; the swing above and below encodes tilt angle.
• Channel P1 — the roll (Y) voltage from the same inclinometer.
• Channels P2 & P3 (differential) — the guided-wave radar level signal. The radar is a 4–20 mA loop instrument; the loop current is dropped across a 5 Ω shunt resistor, turning 4–20 mA into roughly 20–100 mV, which the ADS1115 reads differentially across the shunt.

With tank geometry entered once (diameter, length, end-cap shape, sensor positions), the Pi turns "how tilted is the tank" plus "how high is the liquid" into a precise gallon and barrel figure. Because the sensors were not connected on the bench, these three rows are the intended wiring proven by the code, not something visible in the photos.

Safe shutdown — the part that matters off-grid

A Raspberry Pi hates losing power mid-write; do it enough times in the field and the SD card corrupts. The Juice4Halt module solves this. It sits on the GPIO header and uses two lines:

• GPIO25 — a board-status / boot-finished handshake. After boot the Pi pulses this line to arm the module; if the module later pulls it low, the Pi knows to shut down; the final low-to-high transition tells the module the Pi is safely halted so it can cut power.
• GPIO22 — input-power-fail detection. The module flags the instant the main supply drops, giving the Pi a few seconds of hold-up to flush and power down cleanly.

Separately, a panel-mounted momentary button on GPIO26 lets a person request a clean shutdown by holding it for three seconds — no SSH, no screen needed.

Display and connectivity

The Pi drives an 800×480 HDMI LCD over a standard HDMI cable for video, with a separate 5 V lead for the panel's own power; it is a non-touch kiosk display. For data off the box, the original Gemini design leaned on onboard Bluetooth, advertising the current truck/tank volume to a phone app; an Ethernet (RJ45) port was available on the LAN as well. Notably, no industrial protocol — no Modbus, no WirelessHART — appears anywhere in the original firmware; that is a Clark Farming Company addition in the box's second life.

Signal, control & connectivity wiring

Link	From → to	Carries	Basis
I²C	Pi GPIO2/3 → ADS1115	Digitised sensor readings	seen
HAT power	Pi 3V3/5V/GND → ADS1115	Logic power + reference	seen
Analog P0	Inclinometer pitch → ADC P0	Tilt voltage (2.5 V = level)	code
Analog P1	Inclinometer roll → ADC P1	Tilt voltage	code
Analog P2/P3	Radar → ADC P2/P3 across 5 Ω	4–20 mA → 20–100 mV level	code
HDMI	Pi HDMI → LCD	800×480 video	seen
GPIO25	Pi ↔ Juice4Halt	Boot / shutdown handshake	code
GPIO22	Pi ↔ Juice4Halt	Input-power-fail detect	code
GPIO26	Button → Pi (other side GND)	Hold 3 s → power off	seen
Ethernet	Pi RJ45 → LAN	Network (DHCP)	seen
Bluetooth	Pi radio → phone app	Volume readout (original egress)	code

Bill of materials

Every part, with the marking actually read off it where legible.

Ref	Component	Part / marking	Job	Basis
Pi	Raspberry Pi 3B+	—	Controller, Bluetooth, kiosk	seen
ADC	ADS1115 ADC HAT	4 CH 16bit ADC with PGA for RPi	Analog → I²C	seen
PS5V	CUI DC-DC converter	PYB20-Q24-S5 · 9–36 V → 5 V/4 A	Pi 5 V rail	seen
PS24V-A	CUI DC-DC converter	PDQE6W-Q24-S24-T · 9–36 V → 24 V/250 mA	Field loop supply A	seen
PS24V-B	CUI DC-DC converter	PDQE6W-Q24-S24-T · 9–36 V → 24 V/250 mA	Field loop supply B	seen
TS1 / TS2	Barrier terminal strips	jumpered + / − rails	Raw-DC distribution bus	seen
FUSE	Inline blade-fuse holder	rating not legible	Input overcurrent protection	seen
LCD	800×480 HDMI display	non-touch	On-box readout	seen
J4H	Juice4Halt UPS	—	Safe shutdown (GPIO22/25)	inferred
BTN	Momentary push-button	panel-mount	Manual shutdown (GPIO26)	seen
INCL	DAS-10A inclinometer	not in photos	Pitch / roll tilt	code
GWR	Guided-wave radar (4–20 mA)	not in photos	Liquid level	code

A note on the GPIO stack

The Pi's 40-pin header carries more than one board. The red ADS1115 HAT sits at the top, and underneath it — in several of the photos — there is a second green board with a distinctive circular silk-screen pattern. The most likely identity is the Juice4Halt module itself: that circular footprint is consistent with its hold-up supercapacitor, and it would explain why no separate Juice4Halt board appears anywhere else, and why the GPIO22/25 shutdown lines have somewhere to land. The silk-screen text was not legible in any frame, so this is the one structural detail held at "inferred" rather than confirmed — popping the stack and reading the label would settle it.

How we know — and what we don't

Each claim above is tagged by its evidence: seen means it is legible or visible in the photographs; code means it is proven by the device's own firmware (the intended wiring, even where the part was not on the bench); inferred means it is the best reading but not conclusive. The component identifications were cross-checked by independent review passes over the photographs before being committed here.