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Chapter 7 — Electronics Mounting

Chapter 7 of 10  ·  ~45 minutes  ·  Mounting and connecting all boards

For this chapter, prepare:

  • V4 PCB (1×)
  • SSR DA40 with safety cover (1×)
  • Thermocouple probe (1×)
  • ToFnLED module (1×)
  • Scales PCB + load cells (2×) — Complete Kit only
  • Phillips screwdriver PH2 (1×)
  • M4 screws (2×) — for SSR mounting
  • 3D-printed enclosure — ordered separately from Espressio (EU) or Hudson Creative Group (US)
  • The machine, unplugged

3D-printed enclosure — must be ordered before this step

The V4 PCB and HMI screen require a printed housing. This is not included in the Peak Coffee kit. Order from Espressio (EU) or Hudson Creative Group (US) at least 1–2 weeks before your build session.

Internal mounting (inside the machine) and external mounting (on the drip tray) have different enclosures. Decide which before ordering. External is recommended for first-time builders — easier to troubleshoot and replace.

STEP 1 — Mount the SSR

The SSR mounts directly to the machine chassis (the metal side panel or base) using the two M4 screws.

SSR MUST contact bare metal

The SSR uses the machine chassis as a heatsink. It must have direct metal-to-metal contact — no rubber feet, no plastic spacer.

An SSR running without a heatsink reaches dangerous temperatures within a few minutes of use.

  1. Choose a mounting location on the metal chassis — flat, accessible, away from water and vibration paths.

  2. Apply a thin, even layer of thermal paste to the SSR's back face before mounting. A pea-sized amount spread across the surface is sufficient — the goal is to fill the microscopic air gaps between the SSR and the chassis, not to insulate with a thick layer.

    Recommended: ARCTIC MX-4 (4g)

    ARCTIC MX-4 — high-conductivity, non-electrically-conductive thermal compound. 4g is far more than needed for one build. Spec: 8.5 W/m·K conductivity, rated −40°C to +200°C.

    Search: Arctic MX-4 thermal compound 4g — amazon.nl B07L9BDY3T

  3. Mount with M4 screws. The SSR should be firmly against the metal surface.

Machine exterior bottom — ventilation slots visible. The two M4 SSR mounting screws protrude here and are visible from outside when the SSR is installed inside.

The SSR has four terminals: - AC terminals (top, higher numbers) — connect to the boiler heater wires from your harness - DC terminals (bottom, 3–32V) — connect to the SSR control wires from the PCB

The PCB controls the SSR via the DC terminals; the SSR switches the boiler heater via the AC terminals.

SSR DA40 test-fitted to the Gaggia Classic Pro E24 back panel — two M4 bolts visible from exterior through ventilation slots, IEC mains socket to the right, SSR body flush against the interior metal wall

STEP 2 — Install the thermocouple

The thermocouple replaces the brew thermostat disc on top of the boiler.

STEP 2a — Remove the brew thermostat

  1. Locate the brew thermostat (the disc on top of the boiler, held by a bracket or centre bolt).
  2. Remove the two wires from the thermostat spade terminals.
  3. Undo the retaining bolt and remove the thermostat.
  4. Clean the thermostat mounting point on the boiler with isopropyl alcohol and a cotton swab. A clean surface ensures good thermal contact.

STEP 2b — Install the thermocouple jumper

The thermostat created an electrical connection in the heater circuit. Removing it breaks that circuit. Install the jumper wire (included in the kit) to re-bridge the circuit at the thermostat mounting point.

The jumper is a Y-connector: one Faston female on the single end connects to the existing wire, and the two Faston females on the split end connect to the two original thermostat spade terminals. This re-closes the heater circuit without the thermostat.

Thermostat bypass Y-connector — single Faston female at bottom splitter into two at top, all red wire. One end per thermostat spade terminal.

STEP 2c — Install the thermocouple probe

  1. Thread the M4 thermocouple probe into the thermostat mounting hole on the boiler.
  2. Turn by hand until snug.

FINGER-TIGHT ONLY

The thermocouple probe screws into brass. Over-tightening damages the boiler thread or the probe tip.

Finger-tight means: turn until you feel resistance, then stop. Do not use a spanner on the probe itself.

  1. Route the thermocouple cable to the PCB connector labelled THERMOCOUPLE or TC.

Thermocouple probe installed in boiler — probe tip threaded into the M4 mounting hole on the boiler top, black cable routing away

STEP 3 — Mount the PCB enclosure and PCB

  1. Mount the 3D-printed enclosure in your chosen location (external on drip tray, or internal in machine).
  2. Seat the V4 PCB in the enclosure — it should click into the mounting posts.
  3. Connect the wiring harness connectors to the PCB screw terminals:
  4. Each terminal is labelled on the PCB silkscreen (L, N, PE, PUMP, SSR, etc.)
  5. Match the label to the wire from your harness
  6. Tighten screw terminals with a small flat-head screwdriver until snug

Screw terminal torque

Screw terminals should be tight enough that you cannot pull the wire out with moderate force. Hand-tight with a small screwdriver is approximately correct. Do not over-torque — you will strip the terminal.

STEP 4 — Connect the HMI screen

Back of HMI board — orange PCB with LCD1 ribbon cable connector (left), USB1 USB-C port (right), P1 UART connector (GND/RXD/TXD/5V), and P3/P4 expansion headers along the bottom. Yellow/blue wires connected to P1.

The back of the HMI board has several connectors. You use two of them:

  • LCD1 (large flat ribbon connector, centre-left) — the display ribbon cable. Already connected from the factory.
  • P1 (4-pin white connector, right side, labelled GND / RXD / TXD / 5V) — this is the UART connection to the V4 PCB. The yellow and blue wires in the photo connect here.

The labels IO17, IO18, IO19, IO20 visible on the board are GPIO expansion headers — not used in this installation. Ignore them.

  1. Route the HMI cable from the V4 PCB HMI connector to the screen's P1 connector.
  2. Connect both ends — polarity matters. Match GND→GND, 5V→5V.
  3. The ribbon cable from LCD1 should already be connected on the back of the screen — do not disconnect it.

The ribbon cable connector has a locking latch. Ensure the cable is fully seated before closing the latch.

STEP 5 — Install the ToFnLED module (Complete Kit)

The ToFnLED module goes inside the water tank area. It measures water level via an optical Time-of-Flight sensor and provides LED illumination.

  1. Mount the module in the water tank cavity — it should face upward to measure the water depth.
  2. Connect the ToFnLED cable to the matching connector on the PCB.
  3. Route the cable so it does not interfere with tank removal.

STEP 6 — Install the scales (Complete Kit)

The scales go under the drip tray. This requires the external enclosure option.

  1. Mount the two load cells according to the scales PCB instructions included in the Complete Kit.
  2. Connect the load cell cables to the Dual Scale Board PCB.
  3. Connect the Dual Scale Board PCB to the V4 PCB via the provided cable.

Load cell wiring colour code: - Red — Excitation positive (E+) - Black — Excitation negative (E−) - White — Signal positive (A+) - Green — Signal negative (A−)

Load cell colour sequence is critical

Reversing any load cell wire pair produces incorrect readings or no readings. The colour sequence above is the standard. Verify before powering on.

STEP 7 — Final cable routing

Before closing up:

  1. Route all cables away from the pump (vibration degrades connectors over time)
  2. Route AC cables (orange and blue) away from DC signal cables (thermocouple, pressure sensor)
  3. Secure cables with small cable ties or velcro straps — tidy cable routing is functional, not just aesthetic
  4. Verify no cables are pinched or crossing moving parts

STEP 8 — Bench test before closing the machine

Do this before routing cables and closing the top panel. This is the moment to find any problem — while the machine is still open and accessible.

USB-C power only for this test — machine stays unplugged from the wall

You are powering the PCB only via USB-C (a phone charger or computer USB port). The machine mains cable must remain unplugged from the wall for the entire bench test.

Connect for the test:

  • HMI screen connected to PCB (P1 cable and ribbon)
  • Thermocouple connected to PCB thermocouple connector
  • USB-C cable connected to PCB (power only — from a phone charger, not the machine)

Power on and verify:

  1. Plug in the USB-C. The HMI screen should light up within 3 seconds and show the Gaggiuino boot sequence, then the profile selector.
  2. Check the temperature reading on the home screen. It should show approximately room temperature (18–25°C).
  3. Hold the thermocouple probe tip between your fingers for 30 seconds. The temperature reading should rise toward body temperature (~37°C). This confirms the thermocouple is reading and not reversed.
  4. If the screen is blank: recheck the HMI P1 connector — both ends. Verify the ribbon cable latch is fully closed.
  5. If temperature reads -200°C or a fixed wrong value: recheck the thermocouple connector at the PCB. If reversed, unplug and re-insert with opposite orientation.

Gaggiuino HMI first boot — profile selection screen showing IUIUIU Classic, Filter 2.1, Zer0, Blooming, Londinium, Adaptive profiles. This is what a successful bench test looks like.

Pass: Screen boots, temperature reads room temp and rises when probe is held.
Fail: Stop. Fix before closing. Much easier now than after reassembly.

Disconnect USB-C when done. The machine stays unplugged.

Checkpoint

  • SSR mounted to bare metal chassis, firmly secured
  • Thermocouple installed finger-tight in boiler
  • PCB seated in enclosure, all harness connectors tightened
  • HMI screen connected, ribbon cable fully seated
  • ToFnLED connected (if Complete Kit)
  • Scales installed and connected (if Complete Kit)
  • Cables routed away from pump and heat sources
  • Bench test passed: screen boots, thermocouple reads room temperature

You're 75% done. Physical build verified. Firmware next, then first power-on from mains.

Next: Chapter 8 — Firmware