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Electronics Guide

⚠️ Work in progress — see TODO.md for known open issues (optical-endstop polarity / $5, solenoid holding-current tuning, and on-hardware verification).

This document covers the controller setup, wiring, pin mapping, and component details for the G-code Pen Plotter. For firmware and GRBL configuration see firmware/README.md. For the hardware test procedure see testing.md.

Table of contents

Machine configuration (canonical)

Single source of truth for the electrical build. firmware/src/config.h, the standalone test sketches (firmware/test/) and testing.md must all match this table.

Axes & endstops

Axis Function Endstops Homes?
X Carriage 2 × optical: X_MIN + X_MAX yes → toward MIN
Y Paper feed none no
Z (unused) none no
  • Both X switches are wired in parallel onto the X_LIMIT pin (D9). GRBL 1.1 on the Uno has only one limit pin per axis, so it cannot tell X_MIN from X_MAX — it only ever reports "X limit tripped".
  • $H homes X toward X_MIN. X_MAX is therefore only a hard-limit backstop. The usable X length must be measured once and stored in $130 — see testing.md → TC5b-G "Teach the X-axis length".
  • Y has no reference. Y = 0 is wherever the axis sits at power-up or after $X. Load the paper, then treat that as the origin.
  • Firmware: HOMING_CYCLE_0 = (1 << X_AXIS) (X only).

Solenoid polarity

  • De-energized = pen DOWN (drawing). This is the safe resting state — no coil current, no heat.
  • Energized (M3) = pen UP. Never leave it energized: keep travel moves short and make sure every plot ends on M5 (see pipeline/configs/grbl_a4_pen.toml).
  • The G-code profile (grbl_a4_pen.toml) pulls the pen up at full drive (M3 S1000) then holds it at M3 S350 (~35 % PWM) to limit coil heat — the hold value still needs tuning on the real machine, see Solenoid pen lift.

Controller and shield

  • Arduino Uno (ATmega328P) running GRBL v1.1
  • Arduino CNC Shield v3 stacked on top of the Uno

Only 2 of the 4 possible axes on the CNC Shield are used:

  • X axis -- carriage movement via timing belt, with X_MIN + X_MAX optical endstops
  • Y axis -- paper feed (no endstop)

Stepper drivers

  • A4988 (or DRV8825 as drop-in replacement) for X and Y axes
  • Attach heat sinks before first use
  • Set the current limit via the trimmer pot before connecting motors under load -- set to ~70 % of your motor's rated current per phase

Power supply

  • 12 V DC for NEMA 17 motors and solenoid
  • Use a supply rated for the combined peak current of both stepper motors and the solenoid

12 V power supply

The 12 V supply connects to the screw terminals on the CNC Shield (labelled GND and V+), not to the Arduino's barrel jack. From those terminals the voltage is passed through to the Arduino Uno's VIN pin via the shield's stacking header. The Arduino's on-board voltage regulator converts VIN to the 5 V and 3.3 V rails used by the microcontroller and peripherals. Use the 12 V supply to also power the MOSFET circuit power input as it will also drive the solenoid.

USB and 12 V simultaneously

The Arduino Uno contains a power-path selector between the VIN from Stepper shield (and for its own barrel-jack) supply and the USB 5 V rail:

  • 12 V via CNC Shield only — the on-board regulator powers the Arduino; USB is not connected.
  • USB only (no 12 V via barrel jack/stepper shield) — the Arduino runs from USB 5 V; the stepper drivers and solenoid are unpowered. Use this mode when flashing firmware without the motors connected.
  • Both 12 V and USB simultaneously — this is the normal setup (UGS command sending from PC while the machine runs). The on-board Schottky diode prevents current from flowing back from the Arduino's 5 V rail into the host PC's USB port. No special precautions are needed.

5 V / GND distribution board

Several consumers need a regulated 5 V supply and a common GND:

Consumer Supply needed
Optical endstops (× 2, both on X) +5 V, GND
MOSFET gate circuit GND (gate pull-down)
Button / switch strip (if fitted) +5 V, GND

The CNC Shield exposes only a limited number of 5 V and GND pins on its headers, which is not enough for all consumers at once. The recommended solution is a small stripboard / perfboard distribution board.

Keep the total current draw of 5 V consumers well below 500 mA — the Arduino's on-board regulator (when powered from 12 V via the shield) can supply roughly 300–400 mA at 5 V before thermal throttling. Optical endstops typically draw < 20 mA each, so two endstops + a MOSFET pull-down are well within budget.

Pin mapping (CNC Shield v3 -> Arduino Uno)

Function CNC Shield label Arduino pin Notes
X Step X.STEP D2
X Direction X.DIR D5
Y Step Y.STEP D3
Y Direction Y.DIR D6
Z Step (unused) Z.STEP D4
Z Direction (unused) Z.DIR D7
Stepper Enable EN D8 active LOW
X endstops X_MIN + X_MAX X- D9 both switches in parallel on this one pin
Y endstop (none) Y- D10 GRBL Y_LIMIT — leave unconnected
Pen-lift solenoid (see below) D11 GRBL SPINDLE_PWM (needs VARIABLE_SPINDLE). Not on the "SpnEn" header — that is D12. Tap D11 at the Z-endstop header (hard-wired to D11 on the shield) or solder to the D11 pin.
(Z_LIMIT — unused) Z- D12 relocated here by VARIABLE_SPINDLE; leave unconnected
Abort / Reset -- A0 optional
Feed Hold -- A1 optional
Cycle Start -- A2 optional

Verify pin assignments against firmware/src/config.h and the canonical table above.

Microstepping jumpers

Leave all MS jumpers unpopulated (MS1, MS2, MS3 all open) for full-step operation. This matches $100/$101 = 5 steps/mm (see firmware/README.md → Key GRBL settings).

MS1 MS2 MS3 Mode
- - - Full step ← use this
x - - 1/2
- x - 1/4
x x - 1/8
x x x 1/16

Wiring steps

  1. Stack the CNC Shield v3 onto the Arduino Uno.
  2. Leave all MS jumpers unpopulated (full-step operation, no jumpers needed).
  3. Insert the A4988 stepper drivers for the X and Y slots; leave Z and A empty.
  4. Attach heat sinks to the drivers.
  5. Build and mount the 5 V / GND distribution board (see section above); connect it to the Arduino's 5 V and GND pins.
  6. Route stepper motor and endstop cables through the frame openings into the enclosure.
  7. Connect stepper motors to the X and Y terminals on the shield.
  8. Connect both X optical endstops (X_MIN and X_MAX) to the X- endstop header (3-pin: GND / 5 V / Signal), signal lines combined onto the one pin; take GND and 5 V from the distribution board. Leave the Y- header unconnected.
  9. Connect the solenoid MOSFET gate circuit to D11 (tap the Z-endstop header or solder the D11 pin — see pin mapping); connect the gate pull-down GND to the distribution board (see solenoid section below).
  10. Connect the 12 V supply to the screw terminals on the CNC Shield (not the Arduino barrel jack).
  11. Secure all cables with cable ties and cable management clips to avoid interference with moving parts.
  12. Screw the Arduino/Shield assembly onto the PCB holder, slide it onto the rods, and attach the housing.
  13. Make sure the USB port and the CNC Shield power terminals remain accessible.

Endstops

The design uses two optical endstops, both on the X axis — X_MIN at the home end and X_MAX at the far end. The Y axis (paper feed) has no endstop. See the canonical table.

  • Both X switch signal lines are combined onto the single X- header pin (Arduino D9 = GRBL X_LIMIT). GRBL cannot distinguish the two; it only reports "X limit tripped". $H homes to X_MIN; the X_MAX position is taught into $130 (see testing.md → TC5b-G).
  • The Y- header stays unconnected. If hard limits ($21=1) are enabled, make sure the unused Y_LIMIT (D10) and Z_LIMIT (D12) pins are not floating into a "triggered" state — GRBL's internal pull-ups plus the correct $5 value should leave them reading "clear".
  • Optical endstops need the +5 V supply pin; mechanical microswitches only need GND and Signal.
  • Internal pull-ups are enabled by GRBL; no external resistors needed for the signal line.

TODO (TODO.md): the correct $5 (invert limit pins) value is unverified. In GRBL 1.1, $5=1 means "pin HIGH = triggered", $5=0 means "pin LOW = triggered". Measure the real module output (beam clear vs. blocked) and set $5 accordingly. Also confirm the two parallel X switches can share D9 without an output clash (open-collector / wired-OR).

Solenoid pen lift

The solenoid is switched by the GRBL spindle PWM output on D11 via an N-channel MOSFET.

Which pin, and where to tap it. GRBL's PWM/enable output is on D11 only when VARIABLE_SPINDLE is enabled in firmware/src/config.h (it is — and it also moves the unused Z_LIMIT to D12). D11 is not the CNC Shield "SpnEn" header (that pad is D12). Get D11 from:

  • the Z-endstop header (Z- / Z+), which is hard-wired to D11 on the CNC Shield v3 — the same trick laser add-ons use; or
  • a wire soldered directly to the Arduino D11 pin.
D11 --[100 Ohm]-- MOSFET gate
MOSFET drain     -- solenoid (-)
solenoid (+)     -- +12 V
Flyback diode    across solenoid coil (cathode -> +12 V)
MOSFET source    -- GND
gate --[10 kOhm]-- GND   (pull-down: MOSFET off at power-up)
  • Use a logic-level N-channel MOSFET (e.g. IRLZ44N, 2N7000).
  • The flyback diode (e.g. 1N4007) is mandatory to protect the MOSFET from inductive kick.
  • Test the solenoid switching with a bench supply and current-limited setup before connecting to the full system.
  • If you use an opto-isolated MOSFET module rather than a bare MOSFET, note that GRBL's spindle PWM carrier is ~1 kHz — fine at S1000 (steady on), but a slow module may not pass a partial duty cycle cleanly (relevant only for the holding-current idea below).

Polarity (this build)

De-energized = pen DOWN (drawing). No coil current in the resting state, so no heat. Energized (M3) = pen UP. The solenoid must not stay energized for long — the coil overheats.

G-code Solenoid Pen
M3 S1000 energized (PWM ~100 %) UP
M5 de-energized DOWN (drawing)

Reduced holding current

A pull solenoid needs full current only to move the plunger; holding it needs far less. pipeline/configs/grbl_a4_pen.toml exploits the PWM on D11: each pen-up pulls in at M3 S1000, dwells 50 ms, then drops to M3 S350 (~35 % duty), and every file ends on M5. This keeps the coil at roughly a third of full power during travel.

  • Firmware / settings: nothing to change (VARIABLE_SPINDLE gives PWM, $30=1000 maps S1000 → 100 %).
  • Tune S350 on the real machine (S_HOLD in the toml, used in both document_start and line_end): the lowest value that still holds the pen fully up during a travel move. Raise it if the pen drifts down; lower it if the coil still gets more than warm after a long plot.

Hardware verification before flashing GRBL

Once the wiring above is complete, verify each subsystem independently with the standalone test sketches in firmware/test/ (X axis, endstops, Y axis, pen lift) before flashing GRBL. They need no GRBL knowledge and print PASS / FAIL on the serial monitor.

→ Procedure, expected values and failure diagnosis: testing.md → Phase 1.

Only proceed to flashing GRBL once all Phase 1 tests pass.