Hardware guide

A starter parts list and wiring notes. Everything here is commodity stuff you can buy on AliExpress / Amazon / Adafruit.

Power budget

Two rails, one battery:

    [ 3S LiPo 11.1V ]
        │
        ├─────────────►  URT-2 servo rail   (all ST-3032 / ST-3215 power)
        │
        └──► Buck  ──►  5V rail
                         │
                         ├──►  ESP32-S3 VIN (5V pin)
                         └──►  Sensors via 3.3V regulator on the board

  • Never power the servos from the ESP32’s 5V pin or USB — a single ST-3032 stall pulls more than a dev board can source.

  • The ST-3032’s brown-out floor is ~9V. A 3S pack sags to ~9.9V near empty, which still clears it; a 2S pack does not.

  • Tie all grounds together: battery, buck, URT-2, ESP32, sensor breakouts. This sounds obvious but it’s the #1 reason new builds misbehave.

GPIO map (ESP32-S3)

The serial-bus build needs very few pins — that’s most of its charm:

Function

GPIO(s)

Devices on this line

Analog sensors

1–10

The FULL ADC1 bank — the openbricks.drivers.qtr.QTRLineSensor window (wiring below). Buttons and UARTs deliberately live elsewhere so all ten stay analog-capable

I2C0 (SDA, SCL)

15, 16

TCA9548A mux (0x70) + colour sensors behind it, IMU (BNO055, 0x28), shared bus

UART1 (TX, RX)

14, 41

URT-2 serial bus — every ST-3032 / ST-3215 daisy-chained (RX was GPIO 6 until 1.71.0; it moved so the analog bank stays whole)

Program button

39

Start/stop, polled with an internal pull-up (was GPIO 4 until 1.71.0)

BLE-toggle button

38

Bluetooth on/off (was GPIO 5 until 1.66.3)

WS2812 data

21

Addressable RGB LED strip / ×8 stick DIN (openbricks.drivers.ws2812) — on boards that break out the header corner next to 5 V/GND, this is the free pin beside the reserved 19/20 USB pair

Pin gotchas on the ESP32-S3:

  • GPIO 22–25 don’t exist — the pin list is 0–21 then 26–48.

  • GPIO 26–32 (and 33–37 on octal-PSRAM modules) are flash/PSRAM. Do not use them.

  • GPIO 19/20 are the native-USB D-/D+ and 0/3/45/46 are strapping pins.

  • I2C and UART route through the GPIO matrix, so none of these assignments are fixed — they’re the convention the bundled examples use. On a classic ESP32, the usual equivalents are I2C on 21/22 and any two free pins for the UART.

The firmware enforces the hard cases at construction time: a driver asked to wire a nonexistent, flash, or USB pin — or a pin the runtime already owns, like the program button — raises openbricks.pins.ReservedPinError naming the pin, the role, and the reason, instead of failing somewhere far from the mistake. See openbricks.pins.

QTRLineSensor wiring (the standard line-follow window)

openbricks.drivers.qtr.QTRLineSensor bakes the whole rig geometry into the firmware — pins, element positions, and both mode setpoints — so programs just construct it and pick a discipline:

from openbricks.drivers.qtr import QTRLineSensor
qtr = QTRLineSensor()
qtr.set_mode("left")            # or "right"; switchable mid-run
error = qtr.read().edge_error()

Ten channels of a QTRX-HD-15A (4 mm pitch) in a skip pattern, left to right as mounted, onto GPIO 1..10 in order:

QTR channel

15

13

12

11

9

7

5

4

3

1

GPIO

1

2

3

4

5

6

7

8

9

10

x (mm)

−28

−20

−16

−12

−4

+4

+12

+16

+20

+28

That spans a 56 mm window at spacings 8/4/4/8/8/8/4/4/8 mm (the driver’s positions_mm carries the true coordinates, so edge interpolation is exact across the unequal gaps). The two modes:

  • "left" — holds the line’s LEFT edge under channel 12 (x = −16 mm)

  • "right" — holds the line’s RIGHT edge under channel 4 (x = +16 mm)

Either way the ~20 mm line sits inside the middle of the window with ≥3 channels of mat visible on the far side — those far-side elements are the branch watch in the bundled followers, and the whole window going dark is the intersection/ending signal.

One board-level note: GPIO 3 (= channel 12, the left-mode setpoint channel) is a strapping pin, and on the ESP32-S3-COREBOARD V1.4 it optionally carries a 10 kΩ pull-up through the USB-JTAG 0 Ω link. Harmless (per-element calibration absorbs the bias), but if qtr_calibrate.py shows element [2] with a conspicuously narrower span than its neighbours, that link is populated — desoldering it is safe if you never use pin-JTAG.

The QTRX board’s CTRL (emitter enable) can stay tied high; VCC to 3V3, GND to GND.

If the status LED never lights

The onboard WS2812 speaks a write-only protocol — the firmware can’t tell a dead LED from a working one, so a blown pixel looks like software silently failing. Run examples/led_probe.py on the hub: it drives full white at both timings across every plausible RGB pin (48/38/47/21) plus a raw machine.bitstream write. If nothing lights through all of that, the problem is physical. The decisive check is a power-off ohmmeter reading from the LED’s data pin (or its GPIO) to GND: a WS2812 whose DI input has failed as an internal short reads ~0 Ω there and clamps the GPIO — the pin measures millivolts even when driven high, every write “succeeds” into the short, and the symptom history is “worked once, then went dark for good”. Lift the data link to free the pin. Also check the schematic for population options in the LED circuit. The ESP32-S3-COREBOARD V1.4 (docs/datasheets/esp32s3_coreboard_v1.4_schematic.pdf) routes GPIO 48 to the WS2812 through a 0 Ω RGB link and feeds the LED’s VDD from the 5 V rail through a second optional link — if either link is unpopulated (or the board is powered via 3.3 V only, leaving the 5 V rail dead), the LED never lights while every write succeeds. Bridge the link / feed 5 V, or wire any external WS2812 to 3.3V / GND / GPIO 48 — it becomes the status LED with no firmware change.

Sensor wiring (I2C)

The baseline build has several colour sensors (a line-follower / zone-detection array) plus one IMU. The TCS34725’s address is fixed at 0x29 — no address-select pins — so two of them collide on a bare bus. The TCA9548A multiplexer solves this: it sits at 0x70 and fans the bus out to eight isolated channels, one colour sensor per channel. The IMU has its own address (0x28) and connects straight to the bus.

Wire the mux to the ESP32-S3, then each sensor to a mux channel:

TCA9548A pin

Connect to

Notes

VIN

3.3V

GND

GND

Common ground with everything else

SDA / SCL

GPIO 15 / 16

The main bus — shared with the BNO055

SD0/SC0 … SD7/SC7

one TCS34725 each

Isolated channels; every sensor sits at its own 0x29

TCS34725 / BNO055 pin

Connect to

Notes

VIN (or VCC)

3.3V

The breakouts have onboard regulators, but the board’s 3.3V is cleanest

GND

GND

SDA / SCL

mux channel SDn/SCn (colour sensors); GPIO 15/16 directly (IMU)

The Adafruit breakouts include ~10 kΩ SDA/SCL pull-ups, so for a handful of devices you don’t need to add your own. In code, mux[n] behaves like an I2C bus, so the sensor driver is constructed exactly as it would be on a bare bus:

from machine import I2C, Pin
from openbricks.drivers.tca9548a import TCA9548A
from openbricks.drivers.tcs34725 import TCS34725
from openbricks.drivers.bno055 import BNO055

i2c = I2C(0, sda=Pin(15), scl=Pin(16), freq=400_000)   # ESP32-S3
mux = TCA9548A(i2c)                                    # 0x70 by default
sensors = [TCS34725(mux[ch]) for ch in range(3)]       # left, mid, right
imu = BNO055(i2c)                                      # 0x28, straight on the bus

For a complete program — a 2-sensor array that combines each sensor’s ambient() and rgb() readings to name the colour under it (red / blue / green / yellow / white / black) — see examples/color_array.py.

Simplifications when you need fewer parts:

  • One colour sensor: skip the mux entirely; wire the TCS34725 and BNO055 both to GPIO 15/16 (0x29 and 0x28 coexist fine).

  • Exactly two colour sensors: the ESP32’s second hardware I2C controller also works (I2C(1, sda=..., scl=...) on any two free pins), one sensor per bus — no mux.

TCS34725 LED pin

The colour sensor breakout has two extra pins beyond power and I2C:

  • LED — drives the onboard white illumination LED. On Adafruit boards it defaults on (tied to VIN through the ADC-enable trace). To control it, wire it to a spare GPIO and drive high/low; to force it off, tie LED to GND. Leave it on for consistent colour readings — ambient light alone is unreliable across environments.

  • INT — the interrupt output. The driver polls, so leave INT unconnected.

Serial bus servo notes (ST-3032 / ST-3215)

  • Every servo needs a unique bus ID. Factory default is 1; re-ID one servo at a time with examples/st3215_reid.py (same protocol — it works for the ST-3032 too). The bundled drivebase examples assume left = 1, right = 2.

  • Speed limits. Per the STS3032 datasheet (docs/datasheets/), no-load top speed at 12V is 148 RPM = 888 °/s; under the rated 3.3 kg·cm load it drops to roughly ⅔ of that. The driver’s default max_dps=600 clamps requests at the loaded operating point — construct with an explicit max_dps=900 to chase the no-load number.

  • Mixed fleets: the ST-3215 tolerates lower voltages, but the ST-3032 browns out below ~9V. If you daisy-chain both on one URT-2, the rail must satisfy the strictest member: 12V.

  • Drivebase: ST3032Motor drops straight into DriveBase:

from openbricks.drivers.st3032 import ST3032Motor
from openbricks.robotics import DriveBase

left  = ST3032Motor(servo_id=1, uart_id=1, tx=14, rx=6)
right = ST3032Motor(servo_id=2, uart_id=1, tx=14, rx=6, invert=True)
db = DriveBase(left, right, wheel_diameter_mm=65, axle_track_mm=120)

Bench-test a fresh build with examples/st3032_drivebase_test.py.

Calibrating the drivebase

wheel_diameter_mm and axle_track_mm are the two physical parameters that matter for straight-line distance and turn accuracy. Measure them with calipers or a ruler (wheel contact patch to wheel contact patch for axle track, not hub to hub). If straight(1000) undershoots, your wheel diameter value is too large; if turn(360) overshoots, your axle track is too small.

High-torque 12V servos also deliver the default launch profile much more stiffly than small DC motors — if the chassis pitches or lifts its rear when a move starts, soften the ramp:

db.settings(acceleration=180)   # wheel-deg/s²; default 1500

Alternative: DC gear motors with encoders

The original starter build — still fully supported. (Both this and the serial-servo build run in the MuJoCo simulator via the driver shim.)

Part

Qty

Notes

JGB37-520 DC motor with encoder (1:30 gearing)

2

Pick the 12V version; runs fine off 7.4V 2S LiPo

L298N dual H-bridge module

1

Cheap and robust. TB6612FNG is a better choice if you can find it

2S LiPo (7.4V) + buck converter (→5V, ≥2A)

1 each

Don’t power the ESP32 from the L298N’s onboard 78M05 regulator — it’s good for ~300 mA and browns out the moment the motors draw current

Wiring topology:

    [ 2S LiPo 7.4V ]
        │
        ├─────────────►  L298N  Vmotor   (motor power)
        │
        └──► Buck  ──►  5V rail
                         │
                         ├──►  ESP32 VIN
                         ├──►  L298N  +5V (logic only)
                         └──►  Sensors via 3.3V regulator on ESP32

GPIO map (see examples/esp32_drivebase.py for the ESP32-S3 pin assignments; the classic-ESP32 equivalents are in git history):

Function

ESP32-S3 GPIO(s)

Devices on this line

Left motor dir

1, 2

L298N / TB6612 IN1, IN2

Left motor PWM

17

L298N / TB6612 ENA

Left encoder A, B

7, 8

JGB37-520 encoder channels

Right motor dir

9, 10

L298N / TB6612 IN3, IN4

Right motor PWM

11

L298N / TB6612 ENB

Right encoder A, B

12, 13

JGB37-520 encoder channels

The map deliberately leaves GPIO 39 and 38 free — those are the firmware’s default program button and BLE-toggle button (see openbricks.hub.ESP32S3DevkitHub). The launcher polls GPIO 39 as an input, so a motor driver toggling it would read as button presses and stop your program. GPIO 15/16 (I2C) and 14/41 (serial-bus UART) are also kept free so sensors and a serial-servo arm can join the same build unchanged.

Calibrating encoder counts

The default in jgb37_520.py is counts_per_output_rev=1320, which is 11 CPR × 30:1 × 4 (quadrature edges). If you have a different gearbox variant, recompute:

counts_per_output_rev = encoder_CPR × gear_ratio × 4

Or measure empirically: rotate the output shaft by hand exactly one full turn and read motor.angle(). Whatever it reports is what counts_per_output_rev should be, scaled so that one turn = 360°.