BD-Width Filament Sensor, a Technical Buyer's Guide

The BD-Width pairs a linear CCD image sensor with a laser optical tracking chip of the kind used in optical mice, and wraps both in an STM32 microcontroller that exposes a USB CDC serial port over a CH340 interface and a software-bit-banged I2C bus on two general-purpose pins. The case is a printable 3D model, bore diameter 4 mm, published alongside a schematic PDF and STL and STEP files, although no KiCad source, no bill of materials and no LICENSE file are present in the repository. Firmware is released as dated hex files, with visible releases dated 2025-07-08, 2025-09-03, 2025-11-06, 2026-01-18, 2026-02-21 and 2026-03-13; there are no Git tags and no changelog.[26]

The first load-bearing design choice is the CCD linear array with a light-diffraction compensation algorithm. The author describes it as a unique algorithm that uses light diffraction to automatically compensate for filament shadows on the CCD sensor, even when the filament moves at different distances and angles. In practice this means that the pixel-level shadow thrown by the filament onto the CCD line is not simply thresholded; the algorithm reconstructs the implied edge location after the diffraction envelope, which is what permits a 0.005 mm pixel pitch to translate into a meaningful reading on a 1.75 mm target.[26]

The second load-bearing design choice is the FIFO delay buffer on the host side. Because the sensor measures the filament where it enters the case, and the extruder actually melts the filament several hundreds of millimetres downstream, any diameter reading has to wait for the measured piece of filament to reach the hot end before its value is applied to the flow. The BD-Width driver implements this as a length-indexed FIFO keyed on the Klipper parameter sensor_to_nozzle_length, default 750 mm, and it also exposes a runout_delay_length of 8 mm and a flowrate_adjust_length of 5 mm so that the compensation fires at a finer granularity than a full FIFO flush. This mirrors the architecture that Klipper's upstream hall_filament_width_sensor uses with its measurement_delay field, and that Marlin exposes under MEASUREMENT_DELAY_CM, documented at 14 cm by default in Configuration_adv.h.[26][38][42]

Klipper converts width readings into a flow multiplier via an inverse-square area formula, percentage = round(nominal_filament_dia squared divided by filament_width squared times 100), which it then injects as an M221 S command. Readings are exponentially smoothed with the running update d = (5 times previous_d plus new_d) divided by 6, and fall back to M221 S100 whenever the reading exits the nominal plus or minus max_difference band. ADC samples are taken at roughly 0.5 second intervals, fifteen samples per report.[44]

Within the set listed above, and on the vendor-documentation evidence captured on 2026-04-19, the BD-Width is the only unit in the comparison whose vendor documentation states it measures both filament diameter in millimetres and filament motion in millimetres per second in the same device. The Bambu Lab AMS does not publish a diameter-measurement claim, the Duet3D Rotating Magnet monitor senses motion only, and the Prusa, Creality, LDO, Stealthburner and Orbiter devices are runout or presence detectors. Those are different problem scopes, and each has a legitimate use case; the table is a scope map, not a ranking.[26][60][40][41][61][62][63][59]

Four limitations should be stated plainly before any purchase. First, the BD-Width cannot read the width of fully transparent filaments; motion and runout detection continue to work, but flow compensation is disabled for those materials, per the author's product page. Second, the sensor reports a projected width, not a cross-sectional shape; an oval filament of the same projected width reads the same as a perfectly circular one, a point flagged by Tindie Blog in their 2025 coverage. Third, no independent third-party test of the published width accuracy has been located as of 2026-04-19; all numeric accuracy figures are vendor-stated and the author himself publishes two different values, plus or minus 0.015 mm on the GitHub README and plus or minus 0.01 mm on the Pandapi3D product page.[43][47][26]

Fourth, the software stack is bound to a single author and a single host firmware. The Klipper integration is not merged upstream, the repository has no LICENSE file and therefore defaults to all-rights-reserved under Berne Convention rules, there is no CHANGELOG and no Git tags. Firmware releases ship as dated hex files only, and the only supported update path is STM32CubeProgrammer over UART. Buyers who rely on long-term code availability, auditable release notes, or permissive licensing should weigh these points honestly against the sensor's hardware advantages.[26]

MABS 3D is a Brescia-based 3D printing service and reseller. We import the BD-Width and make it available in our FDM shop at EUR 39, verified on 2026-04-19, with EU-side stocking that removes the 8 to 15 day direct-from-China shipping window. We will re-verify every comparative claim in this article on a quarterly cadence, with the next scheduled review on 2026-07-19, and will update the competitive landscape table as competitor documentation changes.