PIMD — Pulse Induction Metal Detector

A from-scratch, multi-band pulse-induction metal detector built for autonomous, position-tagged ground survey.

Above: a single live frame with a steel spanner and a copper pipe under the coil. The two materials separate spatially inside the decay-space matrix — non-ferrous (blue) collapses early, ferrous (red) persists late. Getting that separation out of a pulse-induction detector is the whole point.

Up to date Build diary: https://makies.com.au/pulse-induction-metal-detector/

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What it is

A pulse-induction (PI) metal detector designed and built from scratch since November 2023 by a maker with no prior analogue-electronics background. It is not a clone — several design choices are deliberately unconventional.

In the language of the field, it's a low-cost, monostatic time-domain electromagnetic (TEM) spectrometer doing cued interrogation — though it started life as nothing more ambitious than "let's build a metal detector with no experience." It is one payload of a larger system: the coil is towed on a trailer behind Roverling, an autonomous RTK-GPS ground robot, so every detection event can be tagged with centimetre-level position and streamed back over LoRa. It therefore has to be quiet, stable and remotely controllable, not merely sensitive.

What makes it unusual among PI detectors is that it doesn't optimise a single operating point. It cycles through a user-defined grid of operating points and reports, for every frame, a two-dimensional decay-space matrix — frequency/pulse-width on one axis, decay-amplitude (time) on the other — that is designed from the ground up as an input to a classifier.

How it works (in brief)

A high-current pulse energises the TX coil; on hard cutoff the collapsing field produces a flyback and then a decaying eddy-current response. A nearby target perturbs that decay. The conditioned RX signal is sampled at precise, calibrated delays.

• Mode 1 — a single filtered (32-bit) sample at one held delay: the low-noise telemetry path, used for baselines and field tests.
• Mode 2 — an interleaved sweep across a profile of (frequency, pulse-width, sample-delay) points, reported as one frame per sweep. The current locked profile (cal_63_air_v1) spans 7 pulse widths × 9 sample delays = a 63-cell matrix. Pulse widths are spaced geometrically (9 → 100 µs, ×1.5 per step) so each band interrogates a distinct slice of target time-constant space; repetition rates are chosen to hold duty near 30%. (An eighth 6 µs band was dropped: it carried no target information not already present in the other bands and was notoriously noisy.)
• Sample delays are amplitude-calibrated — anchored to a top-dense ladder of voltages (4.9 → 0.5 V) on the region of the decay where this coil carries the most discrimination information — and snapped to the 8 ns PWM grid, deliberately avoiding the LC-ringing dead zones and the measured ~4.45–4.65 V noise keep-out zone.
• Polarity convention: ferrous targets read positive (stored magnetic energy reinforces the decay); non-ferrous read negative (opposing eddy currents weaken it). A third class turns out to be common in practice: crossover targets (cast iron, some stainless, real jewelry with steel fittings) read negative at short pulses and positive at long ones — the pulse-width axis resolves what a single operating point would misclassify.

Highlights

• 63-cell decay-space matrix recovering multi-time-constant discrimination from a PI platform — something most commercial PI detectors don't attempt.
• Validated on a 17-target labelled corpus (July 2026): signature shape is invariant with target distance (5–15 cm, 30× amplitude range) and repeats across sessions and recalibrations to within a few percent — the property the classification layer is built on.
• Targets fall into three shape families — ferrous, non-ferrous, and crossover — and overlapping targets combine linearly (a spanner + copper pipe frame decomposes back into its parts at 0.99 correlation), so unmixing is on the table, not just classification.
• Phase-locked TX/sample timing on one RP2040 PWM slice — ~5 ns sample-trigger jitter (measured).
• LTC2508-32 dual-output front end (filtered 32-bit + raw 14-bit no-latency).
• A dedicated delay-calibration tool that measures the clip-release point per band and exports calibrated profiles.
• Three PyQt6 desktop tools: live Mode 1 GUI, the Mode 2 heatmap + ML-data bridge (ClassViz, including a self-describing session recorder for training data), and the delay calibrator.
• Built for autonomous use: RTK-GPS position tagging, LoRa telemetry, single dedicated low-noise supply.

Repository layout

README.md                  This file — start here
DESIGN.md                  Full engineering reference (hardware, firmware, protocol, test log)
CHANGELOG.md               Project change log
CLAUDE.md                  Contributor / agent conventions (version bumps, changelog discipline)

USAGE.md                   Per-app usage guide — intent, operation and pipeline flow

mcu/                       MicroPython firmware for the RP2040-Zero
  pimd_mcu.py  main.py

src/                       PC-side tools (PyQt6)
  pimd_gui.py              Mode 1 filtered-telemetry GUI
  pimd_classviz.py         Mode 2 decay-space heatmap + ML logger
  pimd_delaycal.py         Delay-calibration sweep tool
  pimd111.ui  pimd111_ui.py
  requirements.txt
  data/                    Settings, captured CSVs and calibrated profiles (runtime-generated)

Electronics/PIMD604/       KiCad project — schematic rev 6.04 (as-built)

References/                Schematic export, scope captures, sample heatmaps

Quick start

No build step for either the PC tools or the firmware.

PC tools

python3 -m venv .venv && source .venv/bin/activate
pip install -r src/requirements.txt
cd src
python pimd_gui.py        # Mode 1 GUI (filtered telemetry)
python pimd_classviz.py   # Mode 2 decay-space visualiser + ML bridge
python pimd_delaycal.py   # delay-calibration sweep

The tools connect to the board on /dev/ttyACMx at 115200 baud.

Firmware

# No build step — copy onto the board, then power-cycle it.
.venv/bin/mpremote connect /dev/ttyACM0 fs cp mcu/pimd_mcu.py :pimd_mcu.py + fs cp mcu/main.py :main.py
# (mpremote reset does not re-enumerate USB reliably — power-cycle the board.)
.venv/bin/mpremote connect /dev/ttyACM0 repl

Bench-test over a serial terminal (115200)

V                              identify / firmware version
L                              list profiles
Q5   then  G                   Mode 2: stream the loaded operating profile (W5 records);  E to stop
*5000,40000,8400,256  then S   Mode 1: stream (* records, ~20/s);  E to stop
A32                            one 32-sample boxcar average (R record)

Protocol note (firmware v4.23+): the * command takes freq in Hz, pulse/delay in ns; the D dynamic-profile command takes µs. See DESIGN.md §9 for the full contract.

Hardware

• Schematic: the published KiCad project is rev 6.04, which represents the board as-built. The PCB itself was fabricated from rev 6.01; 6.04 captures the corrections — there were a few PCB issues that needed rectifying after manufacture.
• Front end / drive: functional but a known rework target — a future revision will replace the TX switch and gate driver (planned: STP10NM60N + TC4420/TC4429) to retire the current FET being run past its rated SOA.
• Thermal: a thermistor is now fitted to the TX damping resistor. Thermal drift settles to effectively zero once the resistor reaches ~80 °C, which is the dominant warm-up transient.

Roadmap

This release is Phase 2 — publish the work to date. Active and planned work:

• Phase 3 — machine learning. Use the 63-cell decay-space matrix for filtering, classification and (the stretch goal) ground discrimination. First milestone reached (July 2026): a locked, delay-calibrated operating profile and a 17-target labelled signature corpus recorded at three distances, with distance-invariance, cross-session repeatability and mixture linearity all verified in air.
• Faster response. Investigate reducing cell count and per-cell averaging for quicker frames — the corpus so far says different targets go redundant in different cells, so nothing is cut yet.
• Front-end revision (TX switch + gate driver, as above), plus a scope measurement of TX coil current vs pulse width to settle whether the longest band is past the coil-current plateau.

See DESIGN.md §14 for the full open-problems list.

Documentation

• DESIGN.md — the complete engineering reference: measured operating envelope, coils, drive and receive chains, timing, serial protocol, power, invariants and curated test log. Self-contained; a new reader (human or agent) can pick up the project cold from it.
• USAGE.md — per-app usage guide: intent, operation and pipeline flow for the firmware and each PC tool.
• Build diary — the full chronological story, with photos and dead-ends: https://makies.com.au/pulse-induction-metal-detector/
• Video — https://www.youtube.com/@markmakies

Licence

This project uses two licences, matched to the kind of work:

• Code (src/, mcu/) — GNU GPL v3.0-or-later. See LICENSE.
• Hardware and documentation (Electronics/, USAGE.md, DESIGN.md, schematics, images) — Creative Commons Attribution-ShareAlike 4.0 (CC BY-SA 4.0). See LICENSE-docs.

Pulse Induction Metal Detector © 2022–2026 Mark Makies.

Author

Built by Mark Makies (Australia). Find more work at https://makies.au/.