GitHub link: https://github.com/bigcrimping/bhangmeterV3

An open-source optical bhangmeter — a fast photodetector that recognises the unique two-peaked light curve of a nuclear detonation and estimates its yield from the timing of that signature. This repository holds the complete build: firmware, host software, PCB, and enclosure.

A bhangmeter estimates yield from the optical waveform alone. No other natural or artificial event reproduces the double-pulse signature, which makes the waveform both the detection trigger and the measurement.

The double-pulse signature

A high-yield fireball radiates ~35–45% of its energy as light, but not smoothly. Three phases produce two distinct optical peaks:

First pulse — radiative fireball. Short (~1 ms) and nearly independent of yield.
The minimum. A hydrodynamic shock detaches and ionises the air into an opaque shell, dropping visible emission about one order of magnitude.
Second pulse — shock breakaway. Lower peak, but lasting hundreds of ms to seconds, carrying ~99% of the integrated thermal yield.

The gap between the two peaks scales with yield — and that gap is what the instrument measures, using standard low-altitude Glasstone & Dolan scaling. The device reports a yield only when the timing lands in the valid window (t_inter 90–1050 ms, t_min 5–130 ms), i.e. the 10 kt – 1 Mt design envelope.

Yield	T_min (peak→min)	T_inter (peak→peak)
10 kt	7 ms	115 ms
50 kt	18 ms	233 ms
100 kt	26 ms	316 ms
250 kt	45 ms	473 ms
500 kt	67 ms	642 ms
1 Mt	100 ms	871 ms

How the instrument works

The comparator captures the sub-µs leading edge and triggers the ADC capture. Both t_min and t_inter are then measured from the ADC waveform. A detection requires all of: a fast leading edge, then a peak → shallow-minimum → second-peak shape, with t_inter falling in the valid band. That combination rejects the plausible impostors — lightning (single peak), camera flashes (no second peak), and meteors (one broad envelope).

Hardware at a glance

Block	Part	Why
Photodiode	VEMD2704 (Si PIN, ~400–1100 nm)	Captures the broadband thermal flash, left unfiltered to gather as much light as possible.
Logarithmic amp	AD8304	Log-compresses the photodiode current so one fixed-gain front-end spans several decades — no gain switching mid-event.
Edge timing	LT1711 comparator	Sub-µs timestamp of the leading edge regardless of amplitude.
Waveform capture	AD7091R ADC (SPI)	Records the analogue shape so peak/minimum/second-peak can be analysed.
Compute / comms	RP2350 (Pico 2 W)	Detection + control on core 0, WiFi/NTP/HTTPS upload on core 1.

Repository layout

code/ — firmware (RP2350 / Pico 2 W) and the companion app (a PySide6 desktop operator console)
PCB/ — the carrier board and the BHG-3000 sensor head: schematics, assembly drawings, BOMs and Gerbers
mech/ — 3D-printable enclosure and optical mount

Getting started

Build the hardware from PCB/ and mech/.
Flash the firmware — a prebuilt .uf2 is included, or build from source.
Configure on first boot over USB serial (WiFi + a GitHub repo to publish to). No credentials live in the firmware.
Drive and view the device with the companion app, or read the published JSON from your GitHub repo.

Status & scope

Work in progress. Yield estimation from timing is intrinsically coarse, so the sharp t_inter feature is the primary estimate and t_min only a coarse cross-check. The firmware’s TLS upload path does not yet verify the server certificate; use a dedicated, fine-grained GitHub token.