BHG-2000 — The Sensor Inside

Bhangmeter V2 was designed around a single chip: the HSN-1000L Nuclear Event Detector. It’s a lovely part — and it’s no longer made. The BHG-2000 fixes that. It’s an open-source, drop-in replacement you can build yourself, so V2 doesn’t depend on a component you can’t buy any more.

What it does

The job is simple to state and hard to do: notice the prompt gamma flash of a nuclear detonation, and fire a single clean pulse when it happens. The BHG-2000 is pin-compatible with the old HSN-1000L, so as far as the rest of Bhangmeter V2 is concerned, nothing has changed.

How it works

Instead of one specialised chip, the BHG-2000 does it with ordinary silicon photodiodes and some careful analogue design:

• Four BPW34S PIN photodiodes, reverse-biased, act as the gamma sensor. A gamma ray passing through the silicon knocks loose a tiny burst of charge (via Compton scattering) — a signal far too small to use directly.
• A transimpedance amplifier turns that tiny current into a voltage, and a comparator stage cleans it up into a crisp digital edge.
• The output is an active-low pulse — exactly what the HSN-1000L produced, so Bhangmeter V2 reads it without any changes.
• A built-in self-test flashes an LED at the diodes to prove the whole chain is alive, without needing an actual nuclear explosion to check.

It’s built on a compact 4-layer PCB, with schematics, gerbers, assembly drawings and a 3D-printable cover all published.

Status

The circuit works and detects, but it hasn’t yet been calibrated against a known gamma source (such as Cs-137 or Co-60) — so for now it’s a proven detector rather than a precisely graduated instrument. That calibration is the next step.

Build one yourself

Everything is open-source and on GitHub: BHG-2000 Nuclear Event Detector.