Block Detector

Why

The off-the-shelf detectors on the layout are active low. The feedback nodes’ inputs are pulled up, so a broken cable or connection under the layout floats high and reads exactly like a clear block, the permissive state.

This board is active high: occupied drives 3.3V and clear drives 0V, so the same broken wire reads as occupied. The decision is recorded as build our own active-high block detector.

The board

• Two independent channels, each taking an external current transformer on the track feed.
• Each channel runs: CT → burden resistor → ESD clamp → precision peak detector → comparator with hysteresis → push-pull output.
• Channel 1’s burden is switchable with a jumper: 100Ω for standard track, or 1.1kΩ for the programming track’s service-mode acknowledgement pulse. Channel 2 is fixed at 100Ω.
• Thresholds are set per channel on a multi-turn trimmer, with test points for the envelope and the threshold.
• 48 × 38mm, 2 layers, single 3.3V supply fed through the output header.
• A 3D-printed DIN-rail enclosure (OpenSCAD) with slots for the CT leads and the output cable.

Status

Revision 1.0: 10 boards ordered from PCBWay, awaiting delivery. Nothing has been measured on hardware yet.

A known concern going into bench testing: the signal chain has no gain, so the comparator threshold sits in the millivolt region, a small fraction of the trimmer’s travel. A series resistor to narrow the trimmer’s range was suggested but isn’t in revision 1.0; sizing it needs the CT’s actual turns ratio.

Checks

ERC and DRC run in CI on every change, using the same KiCad release the board was designed in.