Anti-Theft Immobilization Circuit

The factory ignition system on the Datsun 280ZX offers almost no resistance to hot-wiring, especially when the ignition cylinder is worn. To solve this elegantly, I designed a hidden anti-theft circuit that disables the fuel pump unless a concealed switch is toggled at startup.

The circuit uses a self-latching automotive relay and logic-level output from a compact PCB I designed and assembled in Altium. It’s built around a fail-safe “no-voltage return” system: the moment power is cut—whether due to tampering or shutdown—the fuel pump is disabled and can’t be reactivated without the correct rearming sequence. The installation is clean, reversible, and completely hidden, requiring no visible switches or manual steps once the car is started. Unlike cheap mechanical cutoffs, this design integrates invisibly into the existing harness and operates with near-zero current draw when idle. It provides meaningful security for classic cars in a way that’s subtle, modern, and designed from first principles rather than off-the-shelf hacks.