Opto-Isolated Tach/RPM Input Schematic (4N25) - 5V & 12V Mods, Subaru COP Swap

This document is an annotated electronic schematic showing an RPM/tachometer input circuit built around a 4N25 opto-isolator (U3). The signal path runs from a TachIn input through a TachSelect jumper and an OptoIn jumper, then through two 1N4001 series diodes (D1 labeled "Wing" and D2 labeled "John"), through resistor R12 to the LED side of the 4N25 opto-isolator. The output side of the opto is pulled up to VCC via a 4.7K resistor (R13), filtered by capacitor C11 (0.01), and routed through OptoOut and Tsel jumpers to an IRQ-1 output. Other jumpers shown include XG1, XG2 (opto-return jumper, Note 4), VrOut, and VrOutInv. Hand-drawn annotations (black lines, green wire, X marks, and a "Removed C30" note) indicate modifications made to the board. A notes table gives two configuration options: for a 5V trigger signal, replace R12 with a 470R resistor, remove C30, and jumper 5V from the proto area to the top hole of C30; for a 12V trigger signal, replace R12 with a 1K resistor, remove C30, and jumper from s12c to the top hole of C30. This appears to be a Megasquirt-style ECU tach input modification guide, filed with a Subaru coil-on-plug conversion document set.