# Connector pinout and operating concept

Viewed in KiCad's top-side PCB orientation, each square pad is pin 1. The silkscreen spells out each contact function and J1 polarity; verify physical connector keying before any build.

| Connector | Pin 1 | Pin 2 | Connection |
| --- | --- | --- | --- |
| J1, 5 V input | `V5`, +5 V DC | `GND`, 0 V return | Regulated, isolated, current-limited 5 V DC source only. Reverse polarity is not protected. |
| J2, contact A | `A_SEND`, from R2 | `A_RETURN`, to D2 anode | Connect only an isolated, normally-open dry contact between these pins. |
| J3, contact B | `B_SEND`, from R3 | `B_RETURN`, to D3 anode | Connect only an isolated, normally-open dry contact between these pins. |

R1 and D1 form a power-on branch from `V5` to `GND`. R2 feeds J2.1; closing contact A bridges J2.1 to J2.2 and drives D2 to ground. R3/J3/D3 repeat this for contact B. An open contact leaves its indicator off. There is no active contact input, logic output, debounce, short-circuit protection, reverse-polarity protection, or isolation between the board's branches.

Each resistor is 1 kΩ. A 5 V source and an assumed 2 V red-LED drop give approximately `(5 − 2) / 1000 = 3 mA` in an illuminated branch. The LED datasheet specifies its forward-voltage range at 20 mA, so 2 V at 3 mA is a **calculation assumption**, not a measured value or guaranteed bin. Even if the LED were replaced by a short, the resistor bounds one branch to 5 mA and dissipates `5² / 1000 = 25 mW`; three closed branches then draw at most 15 mA under ideal 5 V conditions. These bounds do not cover supply transients or wiring faults outside the intended isolated dry-contact loop.