WALKIN PIR IF

Battery-Powered PIR Motion Interface (Isolated Dry Contact)

Overview

Battery-powered PIR interface designed to trigger motion-activated overhead lighting via an isolated dry input on an XWA11V industrial controller. Intended for installation in a walk-in cooler where running mains power to the sensor is undesirable.

The board detects motion using an AM312 PIR and signals the controller by closing a dry contact (DIN ↔ COM) through a PhotoMOS relay. It also provides a pass-through connection for an external NTC temperature probe, allowing temperature sensing to share the same cable run.

All lighting logic and power switching are handled by the controller; this board functions as a low-power, robust peripheral.

---

Design Summary

• Power: 3×AA battery (targeting multi-month to ~1 year life)
• Isolation: PhotoMOS relay provides a true dry contact
• Protection:
  • Reverse polarity protection on battery input
  • TVS diodes on all field wiring (DIN, COM, NTC probe)
• Environment: Cold (~35–40°F), ~6 ft cable runs
• Assembly: Hand-solderable THT + simple SMD parts

---

Design Rationale

• Battery power avoids mains wiring near the sensor
• PhotoMOS isolation prevents ground loops with industrial controls
• TVS protection improves robustness for long, exposed wiring
• Conservative, low-power design prioritizes reliability over complexity

---

Hardware Documentation

All design, assembly, and fabrication artifacts are published here:

➡️ WALKIN_PIR_IF Documentation (MkDocs)

---

Hardware Status

Rev B sent to fabrication ✅
Simplified prototype validated (video below).

• Rev A: PIR output sags under direct PhotoMOS LED drive → unreliable switching.
• Rev B: Added NPN low-side driver + LED current-limit resistor to isolate PIR output from load.

https://github.com/user-attachments/assets/d97296ae-1a20-4d2e-b4a4-3fdf33289da8