What the AQY221N3TW is and what it switches
The AQY221N3TW is a PhotoMOS solid-state relay (optomos) in the AQY2 series — Panasonic's signal-level SSR family — with a single SPST-NO (1 Form A) contact, meaning it makes a normally-open AC or DC switching path when the LED input is energised at 1.14 VDC. Maximum load current is 150 mA and the load voltage range spans 0-25 V, so this part sits squarely in the low-power signal domain — ideal for sensing lines, analog frontend switching, or routing reference voltages where a mechanical relay's contact bounce would be unacceptable.
Load ratings and on-state resistance — what 7.5 Ω means for signal integrity
On-state resistance is specified at a worst-case 7.5 Ω maximum. For a 150 mA signal path that is not a problem, but in a low-level analog switch role — thermocouple, strain gauge bridge, or source-measure reference — the voltage drop across the relay when carrying a few milliamps adds directly to the measurement error budget. A solid-state relay in this resistance class is the right fit for digital logic-level switching, not precision analog multiplexing. The 0-25 V load rating covers standard 5 V, 12 V, and 24 V logic rails directly. At 1.14 V input, the LED drive threshold is low enough for direct logic-fet drive from a 3.3 V microcontroller pin — no level-translation required.
Package and board-fit for repair or production
The 4-VSSOP (4-SMD, 0.071-inch / 1.80 mm body) package is a standard JEDEC footprint — widely supported by hot-air rework stations and fine-pitch SMT nozzles. The tape-and-reel packaging (TR) is the production-format variant, so reels sourced here are the same reel format used on the original build. No orientation key ambiguity: pin 1 is defined by the package geometry, and the tab silhouette on the silkscreen makes it readable under magnification before power is applied. Surface-mount termination with the SMT tab style means the part sits flat to the pad — reflow or hot-bar compatible without a socket. Because this is a solid-state relay with an internal LED, the part is polarity-sensitive on the input side: pin assignment matters and must match the drive circuit schematic before assembly.
Active sourcing posture — what the lifecycle entry means for your BOM
No official successor or cross-reference is entered in the ledger, so there is no manufacturer-named pin-compatible alternative on file. The AQY2 series is well-established in the Panasonic relay lineup, but confirming an alternative from the same family is a check worth doing against the full series parametric table before committing to a single-source BOM line.