• Status: Draft schematic in progress
• Device: 12–24 VDC powered controller that energizes two relay channels while a GPS receiver reports a valid satellite fix.
• Relay behavior: Both relays turn off immediately when GPS fix is lost.

• Prototype / R&D board for switching two external 12 V or 24 V loads.
• External GPS antenna is required; current schematic uses an external GPS module/header approach so the GPS receiver/antenna assembly can be mounted off-board or at the enclosure edge.

• Accept 12–24 VDC input power.
• Regulate input power to 5 V for relay coils and controller supply.
• Read GPS fix status via UART/NMEA and/or a GPS FIX/PPS/status pin.
• Energize both relays only while GPS fix is valid.
• De-energize both relays immediately when fix becomes invalid, power is removed, or firmware stops driving the outputs.
• Switch external 12/24 V loads up to 3 A using isolated relay contacts.

• 12–24 VDC input with fuse and TVS protection.
• Raspberry Pi Pico / RP2040 controller.
• External UART GPS module header for VCC, GND, TX, RX, FIX, and PPS/status.
• Two SPDT relay outputs with COM/NO/NC screw terminals.
• Low-side MOSFET relay coil drivers with flyback diodes and default-off pulldowns.

Diagram

• Power input/protection: 12–24 VDC screw terminal, resettable fuse, TVS clamp, buck regulator.
• Controller: Raspberry Pi Pico module powered from 5 V/VSYS; firmware parses GPS validity and drives relay GPIOs.
• GPS interface: 6-pin 2.54 mm header for an external GPS receiver module with its own antenna connector/antenna cable.
• Relay outputs: Two SRD-05VDC-SL-C 5 V SPDT relays; contacts routed to 3-pin 5 mm screw terminals.
• Drivers: N-MOSFET low-side switches, gate resistors, gate pulldowns, and flyback diodes.

• Input range: 12–24 VDC.
• 5 V rail estimated peak: ~300 mA to 400 mA for Pico, GPS module, and two relay coils.
• External switched loads are isolated by relay contacts and are not powered by this board unless the user wires them through the relay terminals.

• Prototype-friendly through-hole modules/connectors are preferred where available.
• Relay load terminals and input terminal use 5 mm screw-terminal style parts rated well above 3 A.
• Final layout should separate relay/load copper from low-voltage logic and keep GPS signals away from relay coil switching paths.

• Firmware must set relay GPIOs low during boot.
• Relay outputs must default off due to MOSFET gate pulldowns.
• Firmware should parse GPS NMEA valid-fix status and optionally cross-check GPS_FIX/PPS.
• Immediate dropout behavior: de-energize relays on invalid fix, missing GPS messages, GPS serial timeout, or MCU reset.

• External GPS antenna/module should be mounted away from relay wiring and metal obstructions.
• Screw terminals should be placed at board edges for field wiring.
• Add test points for VIN, 5 V, 3V3, GND, relay drive signals, and GPS UART during layout if space allows.

• Use an RP2040/Pico controller instead of a Raspberry Pi SBC because this is simple, lower power, faster boot, and safer for relay fail-off behavior.
• Use 5 V relay coils from a buck regulator so the board works from either 12 V or 24 V input.
• Use firmware-controlled immediate dropout, with hardware pulldowns making relay-off the fail-safe state.

• The two relays should energize together from one GPS-valid condition.
• Load current is up to 3 A at 12 or 24 VDC per relay contact set.
• The external GPS receiver can provide UART NMEA data and preferably a FIX or PPS/status output.
• Exact GPS module may be finalized later; the current design exposes a compatible header.

• 2026-07-08: Captured requirements: 12–24 VDC input, GPS valid satellite fix trigger, 12/24 V 3 A loads, external GPS antenna, immediate relay dropout on fix loss.