• Remotely activate a low-voltage horn.
• Receiver is deployed once and remains armed for several weeks or months.
• Production intent prioritizes low unit cost and compact size over immediate response time.

• Remain in ultra-low-power standby.
• Periodically listen for a valid addressed 868 MHz command.
• Reject packets with invalid address or CRC.
• On a valid command, charge a pulse-storage capacitor, then switch the horn output for 10 ms.
• Return to standby after activation unless firmware is configured as one-shot.

• EU 868 MHz long-range packet receiver.
• Wire quarter-wave antenna connection, approximately 86 mm.
• Internal A23/23A 12 V alkaline battery.
• Duty-cycled receiver for multi-month standby.
• Switched capacitor charging to eliminate storage-capacitor leakage during standby.
• Low-side MOSFET horn driver.
• Two-pin horn output and programming/test pads.

Diagram

• RF: SX1262-class 868 MHz transceiver module operated in receive-duty-cycle mode. A module is preferred over a bare RF IC to reduce RF matching risk and PCB area devoted to matching.
• Control: Small low-power MCU with SPI, timer, and deep sleep, provisionally ATtiny202/402 class.
• Logic power: Nano-power 3.3 V regulator rated for a 12 V alkaline source, provisionally TLV70433 class.
• Pulse supply: 4700 uF to 6800 uF, 16 V low-ESR electrolytic charged only after a valid command.
• Output: Logic-level N-channel MOSFET rated at least 30 V, with gate resistor, pull-down, and inductive transient clamp appropriate to the horn.

• 868 MHz antenna pad or compact connector.
• Two-pin horn connector: switched 12 V pulse and return.
• Programming pads for MCU power, ground, reset/program data, and clock as required.

• Battery: A23/23A 12 V alkaline, nominal capacity approximately 50-55 mAh at light load.
• Target average standby current: no more than about 10-15 uA for roughly 3-5 months of practical service after derating.
• RF wake latency is traded for battery life: transmitter sends a long preamble while the receiver briefly samples the channel periodically.
• Pulse-capacitor charge adds approximately 2-5 seconds of command-to-horn latency.

• Two-layer PCB using low-cost SMD assembly.
• RF module used to avoid custom RF matching and simplify certification risk.
• Large electrolytic capacitor and battery holder dominate size.
• Minimize BOM; omit LEDs and user controls unless required.

• SPI connection to RF module.
• RF interrupt line to wake MCU.
• GPIO for capacitor charge enable.
• Timer-controlled 10 ms horn output.
• Fixed receiver address and packet CRC; repeated command handling must not unintentionally retrigger.
• Receive-duty-cycle interval and transmitter preamble length must be configured as a matched pair.

• Keep antenna region clear of copper and battery/capacitor metal.
• Place pulse loop—storage capacitor, horn connector, and MOSFET—tightly together with wide copper.
• Approximate minimum board envelope is driven by the selected RF module, A23 holder, and 16 V electrolytic capacitor.

• Use 868 MHz LoRa/sub-GHz packet modulation rather than simple OOK for reliable 1 km line-of-sight operation.
• Use duty-cycled reception to achieve months of standby.
• Use an A23 battery because it directly supplies the 12 V charging rail and is inexpensive and compact.
• Keep the large pulse capacitor disconnected while idle so its leakage does not consume the small A23 battery.

• The horn tolerates a pulse that begins near 12 V and droops during the 10 ms interval.
• Several seconds of activation latency is acceptable.
• The transmitter can send a sufficiently long preamble and compatible addressed command.
• This project covers the receiver only.
• A 10 ms pulse may produce only a click on many conventional horns; acoustic performance must be tested with the actual horn.

• Initial requirements captured: 868 MHz EU/France, about 1 km line of sight, internal disposable battery, several weeks/months armed time, and nominal 12 V/1 A/10 ms horn pulse.