Ultra-Cheap Speed-Triggered Speaker

I’m building a compact, medium-speaker module that runs on 12–24 V DC and plays short voice announcements whenever speed thresholds are crossed or at preset time intervals. The entire bill of materials must stay below US $8 in production quantities, so every component choice matters.

Here’s the concept in a nutshell: a simple motion sensor—3-axis accelerometer, GPS pulse, or any equally economical option—feeds a low-cost MCU or voice-playback SoC. When the sensed speed or elapsed time matches stored values, the board drives a small amplifier and pushes a clear, intelligible voice prompt through the speaker. No fancy music, just clean announcements.

I’m flexible on the exact parts; you can mix-and-match accelerometer vs. GPS, SPI flash vs. SD card, or ESP32 vs. STM32—whatever lets us hit the sub-$8 BOM while keeping the circuitry stable on 12–24 V vehicle power.

Deliverables
• Schematic and PCB (Gerber + fabrication notes)
• Itemized BOM with vendor links and pricing evidence for the <$8 target
• Firmware source, binaries, and simple config tool or instructions to swap audio files
• Quick start guide covering wiring, thresholds, and update procedure
• One revision round after initial review

Acceptance criteria: a bench prototype that powers from 12 V, detects speed/time reliably, and plays at least two distinct voice prompts through a medium speaker with no audible distortion.

In your proposal, let me know which sensor family you’d pick and how you’ll keep the BOM on budget.