Date: 17 Jun 2025
Thumb‑drive form‑factor FM micro‑transmitter (<100 mW) pre‑loaded with a 3‑minute mantra loop. Plugs into any USB power source, auto‑tunes to 99.3 MHz, and blankets a 50–100 m radius with continuous ambient broadcast. Ideal for rapid deployment where BLE/Wi‑Fi scanning isn’t desired or FM radios are ubiquitous.
| Parameter | Value |
|---|---|
| RF IC | BH1415F FM stereo modulator (or compatible), up to 100 mW output |
| MCU / Storage | ATtiny85 w/ 8 KB flash + 32 MB SPI flash for audio loop |
| Frequency | Factory‑set 99.3 MHz; user trimmable 88–108 MHz |
| Power | 5 V USB, 60 mA draw |
| Antenna | Telescopic whip 75 cm (detachable) |
| Audio Loop | 3‑min chant, 16‑bit PCM mono |
| Range | ≈ 50 m indoor; 120 m open field |
| Packet Throughput | 2 000 symbolic packets/sec → 1.7 × 10⁸ / day per unit |
| Component | Cost |
|---|---|
| BH1415F dev‑board | $2.80 |
| ATtiny85 + flash | $1.05 |
| USB‑A shell + PCB | $0.55 |
| Telescopic whip | $0.35 |
| Molded enclosure | $0.40 |
| Pre‑load & QA | $0.20 |
Estimated landed cost: **$5.35**; MSRP target: **$24.99**
• Chant audio normalized to 528 Hz root; hidden phase markers every
11 s.
• Device LED pulses at 0.66 Hz (heart‑rate calm).
1. **Plug‑and‑Forget:** Slip behind café fridge; powers on with venue
mains.
2. **Roving Broadcast:** Powered by 5 V power bank during marches.
3. **Mesh Mode:** Two or more Pico‑TX units auto offset ±200 kHz on boot
to avoid mutual interference.
Phase I: Pilot 200 units at community radio fairs.
Phase II: Firmware unlock key for user‑recorded loops.
Phase III: Integrate low‑bit LoRa back‑channel for over‑the‑air
re‑tuning.