16-SIM polling
Shared TX, multiplexed RX: one ESP32 UART serves sixteen SIM800L modems in turn.
Open-source ESP32 firmware
Sixteen SIMs.
One tiny board.
An SMS gateway you can hold in your hand. It polls up to 16 SIM800L modems through a single multiplexer, forwards every message to your backend, and updates itself over the air.
- SIMS
- 16
- UARTS
- 1
- OTA
- HTTPS
ESP32 · v1.0.21MUX CD74HC4067
SIM01GLOBE
SIM02SMART
SIM03DITO
SIM04TNT
SIM05GLOBE
SIM06SMART
SIM07DITO
SIM08TNT
SIM09GLOBE
SIM10SMART
SIM11DITO
SIM12TNT
SIM13GLOBE
SIM14SMART
SIM15DITO
SIM16TNT
→ FORWARDED TO BACKEND
waiting for SMS…
// 01 · CAPABILITIES
Built like infrastructure,
sized like a hobby board.
Backend forwarding
Every incoming SMS is parsed and POSTed to your API with SIM, sender and timestamp.
Local web UI
Configure Wi-Fi, monitor SIMs, read logs and run tests straight from the device.
Dual Wi-Fi
Setup access point and backend station mode running at the same time.
HTTPS OTA
Pull new firmware from GitHub Releases or any URL; no cable, no downtime.
Missed-call OTPs
Turns missed-call verifications into OTP messages your backend understands.
USSD balance
Bulk *143# balance checks across every SIM with one tap.
SIM watchdog
Detects unresponsive modems and recovers them automatically.
Persistent logs
Message and error logs survive reboots on LittleFS.
// 02 · SIGNAL PATH
From the tower to your API.
01GSM network
02SIM800L × 16
03CD74HC4067 mux
04ESP32 parser
05Your backend
// 03 · HARDWARE
Spec sheet.
| Controller | ESP32 Dev Module |
|---|---|
| Modems | 16 × SIM800L GSM |
| Multiplexer | CD74HC4067, 16-channel |
| Power | 5 V · 10 A+ (2 A peak per modem) |
| Storage | LittleFS (message + error logs) |
| Updates | HTTPS OTA (GitHub Releases / custom URL) |
| Sync | Heartbeat + full inventory every 30 min |
// PINOUT
Wiring.
| GPIO5 | TX → all SIM800L RX (shared) |
|---|---|
| GPIO4 | RX ← mux COM |
| S0–S3 | Mux channel select |
| EN | Mux enable (active low) |
⚠ Power matters: SIM800L modules spike to 2 A each. Use a 5 V / 10 A+ supply with good decoupling.