FAQ
TL;DR: DIY Bluetooth interfaces for STAG 4/QBOX cost ≈ PLN 20 instead of the PLN 80 OEM unit; "I updated the driver without any problems" [Elektroda, weryfany, post #19361578] Build uses an HC-05 at 9600 or 57600 baud and a 5 V regulator.
Why it matters: Saves money, removes dangling cables, and lets you tune LPG maps from a phone.
Quick Facts
• OEM Bluetooth dongle price: ~PLN 80 [Elektroda, weryfany, post #17741134]
• DIY parts cost: approx. PLN 15–25 (HC-05, 7805/step-down, plug, case) [Elektroda, weryfany, post #17741134]
• STAG 4 baud rate: 9600 8N1 [Elektroda, weryfany, post #17741134]
• QBOX / GoFast baud rate: 57600 8N1 [Elektroda, MACIEK_M, post #17886237]
• Recommended supply range: 7–16 V car rail → 5 V regulator (≤ 50 mA) [Elektroda, weryfany, post #17747760]
Which gas ECU models work with a re-programmed HC-05?
Confirmed: STAG 4, STAG QBOX, Go Fast and CCY BT Basic QBOX when set to the correct baud rate [Elektroda, weryfany, #17743387; #19360967]. Others like Zenit may need different commands and were not fully tested [Elektroda, weryfany, post #17743387]
What AT commands do I need for a STAG 4 interface?
- Enter AT mode (key pin high).
- Send AT+UART=9600,0,0
- Send AT+PSWD=1234
- Send AT+NAME=AC STAG
This keeps the default 8N1 framing and preserves ECU compatibility [Elektroda, weryfany, post #17741134]
How do I switch the dongle between STAG 4 and QBOX?
Install a reed switch on the HC-05 KEY pin. Bringing a magnet closes the switch, letting you re-flash the baud setting to 57600 for QBOX, then remove the magnet to return to normal mode [Elektroda, weryfany, post #17741542]
Why does my connection drop after a few seconds?
Voltage sag or noise on the 5 V rail often causes resets. Replace 78L05 with a buck step-down rated ≥ 500 mA and add 100 µF filtering; users report stable links after the swap [Elektroda, MACIEK_M, post #18468145]
Do I need level shifting between ECU (5 V) and HC-05 (3.3 V)?
Yes. Use a simple 1 kΩ–2 kΩ divider on ECU TX → HC-05 RX, and connect HC-05 TX directly; it has 3.3 V-tolerant inputs. Over-voltage killed one user’s ISA2 RX/TX lines [Elektroda, zbyz, post #17747638]
Is opto-isolation required?
No. Bluetooth already provides galvanic isolation; extra optos add cost and latency. Keep good wiring and a regulator; "there is no point in doing optoisolation" [Elektroda, neo_84, post #18469846]
Can I safely flash ECU firmware over Bluetooth?
Yes, several users updated STAG drivers wirelessly without issues [Elektroda, weryfany, post #19361578] Ensure phone or laptop battery is ≥ 50 % to avoid dropouts; a failed flash can brick the ECU (edge case).
How many connections can the ECU log before locking?
Some non-STAG units count connections and lock when the counter is full, but STAG controllers do not implement this protection [Elektroda, jawa1993, #17741929; weryfany, #17742161].
What’s the current draw of the finished dongle?
HC-05 draws ~30 mA when connected and peaks at 50 mA during discovery [HC-05 datasheet]. A linear 7805 wastes ≈ 0.3 W at 14.4 V, hence the recommendation for a 90 % efficient buck converter [Elektroda, weryfany, post #17747760]
Where can I find a suitable enclosure?
Any 4-button gate-remote case (45 × 35 × 15 mm) fits the PCB and costs < PLN 2 at local shops [Elektroda, weryfany, post #19361578] Tape or heat-shrink are quick alternatives [Elektroda, Folxs, post #19362365]
How do I check the required baud rate if I only have a wired cable?
Connect the OEM USB interface, open STAG Gas Synchro, and note the COM port speed shown in the status bar. Use the same rate when programming HC-05 via AT commands [Elektroda, MACIEK_M, post #17770975]
Quick 3-step build recap?
- Wire ECU TX/RX/GND to HC-05 through a voltage divider and 5 V regulator.
- Enter AT mode, set baud (9600 or 57600) and name/pin.
- Enclose in a small remote-key housing and pair with phone/laptop. Build time: under 30 minutes [Elektroda, thread synthesis].
