STEMMA (classic) ↔ STEMMA QT compatibility
Connecting STEMMA (classic) and STEMMA QT — and the reverse, STEMMA QT to STEMMA (classic). Here's the verdict, the exact part, and the voltage catch.
STEMMA (classic) and STEMMA QT use different connectors (JST-PH vs JST-SH) with no common off-the-shelf cable — bridge them with an adapter, or route both through the JST-SH (Qwiic/STEMMA QT) ecosystem.
Side by side
| Spec | STEMMA (classic) | STEMMA QT |
|---|---|---|
| Connector | JST-PH | JST-SH |
| Pitch | 2 mm | 1 mm |
| Logic | 3v3-5v | 3v3-5v |
Why this verdict
What actually decides STEMMA (classic) ↔ STEMMA QT
The connector. STEMMA (classic) uses a JST-PH at 2 mm; STEMMA QT uses a JST-SH at 1 mm. Different housings and different pitch mean no cable mates them directly — an adapter is doing real work, not just convenience.
The pin order. STEMMA (classic) runs SCL → SDA → V+ → GND while STEMMA QT runs GND → V+ → SDA → SCL. A plug that fits mechanically but disagrees on order is the worst case: it looks right and the bus stays silent, or worse, power meets a data line. Use an adapter made for this pair — see the pinout visualizer to see the two side by side.
The voltage. STEMMA (classic) is 3.3–5 V tolerant; STEMMA QT is 3.3–5 V tolerant. i2c has no negotiation — whatever the host drives lands on every device. If one side is 3.3 V-only and your host is 5 V, a level shifter isn't optional, it's the difference between a working bus and a dead sensor. The compatibility checker surfaces this for any combination.
Bridged, it's still one ordinary i2c bus: unique addresses per device (conflicts have fixes), pull-ups in parallel, and a 400 pF capacitance budget. Nothing answering after you connect? Run an i2c scan — it separates a cable problem from an address problem in seconds.
Questions
STEMMA (classic) & STEMMA QT FAQ
Can you connect STEMMA (classic) to STEMMA QT directly?
Not directly — STEMMA (classic) (JST-PH) and STEMMA QT (JST-SH) use different connectors, so you need an adapter. STEMMA (classic) and STEMMA QT use different connectors (JST-PH vs JST-SH) with no common off-the-shelf cable — bridge them with an adapter, or route both through the JST-SH (Qwiic/STEMMA QT) ecosystem.
Do I need a level shifter between STEMMA (classic) and STEMMA QT?
Only if the logic voltages differ — typically a 5 V host with a 3.3 V-only device. STEMMA (classic) is 3v3-5v; STEMMA QT is 3v3-5v. If both ends run at 3.3 V, no level shifter is needed.
Is the STEMMA (classic) pinout the same as STEMMA QT?
No. STEMMA (classic) is wired SCL / SDA / V+ / GND and STEMMA QT is GND / V+ / SDA / SCL. That difference is exactly why you can't just force a plug that seems to fit — an adapter built for this pairing re-orders the conductors for you.
Can I chain STEMMA (classic) and STEMMA QT devices on one bus?
Physically yes, once the connectors are bridged — electrically it's still one i2c bus, so the usual rules decide it. Every device needs a unique address, all the pull-up resistors sit in parallel, and total cable capacitance has a 400 pF ceiling. Mixing ecosystems doesn't change the bus; it just changes the plug.
Related pairings
Other STEMMA (classic) and STEMMA QT combinations
Every pairing is pre-answered — here are the rest involving either of these two. The verdict on each tile is the cautious one, since it assumes nothing about your host's voltage.
Or start from a standard: STEMMA (classic) and STEMMA QT each list their pinout and every ecosystem they reach. The full pairing index has all 15, and the compatibility checker does it interactively.