I'm using a Nuki Opener in a building equipped with a video intercom/doorbell from BTicino. The Nuki Opener is useful for opening the building entrance door with a mobile phone. It connects to the two-wire bus (SCS) and sends and receives the necessary digital messages to trigger the door opener.
Nuki has constructed the Opener such that it does not draw power from the bus, ostensibly to avoid “power theft” in tenant situations – although the amounts are laughable. Instead, it requires four AAA batteries, and at least in my setup, they only last a few months. Tired of swapping/recharging batteries all the time, and wanting to avoid running separate power supply wiring to the place where the video intercom and the Nuki Opener are installed, I tried to find a solution for the Nuki Opener to get its power directly from the bus.
The SCS bus runs on nominal 27 VDC. Detailed technical specifications don't seem to be officially available, but this thesis has some interesting information (page 11 ff.). Data is transmitted at 9600 bps, zeroes are signaled by pulling down the supply voltage for 34 µs, which is done by connecting 100 Ω + series 10 µH across the bus lines (see here); this is aided by the presence of an inductor in the bus power supply. There is also a baseband audio channel, an FM modulated stereo audio channel (5.5/6 MHz) and an AM modulated video channel (38.9 MHz).
Extracting power from the bus for the Nuki Opener requires one to:
My first idea was to use a big inductor to provide a high impedance for data/audio/video signals while still letting the DC power pass. However, it quickly became clear that given the low frequencies in use, the inductor would have to be huge. Instead, inspired by this post, I built a gyrator circuit to provide the necessary high AC impedance. I still used a 1 mH series inductor from the junk drawer to block the high frequency video signals, in order to disturb them as little as possible.
A tiny switching regulator R-78K-0.5 was used to convert the 27 VDC to 5 VDC efficiently. The circuit was built on a breadboard around a 2N5551 NPN transistor, and installed directly inside the case of the Nuki Opener (battery holder removed). Three wires connect to the positive bus terminal, GND, and the positive connection to the battery holder, respectively.
According to my measurements, the circuit draws around 4 mA at 27 VDC when the Nuki Opener is in its idle state. Typical power supplies for SCS systems provide at least 600 mA, or even 1200 mA. So this additional load is negligible in most cases. The circuit does not seem to have any relevant negative effects on the data/audio/video signals, aside from some faint background noise in the (baseband) audio when the Nuki Opener is actively processing commands from the app, but it's a small installation with fewer than 10 devices on the bus, and I have also not made any efforts to measure the AC load at various frequencies. YMMV.
Last update: 2026-03-28