The cinema that published its wiring: KLAK / Solare Zukunft

A climate-education project, 2016–2019

KLAK — KlimaAktionsKino — began as a climate-education project. Solare Zukunft e.V. in Freiburg and the Berlin-based UfU e.V. ran it under Germany's National Climate Initiative from 1 May 2016 to 30 April 2019, with a grant of €634,468 ^2,3. The public funding carried a condition to publish how the machine worked, in enough detail for a third party to build one; the technical manual is dated 20 December 2018 and remains the current published version in 2026 ¹.

The project's centrepiece was the Fahrradkino-Tour 2018, sixteen cities across Germany, and complete systems were installed for six months in four selected youth hostels ^2,3. After the funding ended in 2019, regional Fahrradkinos across the country took up the demand the project created ²; the Swiss Vélorution collective's VéloKino, a ten-bike battery-buffered rig in Zurich, credits "© fahrradkino.org" as its design source ⁶; and Solare Zukunft remains active in the school market, running the ten-bike system directly ⁷.

Specifications

What do these labels mean?

Field	Value	Confidence	Sources
Pedalling stations	10 roller trainersTen ride simultaneously; the manual asks organisers to bring a pool of at least thirty people.	Documented	¹
Generator rating per trainer	up to 250 W peakThe manual's design figure is an average of 50–60 W per bike, not the peak rating.	Documented	¹
Per-bike average output	50–60 W	Documented	¹
Projector draw	340 WProjector model is not disclosed in the manual.	Documented	¹
Design-point generation, ten riders	500–600 WTen riders at the manual's 50–60 W design average, against a load of 340 W plus audio and laptop.	Documented	¹
Central-unit maximum output	230 V AC / 50 Hz, 500 maximum W	Documented	¹
Buffer chemistry	LiFePO4 (lithium iron phosphate)Selected, per the project's own information sheet, for its low fire risk.	Documented	¹
Buffer capacity	not disclosed	Not disclosed	¹
Auto-shutdown threshold	10 minutes without pedallingAlso shuts down at 100% state of charge; warns below 20%; events should start at 60–70%.	Documented	¹
Project grant	634,468 € (funding code 03KF0034A/B, 2016–2019)	Documented	^2,3
Fahrradkino-Tour 2018	16 cities	Documented	^2,3
Ludwigsburg field output (November 2024)	about 400–500 W from 10 bikesDistrict press release; works out to roughly 40–50 W per bike with rotating schoolchildren, slightly under the manual's design point.	Operator-stated	⁴
Karlsruhe battery draw (12 January 2026)	about 30 % of the auxiliary batteryThe school's own report; bikes were not continuously occupied toward the end of the event.	Operator-stated	⁵

Ten trainers, one central unit

Ten roller trainers, each with a generator whose friction roller ("Reibrolle") presses against the rear tyre on a manual adjusting screw, feed two wiring harnesses of five Anderson Powerpole connectors each — "red to red and black to black" — into a central control unit, the Zentraleinheit ¹. Inside it sits a lithium iron phosphate (LiFePO4) battery, chosen, according to the project's information sheet, because "even under the most extreme conditions there is no explosion risk." The unit converts the stored energy to 230 volts at 50 hertz and supplies a fixed set of loads — projector, sound system, playback laptop — up to a ceiling of 500 watts ¹. The manual gives the reason for the ceiling: connect additional consumers and "the batteries are quickly discharged, because the projector draws 340 watts of electrical power." Each trainer is rated up to 250 watts peak, but the system is designed around an average of 50 to 60 watts per rider; ten people at that average generate 500–600 watts against a load of 340 watts plus audio and laptop, a margin meant to hold the battery's charge steady while every seat is filled ¹.

The central unit shuts itself off after ten minutes without pedalling, preceded by optical and acoustic warnings; it also shuts down at 100% state of charge, warns below 20%, and the manual tells organisers to start an event at 60–70% charge ¹. Riders see their own effort on the screen: a colour-coded bar for each of the ten bikes runs beneath the film.

"Alle zehn 'menschlichen Kleinkraftwerke' sollten im grünen Bereich fahren. … Der gelbe Bereich bedeutet: Teil dir deine Kräfte ein. Der rote Bereich bedeutet: Da geht noch mehr." — All ten 'human mini power plants' should ride in the green zone. … The yellow zone means: pace yourself. The red zone means: there is more in you. ¹

The manual describes the friction roller and its screw tensioning but does not specify what is inside the generator housing, and it does not state the LiFePO4 pack's capacity in watt-hours.

Line diagram of the KLAK power path from riders to projector, sound system and laptop via two harnesses and the central unit — Energy flow, stage by stage, as documented for the KLAK / Solare Zukunft system^1,5,4 Image: BicycleCinema.org, drawn from the research corpus

Energy flow, stage by stage^1,5,4

10 riders, 50–60 W electrical average each (≤250 W peak per trainer)
Roller-trainer generators (friction roller on tire, screw-tensioned)
2 × 5-bike Anderson Powerpole harnesses
Zentraleinheit: charge regulation + LiFePO4 buffer battery
Inverter, 230 V / 50 Hz, 500 W maximum
Projector (340 W) + PA + playback laptop

Two school events, 2024 and 2026

At a school event in Ludwigsburg in November 2024, the district's press release reported that the ten trainers generated "about 400 to 500 watts" with schoolchildren rotating through the seats — roughly 40–50 watts per bike, under the manual's 50–60 watt design point ⁴. At the Helmholtz-Gymnasium in Karlsruhe on 12 January 2026, with students swapping in every five to fifteen minutes under a live wattage display, the school's report recorded that about 30% of the auxiliary battery had been needed over the event, "mainly because the bikes were no longer continuously occupied toward the end" ⁵.

"Trotz des großen Engagements reichte die selbst erzeugte Energie nicht vollständig aus. Rund 30 % des zusätzlichen Akkus wurden benötigt. Ursachen dafür waren vor allem, dass die Fahrräder gegen Ende nicht mehr dauerhaft besetzt waren." — Despite the great commitment, the self-generated energy was not fully sufficient. Around 30% of the auxiliary battery was needed. The main cause was that the bikes were no longer continuously occupied toward the end. ⁵

The film played and the battery held the reserve it was designed to hold; the school's report documents a nominally self-sufficient ten-bike system drawing on its buffer as rider occupancy fell. Ten riders at 50–60 watts each is the figure Electric Pedals' founder cites for an average adult and the one Vienna's CycleCinemaClub designs around; what distinguishes KLAK is the published manual and the field reports of its users.