I want to use the PF Light Unit in a small MOC. The battery box contains six 1.5V batteries and is too large.

How can I use a regular 9V block battery to power the lights (and possibly other 9V consumers)? Which wires have to be connected to the poles of the battery?

2 Answers 2


Most Power Functions pieces will run in classic 9V mode. The IR receiver can't, but motors and lights can.

A hacky solution:

Many people have done this by cutting up an extension wire, and connecting the two outer cables to their power source of choice.

From: Lego PF Hacking – Wire Hacking


  • GND – ground wire – Always connected (0V regardless of battery pack direction)
  • C1 – control wire 1 – 9V when battery pack is forward, 0V when in reverse
  • C2 – control wire 2 – 0V when battery pack is forward, 9V when in reverse
  • 9V – power wire – On when battery pack is on (9V regardless of pack direction)

A purist solution:

If you prefer a purist solution, you could get a classic 9V battery box that takes a single regular 9V battery.

classic 9V batter box

They have the classic 9V wire connectors on the top (a little hard to see in the image, but there are 2x6 studs with metal connectors). Just pair that with a Power Functions extension wire (which has a 9V connector adapter) to connect your Power Functions elements.

Power Function extension wire

That will give you a purist solution which doesn't require chopping up your wire.

  • Thank you, especially for the classic 9V box - PF extension wire hint.
    – Metalbeard
    Feb 23, 2017 at 7:24

Check out Philo's pages for details on Power Functions: http://www.philohome.com/pf/pf.htm

Basically, a Powerfunctions cable consists of 4 wires: 9V, C1, C2 and GND. C1 and C2 are also 9V/GND, but output can be controlled by other equipment (such as the IR receiver or the LiPo battery box which has an output power control). The best way to power your lights is by connecting the 9V battery to the C1 and C2 lines (doesn't matter in which order you connect them to the + and - of your battery), and just to be safe you can also connect the + to the 9V and the - to the GND line, but that is optional.

  • Thanky ou for the link. So it is true that the outer lines are always "on" and the inner lines depend on e.g. switch and remote control signals?
    – Metalbeard
    Feb 23, 2017 at 9:26
  • 2
    @Aziraphale I can provide the following evidence for the outer lines being always on: I connected an IR receiver to a switch, thinking that the switch could disable the IR receiver. It turns out the IR receiver ignores the switch; no matter the switch position the IR receiver powered the motor according to what the remote told it. So probably yes, the power line is always on. Feb 28, 2017 at 15:42

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