Remember TBs TechReview 01, where Fernando showed the possibilities the 8886 Extension Wire offered in interconnecting the new Power Functions system and the old 9V system? Well, turns out the Infrared Receiver doesn't work when powered from an old 9V source.
The discovery was made by simonwillems from Eurobricks, and the ensuing posts on that thread were quite interesting to follow, to see several workarounds brewing.
One of them was suggested by simonwillems, the one Fernando added to the TechReview. It has the advantage of using less parts (good for mobile/cramped constructions), at the cost of having only one of the receiver's outputs available.
The other one is the focus of this edition of TechTips. Lasse D suggested adding an empty Power Functions battery box in the circuit. Since the receiver appears to use the power pins of the PF connector, the adapter cable appears to supply power only to the motor pins, and the battery box can power both motors and receivers, the latter would probably bridge the two incompatible circuits. Sounds complicated? Philo explains it brilliantly: just scroll down to the "Power Functions working" section on his page.
Meanwhile, Lasse D reported success for his method. I tried it to see if it worked, and indeed it did! This is a short video explaining how to do it. Basically, just plug the receiver/adapter cable connection on an empty PF battery box, and turn it on. The polarity should be the same as the power source: if the LED on the battery box doesn't light up, just slide the switch the other way.
Of course, I used an AC adapter and train speed regulator to power the receiver, but any 9V source could be used instead: from the standard old, new and small battery boxes, passing by NXT's (with some power modding), to more esoteric solutions, like Control Centres, train track contacts or even solar panels! This is one of the beauties of LEGO: the imagination is the only limit.
It has been asked in the comments if the same result could be achieved using a PF Pole Reverser instead of the battery box. I tried it in the weekend, and regret to inform that it doesn't work.