Monday, May 11, 2009

TBs TechTips 23 - Connecting the PF Receiver to 9V

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.


TechnicBRICKs said...

Thanks for the video and explanation!

Well done Alex!

I wonder the amount of things we can learn and forget at the same time... :P ;)

Parax said...

I wonder if they could do another adapter with the c1,c2 connectors on the top and the +9/0v connectors on the bottom...

Additionally I see that the 20 inch leads are now in the UK shop.

TechnicBRICKs said...

Hi Mike,

why would you need such specific cable, if I got it right?

I was also trying to order the long cables from S@H on the phone less than one moth ago and it was not possible.
It looks they were waiting for me to buy them via Bricklink, or someone really forgot this reference for some months...

Anonymous said...

Ain't it possible that this trick also works with a nem PF switch?

I think it might work because a PF switch also feeds both motors and reciever.

Alex Campos said...

That should be something interesting to test, too. But, for 9V to be able to power the receiver, the voltage present only at the C1/C2 pins on the 9V adapter needs to reach the 9V/0V pins on the receiver, and I doubt the pole reverser joins them.

But nothing like trying that out! ;)

Parax said...

I was thinking a more universal lead. either for power or control. (could run lights always on from one side and a switched output on the other..) althogh originally I was thinking just a 2x2 block with no wires! just use as an adapter between PF wire and 9v wire, red block for 0&9v and blue block for C1&C2

I would think The PF reverser switch would do this job! it should connect across from C1-9 and c2-0 or vise versa..
(I dont have much 9v to test with)

Thomas J Avery said...

I think you need a tripod for your camera ;-)))

Seriously, thanks for the explanation. That helps!

Rob Sanderson said...

I would love to connect up my old 9V battery box to run my PF parts. In my opinion the old 9V battery box has a distinct advantage over the newer PF model in that it is more compact and therefore easier to fit into models.
Unfortunately this substitution is not possible when using the Infrared Receiver. The only way around this problem would either be to use an empty PF battery box or to buy the new, and quite expensive, rechargable battery pack.
There is another method however...
remove the electronics from a PF battery box (enclosing them in a suitably modified Lego block) and attach this to your old 9V power supply. I have yet to try this method out for real though, as I don't really have a spare PF battery box, but I can't see why it wouldn't work?

Alex Campos said...

@TJ: the shakiness of the image is just one of the problems! I need to figure out a way to enable macro focus for video recording on the phone (which is the device I have with the best video-recording capabilities), and somehow cover that blindingly horrible red recording light... :P

@Rob: in principle you don't need to butcher a battery box. Just use a pair of short wires on one of the adapter cable's PF plugs to connect together C1 to 9V and C2 to 0V (or vice-versa, depending on the polarity you're supplying; the ideal thing would be to use a rectifier bridge like the PF lights). This is a much more compact solution, but it requires some precision skills and non-LEGO components.

Norama said...

This whole exercise seems a bit pointless to me. If you are going to use a PF battery box, why not slap some batteries in and be done with it? The alternate power source then becomes redundant.

Alex Campos said...

Last Saturday I tried using a Pole Reverser instead of the battery box, and found out that it doesn't work. :( The article has been updated with that info.

@Norama: the point in this exercise is exactly to avoid using batteries, which eventually get depleted... With an empty battery box, other power sources can be used. At the end of the article there are some examples.

steve(scna1000) said...

hello all i have found the solution. you need to connect the old 9v box to a pf extension lead. connect this lead to the bottom connection on the ir receiver. now the trick is to use another pf extension lead but connect that to the top of the ir receiver lead. This 2nd lead connects to nothing. Now you have a fully remote control system from the old 9v box.


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