PF Wiring Question

[Blakbird]

I'm in the midst of a project with an interminable amount of wiring and no good place to rout it. As I ponder the Power Functions standard, I find myself with some questions about the purpose of wiring. All the PF components use connectors and wires with 4 conductors, but it is my belief that not every component actually uses all 4. Is it possible to trim 2 of the 4 wires to save space?

• Motor: A DC motor only uses 2 wires, positive and negative. What happens to the other 2? Do they just dead end inside the motor? Which 2 could I remove?
  
• LED: I think the LED only uses 2 wires for similar reasons.
  
• Servo: The servo doesn't transmit it's position back along the wire, it positions itself based on regulated voltage input. Does it only use 2 wires?
  
• IR Receiver: This one is the biggest question for me. It only needs +/- to get power, and it doesn't use modulated input voltage based on the output of the rechargeable battery box, for example. Does it only use two wires?
  
• For that matter, is there any single component which actually needs all 4 wires?
  

Thanks for any guidance.

[DrJB]

The way I understand it, it all depends on which component you're using (as you stated). Simplistically, two of the wires provide external power (Ground and MAX 9V), while the other 2 wires tell the specific component how much of those 9V to use. For example, in the old 9V system, you vary the speed of a motor by changing the voltage on the wires from 0 to 9V. In the PF system, the 9V is fixed, and what fraction you need to use is 'coded' in the other 2 wires. Thus, while you can run a motor from only 2 wires, you won't be able to change its speed. Hope this helps ... and I'm sure there are more 'elaborate' explanations out there.

Edit: Based on subsequent discussions (read below) ... I was a bit off.

Edited April 24, 2014 by DrJB

[Blakbird]

The way I understand it, it all depends on which component you're using (as you stated). Simplistically, two of the wires provide external power (Ground and MAX 9V), while the other 2 wires tell the specific component how much of those 9V to use. For example, in the old 9V system, you vary the speed of a motor by changing the voltage on the wires from 0 to 9V. In the PF system, the 9V is fixed, and what fraction you need to use is 'coded' in the other 2 wires. Thus, while you can run a motor from only 2 wires, you won't be able to change its speed. Hope this helps ... and I'm sure there are more 'elaborate' explanations out there.

I follow that much, but my question is more about where is the "logic" that controls the speed? I think the logic that tells the motor how much voltage to use is probably within the receiver, not within the motor. That would mean the receiver modulates the voltage to the motor, acting as a speed controller. The motor is just a motor, no different than any other. But these are only guesses.

[zux]

Well I think you could try your theory, by attaching an extention PF cables to each device (like 1 for motor and 1 to IR Receiver) and old 9V electric cable between them. I'm thinking if it works, then some of cables cab me trimmed for sure.

[Conchas]

The logic is indeed inside the IR Receiver and inside the rechargeable battery box, which generate the appropriate PWM signals to control the speed of the devices which accept proportional control (it is also the case of the lights for instance).

So if you want optimize space you can get rid of the unnecessary wires in a case by case basis.

I made a kit of specific custom wires wire I can connect only what I need in each situation. This was done mainly for testing purposes and adaptations with other systems.

[Hrafn]

Philo's PF page should have the answers you're looking for; in particular he has this diagram, with dotted lines indicating channels that are not used:

His diagram suggests that the receiver only needs 0V and 9V in, and C1 and C2 out. Motors need C1/C2 in, and I would guess lights are the same. The servo needs all 4: 0/9V for power, C1/C2 for angle.

[1974]

Schematic tells everything. TLG choose a 4 wire system throughout the whole PF system, this simplifies production

Also, with having seperate powerlines and PWM lines you can have motors with active electronic inside, like the servo. Which means they thought about the servo many years ago when they made the blueprints for PF

Four wires does make everything more bulky though and PF in genral is nowher near as flexible as the old 9V system. But with that system it would be difficult/expensive to have modulation/control on top of a 2 wire power scheme

TLG probably did just that when they made reciever prototypes, I guess ..

I could also see the PF system used for other active devices (like the servo). Could be IR controlled sound and light devices. Those parts would have active electronics inside and with the IR reciever one could control different sounds/light patters/colours etc ..

[Blakbird]

Philo's PF page should have the answers you're looking for; in particular he has this diagram, with dotted lines indicating channels that are not used:

Thanks, that's exactly what I needed. It looks like I could indeed eliminate half of the bulk of my wiring.

The logic is indeed inside the IR Receiver and inside the rechargeable battery box, which generate the appropriate PWM signals to control the speed of the devices which accept proportional control (it is also the case of the lights for instance).

Is it really a speed control (PWM) that is being supplied by the receiver or just a reduced voltage? A reduced voltage is not a speed and will still change speed as a function of output drag. It also means the motor operates at less power when at a lower setting. However, a true PWM speed control could have similar power by pulling more current at a lower voltage.

[1974]

It's PWM, you can hear the whining easliy. You need PWM to have some proper torque

[Blakbird]

It's PWM, you can hear the whining easliy. You need PWM to have some proper torque

Interesting! So when I connect a PF XL motor to an old train regulator (like I did when I operated Nico71's braiding machine), then the motor operates at low speed differently than it does when connected to a rechargeable battery box, for example.

[1974]

Yes, the train regulator is standard linear/high loss regulation (LM317)

[Blakbird]

Yes, the train regulator is standard linear/high loss regulation (LM317)

At least it's not a rheostat.

[1974]

Oh, for that you'll get an 'old geezer' tag ..

[Boxerlego]

The PF system has its unnecessary wires mostly on the DC motors. The PF switch is tricky because it sends power by the two outer wires but it doesn't switch the polarity on them because that would probably destroy the IR receiver.

[Conchas]

Yes, it is really PWM.

[DrJB]

Philo's PF page should have the answers you're looking for; in particular he has this diagram, with dotted lines indicating channels that are not used:

His diagram suggests that the receiver only needs 0V and 9V in, and C1 and C2 out. Motors need C1/C2 in, and I would guess lights are the same. The servo needs all 4: 0/9V for power, C1/C2 for angle.

What I struggle with in the above diagram is ... what happens if you connect a motor directly to a battery box? In this case the motor runs at constant RPM, and supposedly uses the max 9V available. Then, the above diagram suggests that wires 9V/0V are not connected at the motor end and wires C1/C2 are not connected at the battery box end ... Not sure then how the motor can operate when connected directly to the battery box ...

Does anyone else see the apparent issue?

Unless 9V/C1 and 0V/C2 are connected at either the battery box or motor ends ...

Edited April 24, 2014 by DrJB

[Blakbird]

Unless 9V/C1 and 0V/C2 are connected at either the battery box or motor ends ...

Bingo. That's why you can use an empty battery box to bridge those terminals to, for instance, connect an old 9V battery box to an IR receiver.

[Hrafn]

Bingo. That's why you can use an empty battery box to bridge those terminals to, for instance, connect an old 9V battery box to an IR receiver.

Interesting! Do you have any more details as to how to do that?

Never mind, I found it. 9V box -> converter cable -> empty PF battery box -> cable -> receiver

Edited April 24, 2014 by Hrafn

[DrJB]

Bingo. That's why you can use an empty battery box to bridge those terminals to, for instance, connect an old 9V battery box to an IR receiver.

This then explains why, a while back, when I tried connecting a PF receiver to the old 9V train transformer (with a 9V/PF converter cable in between) ... the PF Receiver could not get any power from the 9V train box ... My conclusion back then was that it makes no sense to have a PF receiver (supposedly for mobile applications) connected to a stationary box (the 9V train) ... but I could not 'accept' that such limitation was implemented by TLG. Now at last, mystery resolved ... it was much simpler than I had concocted.

Yes, it is really PWM.

Unless there are some built-in electronics to filter the PWM signals ... those motors are bound to be noisy ... but I guess the coil (windings) of the motor itself have electric 'inertia' an thus act as a low-pass filter.

Edited April 24, 2014 by DrJB

[Philo]

Sorry to chime in late, but it seems you figured out everything... much more educational this way

Edited April 24, 2014 by Philo

[Blastem]

Thanks a lot for the info. I have to try a "trick" in an old "bricky" MOC

[JM1971]

With four wires PF is far more likely to fail and is very difficult to repair, lego designed PF this way to make more money?

[DrJB]

Not sure about that ... the one 'problem' I have with PF is the trains ... you spend a fortune on batteries, and the fact that PF and mindstorms are not compatible ... but that's a different discussion.

[zux]

Not sure about that ... the one 'problem' I have with PF is the trains ... you spend a fortune on batteries, and the fact that PF and mindstorms are not compatible ... but that's a different discussion.

There is such cable. I wonder if it allows you to connect Mindstorms motor to old 9V system or old 9V motor to Mindstorms? Would it be possible to achieve some PF compatibility with minstrorms using that cable through PF extension cables?

[Philo]

Sure it's possible! But it's kind of garland like, and cable is no longer available from LEGO. You better do some cable splicing between a NXT/EV3 half cable and a half PF extension cable. Connect the two wires on RJ connector latch side (black and white) of the Mindstorms cable to the middle wires of PF cable.

Another possibility is to use Mindsensors GlideWheel PF Motor controller for NXT or EV3. More expensive, but you get a position encoder...