How to control servomotor without using battery box

[Jonas]

Hi,

I have just purchased a lego servomotor 88004. I tested it with a standard battery box (8881) and switch (8869) and it work well, i.e. it moved to one of the 3 positions -90, 0 or +90 degrees. It worked also without the 8869 switch when I used the internal switch on the battery box.

OK. This is exactly what I need in my project, with only one exception:  I need to use an external 9V power supply, not the battery box. I hoped that I could use the same arrangement like I had used with other (M, L, XL) PF motors, i.e. 9V train regulator (set up for max output voltage), extension wire 8871, PF switch 8869 and the servomotor. And ... it did not work.

I was surprised because I had believed that for this simple non-proportional control no additional electronics is needed, which was proven by my initial test with the servomotor and the battery box only. I looked at Philo's excellent web pages to see the internal schematic of the battery box. I found what I was expecting, i.e. there is no electronics there. The box just outputs 4 signals (0, 9V, C1 and C2) where C1 and C2 are connected either to 0 or 9V according to the position of the switch. So the catch seems to be in the extension wire that does not transmit the C1 and C2 signals when powered from the 9V supply. Am I right?

Do you know a trick how to overcome this problem, so I could control the servomotor by the 8869 switch, yet with the 9V train supply? At the moment, I see just one solution: an empty battery box used as a transducer between the external 9V supply and the switch. But it is not very nice. Anything more clever?         

 

 

[Captainowie]

You've hit the right cause to your problem. The basic solution is to have a battery box in the middle, as you've suggested. If you don't want that, I think your only option is to break out the soldering iron and physically join the C1 and C2 lines to the 0 and 9V lines.

You may want to investigate using the smaller battery box that holds AAA batteries - that might look a little less ugly.

Owen.

[MajorAlvega]

The servo position is proportional to the voltage at C1/C2. It's not always 0 or 9V - with the LiPo battery it goes from 0 to "9V" in 6 or 7 steps (both directions). The AA and AAA boxes are wired in a way that C1 and C2 are connected the way you observed.

The empty battery box trick is the only one I know for a manual switch. Using a Power Functions remote is not an option?

[Jonas]

Thanks for your responses. You helped to confirm my hypothesis. Maybe I should have emphasized in the title that my goal is the simple left-center-right control, not the proportional one.

The core of my problem is: How to bring a 2-wire supply (0, 9V) from the standard 9V plug to the standard 4-contact PF plug (0, 9V, C1=0, C2=9V) as simply as possible. It seems that the battery boxes (AA or AAA) are the only official Lego components where the 2-wire-to-4-wire connection is made.

[MajorAlvega]

"It seems that the battery boxes (AA or AAA) are the only official Lego components where the 2-wire-to-4-wire connection is made."

As far as I know, yes.

If you do need to use a LEGO switch, wiring a custom switch in the same way it is done in the battery boxes is no option.

I guess using a Mindstorms is overkill?

But I think you can do the same "left-center-right" movement with an ordinary motor and some clever mechanical setup, like Isogawa or other master builders (look for stepper motor).

[Philo]

There is indeed no solution without cable surgery or using a battery pack. Extension cable that provides old 9V to PFS conversion powers only C1/C2 for the motors: if it did link to 0/9V lines, electronics in IR receiver or servo motor could get powered backwards (old 9V connector switches polarity depending on its orientation)

[Jonas]

26 minutes ago, Philo said:

There is indeed no solution without cable surgery or using a battery pack. Extension cable that provides old 9V to PFS conversion powers only C1/C2 for the motors: if it did link to 0/9V lines, electronics in IR receiver or servo motor could get powered backwards (old 9V connector switches polarity depending on its orientation)

Thank you, Philo, for this clear explanation. It seems rational and now I understand why the extension wire is produced as it is.

OK, if I prefer a 'fully legal' solution I will use either of the (empty) battery boxes.

Edited February 10, 2017 by Jonas

[Blakbird]

On 2/9/2017 at 0:05 PM, Captainowie said:

You've hit the right cause to your problem. The basic solution is to have a battery box in the middle, as you've suggested. If you don't want that, I think your only option is to break out the soldering iron and physically join the C1 and C2 lines to the 0 and 9V lines.

It doesn't need to be that hard.  I just use a tiny piece of foil to bridge the terminals.

[Captainowie]

On 10/02/2017 at 7:12 AM, MajorAlvega said:

Using a Power Functions remote is not an option?

No, it's not. That suffers the same problem - you'd need to do exactly the same hack in order to get the IR receiver to work.

12 hours ago, Blakbird said:

It doesn't need to be that hard.  I just use a tiny piece of foil to bridge the terminals.

Ah yep, that'd work too.

Edited February 11, 2017 by Captainowie
Grammar

[Jonas]

7 hours ago, Blakbird said:

It doesn't need to be that hard.  I just use a tiny piece of foil to bridge the terminals.

Thanks Blakbird. This is definitely the best and easy to make hack.