Simple battery in series question - how to connect ground?

[tbrun]

My 7 year old son has a toy robotic arm that uses 4 D batteries in series.

He had the idea to replace the batteries with a DC adapter so he could save some allowance $$ by not buying new batteries.

Seemed like a good idea until we opened it up and saw that it's not as simple as just a positive (+ red) and negative (- black). There is also a ground wire that connects to the middle of the battery series (between 2nd and 3rd batteries).

If I connect that ground wire to the negative (- black) line, the robot arm only goes one direction (up, right). If I connect that ground wire to the positive (+ red) line, the robot arm only goes the other direction (down, left). If the ground is not hooked up to either one, the robot arm doesn't work at all. If the ground wire is hooked up to both, of course it shorts the converter.

So the question is: how can I hook up the ground wire to make the robot arm work again in both directions? Or, is there some special property of a ground in the middle of a battery series that I cannot replicate with a DC adapter?

This is nowhere as easy as when I did this with his baby swing 6 years ago!

Thank you for any advice, much appreciated!!

:?::-?

 

[BradtheRad]

It sounds as though your battery arrangement is like this:

One switch is closed, sending current in one direction through the load. When the other switch is closed, current flows in the other direction through the load.

There are circuits that take a single supply, and create a split (+ & -) supply. This may allow you to use your present DC adapter (if you trust your electronics construction abilities).

Otherwise you can try connecting two readymade 3V supplies, as in the diagram.

 

[tbrun]

Thank you, BradtheRad.

So to put it simply. If I find two 3v supplies, where would I put the (+) and (-)?

Would I connect one supply to (+) and ground, and the other to (+) and (-)?

 

[d123]

I'm embarrassed to answer this post. 7 years old you say?...

By any small chance is the robot arm the Velleman KSR10?
(Apparently for +14 years old, but maybe I'm too old to have such a toy, all the same your son clearly has a great future in robotics!)

Attached is the circuit diagram for that one, if it is the Velleman one, maybe it'll help people to advise you on how to proceed with swapping the batteries for an adapter to cover the dual polarity.

 

[hobbyckts]

No it should be like..

First One --> Positive Terminal to (+) & Negative Terminal to Ground

Second One --> Positive Terminal to Ground & Negative Terminal to (-)

The Ground is the point where you need to connect your robotic arm

 

[tbrun]

Thank you for all the info! d123, it is this robotic arm: https://www.owirobots.com/store/cat...ssories/owi-535-robotic-arm-edge-kit-110.html

He's a special 7 year old kid, he's been taking apart electronics since he was 2 years old. Now putting them back together properly is another story...

Hobbyckts - I will try to find two 3v adapters and see if your suggested method works! Thank you!

 

[tbrun]

I'm embarrassed to answer this post. 7 years old you say?...

By any small chance is the robot arm the Velleman KSR10?
(Apparently for +14 years old, but maybe I'm too old to have such a toy, all the same your son clearly has a great future in robotics!)

Attached is the circuit diagram for that one, if it is the Velleman one, maybe it'll help people to advise you on how to proceed with swapping the batteries for an adapter to cover the dual polarity.

Actually, yes, that is the exact same circuit as on our robotic arm. Looks like the Velleman and OWI-535 are basically the same.