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SBrick-based 9V CV power supply
The old 9V train transformer turns out not to be a continuously variable (CV) power supply after all, but it can be made so with the addition of an SBrick.
About this creation
Please feel free to look over the images and skip the verbiage.

No doubt about it, the third-party SBrick Bluetooth receiver greatly enhances the functionality and play value of PF remote control vehicles -- especially boats. But the benefits don't end there.

I'd always thought of the trusty old black and yellow 9V LEGO® train transformer (TT, 2968b) as a continuously variable power supply (CVPS), but what did I know?



Turns out that the 2 TTs I own deliver power, not continuously, but in 5 discrete voltage steps. I discovered this while testing the centrifugal tachometer above (coming soon) and now suspect that many if not all TTs do the same for 2 reasons: (i) Both of my TTs put out very similar voltage steps. (ii) Another 2 TTs owned by building buddy Shawn Kelly also have 5-step outputs.

NB: If you have evidence for or against this generalization, please leave a comment!

The practical import: If all TTs have stepped outputs, that makes the SBrick the most affordable route to a true CVPS in the LEGO® realm today -- and also one of the more readily available.

Of course, the TT still outshines all battery-fed LEGO® power supplies in its ability to deliver a steady 9V of peak power without voltage sag. And that's the really good news here: You can power an SBrick with a TT and have it all -- continuously variable output up to a reliable 9V max with no need to recharge or throw away money on batteries!



So, for MOCs in need of finely adjustable power and able to operate while tethered to a power cord (e.g., some model machines and GBC modules), there's simply no better power supply than the TT+SBrick+phone combo.

The video below pretty much tells the story. Watch how the red centrifugal tachometer needle behaves as the TT dial moves smoothly from off to maximum power and back again.



The only catch: The TT's output polarity has to match the SBrick's input polarity for the latter to power up.

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CVPS alternatives

The old RC Race Buggy (RCRB) remote control system can also be pressed into service as a variable power supply, but whether it's continuously variable, I can't say. What I do know is that RCRB receivers and their dedicated transmitters are rare as hens' teeth and hideously expensive.



With TLG's "EV3 Commander" app on a suitable Bluetooth-enabled Android or iOS phone, the EV3 processor can function as a remote control CVPS right out of the box with no programming or external sensor involved. (Just be sure to update the EV3's firmware before attempting to pair it with the phone.) Stand-alone EV3s are widely available (e.g., from shop.lego.com, generally the cheapest source) but cost even more than the RCRB solution.

With a suitable external sensor for a control interface, an RCX, NXT, or EV3 processor can be also programmed to function as a CVPS, but that can be even more costly -- especially in the EV3's case.

The battery-fed Mindstorms and RCRB options obviate the need for a power cord, but they'll always max out at less than 9V due to voltage sag -- even with fresh alkalines.

Finally, if you have a TT but not an SBrick, you might find use for a variable but not continuously variable PF-based option like my power dog.

Bottom line: If you already own a suitable Android or iOS phone, you can turn a TT and an SBrick into a more powerful true CVPS for a third of what the EV3 and RCRB alternatives cost, and you'll spend less time and money on batteries to boot. The TT+SBrick+phone option will probably weigh less as well.

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Comments

 I made it 
  January 28, 2016
Quoting jds 7777 Very interesting! I just realized Mocpages isn't sending me emails anymore for some reason, so I have a lot of catching up to do! The problem with the RCX you mentioned (which is probably the cheapest besides PF), is that it cannot supply as much power as the other options. It's to bad people think of the retired NXT as "rare" or "collectable", and then proceed to jack the price into the mesosphere. I was hoping cost would go down after the EV3, but I guess I was wrong.
Thanks, JDS! Good points about the RCX and NXT. The EV3 is so much more capable that it may well be worth the extra $$$ for a stand-alone EV3 from shop.lego.com given currently exorbitant NXT prices. My ratings tank every time the e-mail notifications go down, and I'm sure many others are similarly affected. Many thanks for making the effort to catch up.
 I like it 
  January 28, 2016
Very interesting! I just realized Mocpages isn't sending me emails anymore for some reason, so I have a lot of catching up to do! The problem with the RCX you mentioned (which is probably the cheapest besides PF), is that it cannot supply as much power as the other options. It's to bad people think of the retired NXT as "rare" or "collectable", and then proceed to jack the price into the mesosphere. I was hoping cost would go down after the EV3, but I guess I was wrong.
 I made it 
  January 18, 2016
Quoting Oliver Becker Great solutions seem to get possible with your idea, Jeremy! Your really one of the greatest technic-puzzle freaks I know! Hope my dictionary is right translating "Tüfteler" in the correct way... ;)
Too kind, Oliver! Just wanted to show that the SBrick opens up exciting possibilities beyond the usual RC vehicle.
 I made it 
  January 18, 2016
Quoting Didier B Cool Jeremy ! You should make an air supply fan with this. Didier
Thanks for the compliment and suggestion, Didier! Hmmm, my motorized prop cart (http://www.moc-pages.com/moc.php/403294) already doubles as a powerful fan. Repowering it with an SBrick and a TT to bring the input up to 9V (as opposed to the 7.4V of its usual PF rechargeable) would be a snap. Stay tuned.
 I like it 
  January 18, 2016
Great solutions seem to get possible with your idea, Jeremy! Your really one of the greatest technic-puzzle freaks I know! Hope my dictionary is right translating "Tüfteler" in the correct way... ;)
 I like it 
  January 18, 2016
Cool Jeremy ! You should make an air supply fan with this. Didier
 I made it 
  January 17, 2016
Quoting Gabor Pauler Cool!
Thanks, Gabor!
 I like it 
  January 17, 2016
Cool!
 
By Jeremy McCreary
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Added January 18, 2016
 


LEGO models my own creation MOCpages toys shop SBrick-based 9V CV power supply


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