Buck-Boost Wrangling: Part III – Success!


IMG_0570     IMG_0573

Same setup, nothing different, except… the protoboard.  Had I not decided to switch boards (the one I was working with was too heavy to be convenient), I would have never known.  I am frustrated because I essentially burned a week unnecessarily traveling down a rabbit hole.  But, it works.

Before, I was incredibly concerned because I had seen a comment in one of the DIY articles that mentioned the USB’s data pins had to be engaged, which I found strange.  Good thing it was a non-issue.

MAKE excerpt

… also it won’t charge an iphone unless you have the correct voltage ref on the data lines…. other USB devices like Apple products also need correct voltage ref on data pins

Next steps for now till Friday: Integrating the mechanical and electrical systems.  Karen, Allieberry, and Shakeena will be presenting at ISTE 2015 (International Society for Technology in Education).  We’ve got quite a bit of work to do.  Just on the EE side, I have to install the buck-boost, solder and heat-shrink all the joints, and test.  However, all this is on pause until Lindsey prints all the parts for me to install the EE system onto, which will happen at some point tomorrow.

Prior: Buck-Boost Wrangling II


Buck-Boost Wrangling I: Part Selection and Testing

When I joined the project on June 1st, Lindsey had already been working on the mechanical aspect for a couple of weeks.  The electrical system had only been discussed in very high-level terms.  By this point, Erica had suggested using a buck-boost converter circuit.  A buck-boost circuit takes some voltage input and spits out some voltage output.  The natural acceleration involved in riding a scooter would result in variable voltage when backdriving the motor; the buck-boost would act as a voltage normalizer.  For context, a cell phone charges on 5V and anywhere from 0.5-0.9A.

This naturally leads to the QUESTION: Which buck-boost converter should I use?


Choosing a buck-boost:

Quick Google searching resulted in a handful of options, but I decided to test use the LM2596 from Amazon because:

  1. Two-day Amazon Prime shipping = reduced lead time.
  2. Not only would it come in two days, I would also be getting a 2-pack.  Spares are always a good idea.
  3. The input/output voltage range and output current seemed appropriate.  Spec sheet here.
  4. There was a potentiometer onboard so I could customize Vout. Hooray for debugging tools!

Testing the buck-boost:

I didn’t have high hopes for the buck-boost converter, but testing was particularly frustrating.

This was the setup:

WP_20150618_001     WP_20150612_003

 (Right: The computer is not the power source; the power source unit is outside the picture frame.)

I set the power source to 5V and… nothing happened.

Debugging Step 1:  Tweak parameters and probe

I trust the power source to be outputting the correct voltage, so I connected a multimeter to measure the current drawn by phone.  I slowly tweaked the voltage supplied to 12V.  The buck-boost did its job and outputted 5V all the way, but still no current was drawn.

Debugging Step 2: Return to known system

I know that if I plug my phone directly into a computer, my phone charges.  I disconnected my male / female USB ports from the protoboard and reconnected to the computer’s USB, adding the multimeter to get a reading.


Plug in phone, and… no dice.

Disconnect multimeter, plug in phone – still nothing.  Conclusion – borked USB cable.  Find another male/female, strip, splice, heat shrink.  Repeated this process twice until I found a working USB.

The multimeter read 5V and 0.4A.  This is my baseline.  The question now: was it simply the USB cord that was messing up?

Debugging Step 3: Add a layer of complexity

Move system back to protoboard.  This time, the multimeter registered a current draw of 0.1A, which stayed consistent no matter how I tweaked the voltage.  The USB cable is functional; therefore it must be the buck-boost converter that is limiting the current draw.

I was baffled, though, because the LM2596 chip was spec’d to allow a draw of maximum 3A.  I was at an impasse, and decided that I would pick Erica’s brain about the matter the next day at the National Maker Faire.

Continued: Buck-Boost Wrangling II