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:
- Two-day Amazon Prime shipping = reduced lead time.
- Not only would it come in two days, I would also be getting a 2-pack. Spares are always a good idea.
- The input/output voltage range and output current seemed appropriate. Spec sheet here.
- 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:
(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