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• Cheapest Way to Drive Motors

  williamg42 • 06/09/2019 at 23:15 • 0 comments

  Over the course of this project two major methods of driving a motor were used. The first was way more hacky and fun then the second. 

  
  For the first two generations of the robot, 74HC244 Octal buffer/line driver were used to drive the motors. They are cheap, and by using all 8 outputs/inputs tied together software PWM allowed basic speed control. 

  And it worked really well! Except everyone who tested it really wanted to go backwards for some reason. Since every motor driver on digikey/mouser was still 5x more expensive that a 74HC244, we just doubled the number and uses 2 per side! Sadly, by using two the forward and reverse speed became limited by the current sinking capabilities of the 74HC244 and the kindest description of the prototype was sluggish. You can see the dual line drive prototype below.

  After going back to the drawing board, we found the L9110s motors drivers on ebay. The L9110s has two inputs and drives one DC motor at voltages up to 9V. They don't require PWM inputs, and logic high and logic lows are all that is required to control it. 

  In order to fix the sluggishness a cheap boost convert was added to increase the voltage from the battery to something higher to let the robot move faster. 

  An important note: back EMF

  Since the L9110s do not have flyback diodes, there is a huge back EMF when the motors change directions. This normally would do bad things to the boost converter. However to both add protection to the boost convert and add an easy way to set the boost converter to the right voltage, a 5V zener diode and LED were added in parallel to the boost power supply. That way when the LED turns on the boost converter is set to around 6V. In addition, they give the back EMF a path to ground not through the power supply. (And it is noticeable, the LED gets a lot brighter when the motors change direction).

  Theses Generation 4 rovers were used as part of a Sumo Bot competition and performed fairly well (there was issues with the battery moving inside the holder and causing it to disconnect)

  And this is the solution we used for the Generation 5 and 6 Rovers! Each L9110s is less than 50 cents and runs like a champ!

• A warning about "Protected" 18650s

  williamg42 • 06/09/2019 at 21:27 • 1 comment

  As I am going through and documented some of the things I did for this project, I felt obligated to post this one first, since it is DANGEROUS and MISLEADING for the average hobbyists. The VAST majority of cheap "Protected" 18650 cells available on eBay and other sites are not protected, even if they are labelled as protected on the wrapping. 

  As part of this project, 3 different brands of 18650s "Protected" Cells from ebay were bought and tested, and two of them were found not to contain any kind of protection whatsoever. (Note: This testing was done spring of 2017)

  The worst offender was these cells, which are still listed as protected online, but after the university raised hell the "Protected Cell" label on the cell wrapper itself seems to have been removed.  

  They are not protected and definitely are not rated to 6000mAh. We did a discharge test and got 50 mAh out of the cells. Its obvious if you look at the bottom of the cell and notice there is not the bulge of a protection PCB and there is not a strip under the wrapper running up the side to the top of the cell.  

  The other big offender was any of the Skywolfeye brand cells. Again after the University intervened, the "Protected" seems to have been removed from the cell wrapper but they still can be found listed as protected online. 

  Once again they are not 5800 mAh. After testing we got a maximum capacity of 600 mAh. 

  Now the good news is all of the Ultrafire cells we tested were protected, and after disassembly they all used the same cheap IC used in the 18650 TP4056 usb chargers with built in battery protection. What that means is the cut off current is surprisingly low, and any current draw over about 1 amp results in the oven circuit protection tripping. (Which is good ,as I doubt theses cells are safe for much over .5C)

  Basically the rule of thumb is if the battery capacity is over 3400 mAh or costs less than $5 a cell, it's not a reliable cell. In addition, if you do not see a ribbon under the wrapper or a disk on the bottom of the cell, it's not protected. 

  Here is a protected cell with a clear wrapper so you can see what I am talking about

  The cheapest cells we've found were from liionwholesale.com, and they are shipped according the US law. 

  Hopefully someone finds this helpful, and stay safe hacking!

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