It's probably time I worked out how to distribute power to all of BB-8's components, a little late I know. Just a little warning: I am still considering myself a newcomer to electronics so the best I can do with what knowledge I have is get it working and avoid shocking myself at the same time. I think I have a bit of a knack for it so we'll see how well this works out.
In this article I'll discuss my options and what choices I made. It's a little more complicated than you might think. Different components have different tolerances for voltage and current, but some components also work better on higher voltages.
Let's get started ...
The first choice I need to make is the battery. My first thought was just a small 9.6V NIMH 2400mAh battery. Well, if I consider the motors could run at 2A each (worst case scenario), my audio might get up to 1A, and everything else close to 1A, that's around 10A! Obviously it's not going to be that much, but if I run everything full blast I want at least an hour, two would be preferred. I chose NIMH batteries commonly used with R/C cars since I can draw a lot of current from them without worry. I also want a good amount of weight in my hamster and these are cheaper than LiPos.
I finally settled on a couple of 9.6V 5000mAh batteries (will wire in parallel). Since they'll be in parallel I'll need to make sure to keep them both at around the same charge. I hope this wasn't a bad idea! I'll setup the circuit so that I can use just one if I need to. This gives me a total of 10Ah which should be plenty for my purpose.
Now, I need something to mount the batteries to the chassis. In order to balance my weight they'll go opposite the speakers which are also quite heavy. My first attempt at the battery mount was good for a while, but after some testing it was not strong enough to hold the weight! Part of the problem was the way I had printed the parts and the direction of the grain in the plastic made it very weak where the force was applied. The failed mount is in the left-most image.
In the new mount (right-most image) the batteries are moved slightly to one side to make room for the WAV Trigger nearby. I've also printed the bottom piece (gray) 100% infill and flat on the printer. It seems to be considerably stronger!
For the motors and the speakers I would prefer 12V, but my batteries only give me 9.6V. To solve this I'm going to use DROK's 150W DC Boost Converter (Click-here to see it on Amazon. Left-most image.). It can be configured to stabilize the voltage to what I need (12V). I picked this one because it can handle plenty of current, hopefully no worries there. I like how it also has a 5A fuse. To further protect everything I may also add some PTC fuses on the wires that go to my motors from the motor driver.
I also need a mount for this so it can sit flat on the chassis. I'm changing my ways a bit since this circuit board does have screw holes. Two of the screws go through the board, through the mount, and into the chassis. Actually I'm using #4-40 1/2" bolts and matching nuts.
Finally I can move on to actually distributing the power to all the components! For this I'm going to make my own custom circuit board and I'm quite nervous. I'm new to creating circuits so this is my my second board (the first being the power distribution for the audio). Using various wires I have around and bits from LEDs I was able to build something that works quite well.
Before you question my choice of wires on the bottom side of the board, keep in mind some of those wires are thick gauge enamel insulated wire. The insulation is transparent so you might think it's just raw copper, it's not. The main rails going through the center of the board are not insulated and are meant for most of the current since they're about 18 gauge. I tried to keep in mind how much current I might expect on various parts of the board; hopefully I did a satisfactory job. I would rather not see 5 or 6 amps go through this board, but I only think that's possible if every motor fully stalls (assuming I use PTC fuses on the motors) and the audio is full volume. Even if that does happen a 5 amp fuse will blow on the step-up converter (it's replaceable). Also notice the bit of wire that seems misplaced on the left side of the board. That was where I was going to put in a glass fuse, but decided not to. I plan to remake this board on a PCB in the future so I left that. It's probalby not good for 5A that way. It's a start though!
What this circuit does is take power from 2 batteries (in parallel), through a power switch, into some resistors and LED (power indicator), then to all of the devices (Arduino, Boost Converter, Motor Driver, Amplifier, etc.) then finally into a terminal for a current meter (which I may or may not use). The cool thing about this circuit is that if only one battery is connected, it still works just the same.
I've also made a mount for this board as well (click-here to see it). It's more of a slide-in design like some of my audio circuit mounts. When I'm able to manufacture my own circuit boards I'll be able to make them more mountable; one day.
I thought you might want to know what kind of wires I'm using. My personal favorite is the JST SM 2 Pin male and female pairs! They fit well into screw terminals and have a good solid one way connection. The wires are also pretty thick so they should handle the current.
I'm also using various other kinds of wires depending on what it's connected to. To power things like the Arduino and WAV Trigger I'm using male barrel jacks since those components have that already. Earlier in the project I was using small mini micro JST connectors which seem fragile so I stopped using those further into the project. Of course for everything else I'm using standard DuPont Jumper wires! I do consider this project somewhat of a prototype so the jumper wires are not ideal. Suppose my droid hits a wall, that bump causes a very brief open circuit and it might reboot. I think it's just the jumpers doing that, all of the other connectors are very solid connections.
From left to right: JST SM 2 Pin Male & Female, Mini Micro JST 2 Pin Male Wires & Female PCB Connectors, DC Male Barrel Jack (Positive Tip), and DuPont Jumper Wires. Not shown are the Tamiya Wires I use for the batteries that also fit snugly in my large screw terminals.
That covers the power distribution. In the next article I'll start connecting all of the components together and make sure everything works simultaneously!