In the last post I extracted my motors and built some basic prototypes for mounting them to the drone. In this post, I’ll run through all the bits I’ve gathered so far and the specifications we can gather from them.
The Arduino Pro Mini mentioned in the starting brief uses a 3.3V VCC line; that is the high voltage output is regulated to 3.3V. The current output is max 40mA per pin, so here the requirement is for voltage. My SN754410 motor driver similarly requires a minimum of 4.5V, but typically to run motors, a good current supply is needed. Consequently, I need batteries that supply voltage and current, with minimum weight. Normal Alkaline batteries such as AAAs are too bulky to fit inside my tiny Altoids tin, so I have turned to LiPo. Nine Eagles do an excellent and extensive range of parts for RC cars, helicopters and now, drones.
For my drone, I selected a 3.7V battery with 350mAh and 25C, which should supply plenty of current to my motors. When you pick up rechargeable batteries, make sure you get a charger too – and as I forgot, something to plug your batteries into. Nine Eagles do a range of batteries with different heads, so they make very handy adapter cables. These are perfect for any project; you can just snip them and tin the ends to make a perfect jumper cable for your battery. Importantly, it means the connector is wired and colour-coded for you, so there’s no chance of mixing the polarity of the connector.
When I put my jumper wires on, they really didn’t want to come off. I enthusiastically thought, “Hmm, I don’t want to damage my connectors with pliers so I’ll wedge in a trusty screwdriver and prize them apart.” And so I did. You’ve probably seen the slight flaw in my plan, and yes, there was smoke. In lieu of more haste, I’m filing down cable the connectors so that they will come off the battery easier.
I have searched the shops high and low for rotors. These have presented the biggest challenge, but G-Force came to the rescue with some inexpensive rotors. The best bit is that because they are for drones (not RC helicopters), they are designed to mount straight to a standard DC model motor core which makes life very easy.
My hoard so far
When you are dealing with space constraints, it really pays to go to a shop first rather than buy online. That way, you can get a good sense of how big something is and how suitable it is before you buy it. Here are the parts I have amassed thus far:
I also have some 0.3mm and 0.5mm aluminium sheeting for making my rotor arms.
Space inside the drone is very tight (if I keep to the smaller tin) and the battery takes up a lot of space. As such, I’m going to try and see what the Arduino can achieve without a motor driver attached. 40mA would be no where near enough for motors powering wheels, but essentially the resistance to motion comes from the air, not the weight of the drone, so theoretically I can get away with a smaller current. I’ll give it a go anyway. The drone will be lighter without the driver, capacitors, wires a smaller PCB, which is again beneficial.
At this point, it’s a good idea to weigh all your components and see how much power you’ll need to get them off the ground – literally. knowing the weight will help you to calculate the size of the rotors you’ll need, the power and speed of the motors and really help you plan it properly. For now at least, I’m winging it because the rotors and metal I have to hand were cheap enough to warrant just giving them a go.
In the next post, I’ll build a rough-and-ready model to see if my setup can get off the ground. Fingers crossed!