Wednesday, December 12, 2018

RocketBot 1: Brains, Brawn, Barometers and Batteries

What is RocketBot made of? We'll go through each part, explaining what it does and A tiny computer, a servo motor, a barometer chip and a battery. This article explains each of the parts that make it tick - the , and also where to get them.


First of all, we need some brains - but not in a zombie way. 

This tiny bundle of goodness is the Wemos D1 Mini. You're looking at a 32-bit microprocessor, 4MB of flash memory, plenty of digital inputs and outputs, and a USB port for programming it. But the really cool bit is that gold wiggly track, which is the aerial for built-in WiFi networking.

WiFi means you can connect to it from a smartphone, so you don't need to mess about mounting switches and displays on your rocket. And you can control your rocket from a safe distance, and see how high it went before it even returns to earth.

OK, WiFi is cool, but now for the fact that really blows my mind. This thing is made out incredibly pure crystals of silicon which are then blasted with X-rays in a vacuum to etch circuits 90 nanometers wide. You could fit 8,000 of those circuits across one human hair, and they can switch on and off 100 billion times a second - it would take you 2,000 years to switch a light switch on and off that many times. Then they connect tiny gold wires from the silicon to the chip pins, and cast the whole thing in a tiny black plastic package. Now multiply all that by the 4 integrated circuits on the Wemos board. Then those chips, and 18 other components, a reset switch and the USB socket are all soldered onto a fibreglass circuit board that is itself a marvel of intricate engineering.

And think about all the design and programming work, all the millions of dollars spent on incredibly high-tech factories, and the robots that assemble things way to small for humans to manipulate. And at a human level, the person who advertises them on the internet, packages them up and posts them, and adds their profit margin to make a living. And the trucks, ships and planes that move them from the factory to warehouse to shop and then halfway round the world to you.


Wow. Just wow.


If the RocketBot is going to do something to make your rocket come down slower than it went up, it needs some muscle - some way to release a catch or a rubber band, which can then pop out a parachute or streamer. Here is RocketBot's muscle:

This is called a servo. It contains a tiny electric motor and lots of gears, which moves it's arm to any angle between 0 and 270 degrees. It also has a way to measure the angle, so it can return to the same position pretty accurately.

We're going to use a 3.7G servo, for two reasons. It's very small and light (3.7 grams), which hurts less if it happens to fall on you from a disintegrating water rocket. It also works fine with the voltage from a single lithium cell, whereas bigger servos need higher voltages. Higher voltage means a heavier battery which means more hurting, and we don't want that!


RocketBot's brain needs senses, specifically a sense for how high it is. You know how your ears pop when you climb a hill? That's because the air pressure goes down as you get higher. So by measuring air pressure, you can work out how high you are. RocketBot uses this nifty little electronic barometer:

This chip is called a BMP280, and as well as a barometer made out of silicon, it contains a thermometer and it's own processor to let it talk to other digital devices. It's amazingly accurate, to about 25cm of height.

And here's another "wow" moment: the barometer, thermometer and communications processor are all inside that 2.5mm square metal component between the big (actually only 3mm across) gold holes.


All these electronic components need electricity, so we need a battery. We've carefully chosen parts that run happily on 3.7 Volts so we can use a small, light (less hurting, remember?) lithium polymer battery:

LiPo (lithium polymer) batteries come in different capacities, measured in milli-Amp-hours (mAh). A 260mAh battery  is about right - it can supply 260mA for an hour, but because RocketBot only draws 50mA it will last for about five hours. The capacity isn't critical, so if you find a larger or smaller capacity, go for it. Bear in mind that larger capacities will be physically bigger and heavier (more hurting).

You will need to connect the battery to the circuit - and also to the charger. There are several kinds of connectors, but the best one is called a micro JST PH connector. Just to make life interesting there are two kinds of JST connector, but luckily they are different colours (red and white). We want the white ones:

So to sum up, you are looking for a 3.7V  260mAh lithium polymer (lipo) battery with a white micro JST PH connector.

Other Stuff

As well as the components mentioned above, you will need tools for soldering, a hot glue gun, a laptop or desktop computer, a USB cable, and a smartphone.

Let's Go Shopping!

Unless you are lucky enough to live in Shenzhen in China, you probably want to order all the parts online. There are several websites you can use:
I happen to like AliExpress, so I've included their search links below. You can usually choose different shipping options, and you may want to pay a little more to reduce the number of days you'll have to wait for things to arrive.
  • Wemos D1 Mini: Search for "Wemos D1 Mini" with a price below $3. You don't need the Pro (16MB) version, you do need the strips of header pins they almost all come with.
  • Servo: Search for "3.7g Servo" with a price around $2.
  • BMP280: Search for "BMP280 3.3" with a price around $1. You need the 3.3V version, which comes with 6 pins. Do not get the 5V version (usually with 4 pins).
  • Battery and charger: Search for "3.7V lipo battery", which will bring up batteries and chargers. For the battery, look for something around 260mAh,  a price below $2, the white micro JST PH connector, and don't worry about the shape. For the charger, look for a price below $1, and make sure it has the white micro JST PH connector. Some of the charger images show a battery, but don't come with it.  
  • Battery cable: Search for "micro JST PH 2.0 male female cable" with a target price under $1. You only need one female cable, but it seems you can only get them in sets with ten male and ten female cables. 
All up, you should be able to get everything for comfortably under $10, including postage. And if some of your friends want to make a RocketBot too, you can save even more by buying in bulk.

What Now?

Now you wait... and wait... and wait. I've made probably fifty orders from AliExpress, and they all arrived eventually.

In the meantime, it's worth looking at the next step. Beg, borrow or buy the other stuff mentioned above, and if you haven't done any soldering before, you might want to do some practising.
  • RocketBot 2: Soldering On
  • RocketBot 3: Get With The Program
  • RocketBot 4: (Optional) Making Sense of Madness

1 comment:

  1. Where are the instructions for "soldering on" and "get with the program?"