HackSpace_-_April_2020

FORGE

happens that on the LOLIN32 that we’re using, there
are three pins in the right order with pin 13 above 5 V
and ground. Not all servos have the pins in this order,
so check yours before plugging it in. If your servo
doesn’t line up with the pins on your microcontroller,
then just use some header wires, a breadboard, or
solder them in place.
Servos are controlled with PWM signals; however,
the ESP32 handles PWM in a slightly different way
to most Arduino-compatible boards, so you can’t
just use analogueWrite. Fortunately for us, there’s
an Arduino library that does all the heavy lifting –
ESP32Servo. You’ll need to install this via the library
manager; then you can include it with the line:

#include <ESP32Servo.h>

We need a few bits to get it set up and running with
our servo on IO13:

Servo myservo;

int pos = 0;

int servoPin = 13;

void setup() {

myservo.setPeriodHertz(50);

myservo.attach(servoPin, 500, 2500);

}

The two numbers on the myservo.attach are the
minimum and maximum pulse lengths. You might
need to tweak them a little for your servo, but these
worked for us.
The only thing we need to do now is use myservo.
write(pos) to set the servo to pos number of degrees.

We need to convert our number of micrograms of

NO 2 to a position for the servo, and we can do this

with the line:

myservo.write(min(no2.toFloat()*4.5,180.0));

The data for NO 2 that we extracted from the

JSON may look like a number, but it’s actually text

containing the digits. We can use the toFloat()

method to change this into a float data type.

Multiplying this by 4.5 converts it into the position

of the servo, but in the event that Bristol exceeds its

legal air quality limits, we need to use a min function

to make sure that we don’t try and go beyond the

range of the servo.

That’s the code sorted; it’s time to set everything

up physically. Generally, servos come without their

arms attached, so the first job is to attach the arm

pointing in the correct direction. When you first

turn on the build, the servo will go to zero degrees,

so at this point you can attach the arm. Of course,

you don’t want just a bare arm, so you can adorn it

with whatever decoration you like. We went with

an arrow, but you can be more creative if you like.

There should be holes for small screws to attach

decoration to the servo arm, but for something like

this with little force, a drop of superglue does the job

just as well.

That’s all there is to making a simple data

visualiser. What will you make yours display?

Below

Open Data Bristol has a dashboard for viewing the live air

quality figures, as well as an API that lets you get the data

from the back end

Above

It’s easy to over-complicate data displays – sometimes a bit

of card glued to a servo is all you need

DATA

There are loads

of data feeds

around, but

often the most

useful ones

are the ones

local to you. We

looked at using

a weather feed

in issue 24, and

you could easily

adapt this little

pointer to display

temperature,

wind speed, or

something else.

There’s a

wide range of

transport feeds

from trains to

road networks

that could equally

drive a similar

display, should

that information

be useful.

You could

also build your

own sensor and

have it set up to

serve information

as JSON and

create your

own personal

data network if

you wish.

FEEDS