The MagPi - February 2020

07

This might get loud

The siren is capable of an ear-splitting

120 dB. Always wear ear defenders in case you

accidentally set the siren off. This particular model

is tolerant of voltages less than 12 V and the volume

reduces accordingly. We recommend starting no

higher than 5 V, which is still incredibly loud. The

siren requires its own power supply, so you’ll need

to source one (we recommend something with

variable voltages so you can experiment with noise

levels). Before continuing, make sure you’re happy

with the siren’s noise levels. If you don’t want a

loud siren, there are many buzzers and quieter

alarms available that will work to demonstrate the

project and will be much safer for younger makers

to handle.

08

Relay race

Relays are magnetic switches, allowing one

device to control another without their circuits

‘touching’. The Automation HAT comes with three

relays and we’ll use one to switch the siren on and

off. Making sure it’s unplugged, snip the siren’s

positive line (red wire) and strip a bit of wire on

both ends. Now, connect the part going to the

power supply to one of the relays on the terminal

marked ‘COM’ (common). Connect the other part

of the red wire to the same relay on ‘NO’ (normally

open). Double-check everything and make sure the

wires are secure.

09

Software

We can now read an input and create an

output using the relay. Using your favourite editor,

enter the roomguard.py code here. This is a simple

loop that will check the sensor twice a second to

see if movement has been registered. If so, the

relay is switched into place, allowing current to

flow to the siren and it sounds. Once the sensor no

longer registers movement, the relay is deactivated

and it all goes quiet. If you don’t fancy typing it

in, you can download all this code and a few extras

from magpi.cc/roomguard.

To run the code, enter the following on the

command line:

05

Connect the sensor

Although this type of PIR sensor is widely

available, they do vary in wiring. In particular, the

orientation of Vcc (power in) and ground can be

different. The sensor we’ve recommended here

has both clearly marked on the PCB. Connect the

ground pin to any of the GND connectors on the

Automation HAT, then connect any 5 V power

output to the 5 V in pin on the sensor. So that we

know what state the sensor is in, connect the data

line to any one of the buffered inputs on the HAT.

Carefully check each of the screw terminals for a

solid connection.

06

Time for a test

When you installed the software for the

Automation HAT, examples and documentation

were included. We can use these to quickly test and

calibrate the sensor. From the command line, run

the following:

python3 ~/Pimoroni/automationhat/examples/

input.py

You will now be shown the reading from all

the input sensors. Watch the one to which you

connected the data line in the previous step. It

should report a 1 when it is activated (motion has

been detected) and 0 when it ‘times out’. You can

carefully adjust the two potentiometers on the

sensor to fine-tune the sensitivity and timings to

your preferences. Press CTRL+C to stop the Python

script when you’re done.

Always wear ear defenders

in case you accidentally set

the siren off

There’s plenty of

scope for adding

more to the

project, such as an

inexpensive keypad

or a camera

Top Tip

Calibrate

carefully

Be patient when

calibrating

the sensor.

Start with both

potentiometers

turned fully anti-

clockwise, and

work from there.

TUTORIAL

44 magpi.cc Build an internet-connected room guard