The final product

Setting up serial on the RPi

Enable serial communication and disable login over serial, which you likely don’t need.

sudo nano /boot/config.txt
# add this line at the bottom, or make sure it's set to 1:   enable_uart=1

# reboot
shutdown -r now

# remove the login service over serial
sudo systemctl stop serial-getty@ttyAMA0.service
sudo systemctl disable serial-getty@ttyAMA0.service

# edit cmdline and remove the line that looks like this: console=serial0,115200
sudo nano /boot/cmdline.txt

# example contents of cmdline.txt:
cat /boot/cmdline.txt
dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait ipv6.disable=1

# reboot
shutdown -r now

# check that /dev/serial points to ttyAMA0
ls -l /dev/serial*
# output:
lrwxrwxrwx 1 root root 7 Feb  1 22:15 /dev/serial0 -> ttyAMA0

Credits: http://spellfoundry.com/2016/05/29/configuring-gpio-serial-port-raspbian-jessie-including-pi-3/

My serial hardware – an 8 port relay box

In my case, I need to connect to this 8-relay box:

(purchased from: QuasarElectronics.co.uk )

and issue commands to turn on any of the 8 relays, using the on commands: (N1, N2, … N8), or off commands (F1, F2, … F8 )

Testing in Windows

In Windows this works right off the bat, with a regular serial cable, and this realterm program from https://sourceforge.net/projects/realterm/ :

Download a terminal application and change the baud rate to 9600, and OPEN the serial connection.

To turn ON relay 1, use N1, to turn it off, use N0. Example run from Windows to Quasar Electronics relay box, actually working:

Buy a converter

Like this male one: https://www.amazon.com/gp/product/B00OPU2QJ4/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

Solder some wires

Wire up the converter, VCC, ground, transmit, and receive.

If you bought the male converter, you wire up the converter’s transmit to the RPi receive, and the converter’s receive (RX) to the RPi transmit (TX). If you bought the female converter, you wire them up one-to-one, transmit to transmit, receive to receive.

Issue serial commands from the RPi

Use the “screen” command:

screen /dev/serial0 9600

You won’t see what you’re typing, since echo is likely off. I tested passing the “turn on relay 1 command”, or “N1”, and then off, “F1”, as simple text, and it works.

To get out of the “screen” program, you type Control + A  ,  D

Using minicom and then giving up

You can also install minicom: “apt-get install minicom” and then issue commands:

minicom --baudrate 9600 --device /dev/serial0 --8bit

which ignores your settings, and you have to go to options (Control A O )

and don’t forget to type A and change /dev/tty8 to /dev/serial0 , of course you’ll forget.

And then you’ll still be offline, because this doesn’t work, and you decide that using “screen” is better, and you can automate that with scripts.

To exit minicom, Control + A, x, <enter>

Scripts to automate it all

This creates a screen session, and saves its name in a file. Runs only one time, at reboot. All other scripts to turn on/off relays use this screen session.

touch /home/pi/startscreen.sh
chmod a+x /home/pi/startscreen.sh
nano /home/pi/startscreen.sh

#!/bin/bash
sudo chmod a+rw /dev/serial0


# store session in file
myScreenSession="alan1"


echo "$myScreenSession" > /home/pi/session.txt
# set up session
screen -dmS "alan1" /dev/serial0 9600


# list screens
screen -list

Edit your crontab to set it up to run at reboot

crontab -e

and set its contents to:

@reboot /home/pi/startscreen.sh

Try it manually as well:

/home/pi/startscreen.sh

Reboot: sudo shutdown -r now

Check to see that the screen is running:

screen -list

Set up bash scripts to turn ON or OFF each relay

Try it manually to make sure it works. We’ll issue “screen” “session name” “command” and then the <ENTER> character. And we must not forget the semicolon at the end.

screen -S "alan1"  -X stuff "N1"`echo -ne '\015'`;

… and relay 1 clicks on, and the beacon turns.

Let’s quickly turn it off:

screen -S "alan1"  -X stuff "F1"`echo -ne '\015'`;

Relay 1 on, we’ll call this “on1.sh”, and relay 1 off, we’ll call this “off1.sh”

touch /home/pi/on1.sh
chmod u+x /home/pi/on1.sh
nano /home/pi/on1.sh

#!/bin/bash
myScreenSession=`cat /home/pi/session.txt`
if [[ -z "$myScreenSession" ]]; then
  echo "myScreenSession is not defined"
else
  echo "Ok, myScreenSession is: $myScreenSession"
  screen -S "$myScreenSession" -X stuff "N1"`echo -ne '\015'`;
fi

and the off script:

touch /home/pi/off1.sh
chmod u+x /home/pi/off1.sh
nano /home/pi/off1.sh

#!/bin/bash
myScreenSession=`cat /home/pi/session.txt`
if [[ -z "$myScreenSession" ]]; then
  echo "myScreenSession is not defined"
else
  echo "Ok, myScreenSession is: $myScreenSession"
  screen -S "$myScreenSession" -X stuff "F1"`echo -ne '\015'`;
fi

Execute them: