Blog 11

Clock based On/off system

In this post I am going to work with crontab. What I am going to show is how you can make it possible to turn a light on or off on special times. I made this as some anti-criminal technique. So, that when I am away the lights are still on at night or day in the house. With that the house doesn't look like a house that has nobody in it.

For this system I am of course going to try if it works with shorter times otherwise I have to wait 8 hours for a lamp to get on or off.

So when I start the code I plan it 1 minute in the future.

I already made a lamp go on and off in blog 02. So I am going to use that same way for turning on and off the lamp.

Working with crontab has to all be done manually:

crontab -e 

has to be done to open crontab for editing

I then have to put this in to make it work to turn the light on at 13:19 and turn the light off at 13:20:

#  mm hh dom mon dow command 
19 13 * * * /usr/local/bin/gpio write 3 1 
20 13 * * * /usr/local/bin/gpio write 3 0 

To read a certain part:

crontab -l |grep bin/gpio 

( With -v you can also let the other parts without bin/gpio appearing.

For the people who don't understand:

• 1 (mm)Minute 0-59
• 2 (hh)Hour 0-23 (0 = midnight)
• 3 (dom)Day 1-31
• 4 (mon)Month 1-12
• 5 (dow)Weekday 0-6 (0 = Sunday)

So the text from my is now saying:

gpio write 3 1 at 13:19
gpio write 3 0 at 13:20

To manually change it in the command line:

( echo "#  mm hh dom mon dow command" ; echo '19 13 * * * /usr/local/bin/gpio write 3 1' ;  echo '20 13 * * * /usr/local/bin/gpio write 3 0' ) |  crontab 

( You can of course also use it as alarm clock, then you only have to change the command to the location of the audio file you want to play.)