Posted by Carmen Zhang on

Using the Raspberry Pi to control AC electric power

VERY IMPORTANT NOTE: THIS IS NOT A HOW-TO ARTICLE. This article explains how the author used a Raspberry Pi to control electric current. However, the author is NOT an electrician, and just because he did something doesn’t mean YOU should, particularly if you are unfamiliar with how to wire electrical devices safely. WIRING A DEVICE IMPROPERLY CAN RESULT IN FIRE, PROPERTY DAMAGE, BODILY INJURY, AND/OR DEATH! So DO NOT just blindly do what the author has done, unless you are certain you know what you are doing and are willing to take full responsibility for your own actions! This article is for INFORMATIONAL PURPOSES AND DISCUSSION ONLY, and is NOT intended to be a guide to building any device!

WE ARE NOT RESPONSIBLE IF YOU ATTEMPT TO BUILD ANYTHING BASED ON WHAT YOU SEE IN THIS ARTICLE AND INJURE YOURSELF OR HAVE PROPERTY DAMAGE AS A RESULT. STOP READING THIS ARTICLE NOW IF YOU ARE UNWILLING TO TAKE FULL RESPONSIBILITY FOR YOUR OWN ACTIONS!!!! AGAIN, THIS IS ONLY TO REPORT ON WHAT THE AUTHOR OF THE ARTICLE DID, AND WE ARE NOT SUGGESTING THAT YOU DO LIKEWISE, PARTICULARLY IF YOU DO NOT FULLY UNDERSTAND AND APPRECIATE THE DANGER OF WORKING WITH LIVE ELECTRIC CURRENT! PLEASE CONSULT WITH A LICENSED ELECTRICIAN BEFORE ATTEMPTING TO BUILD ANYTHING THAT WILL UTILIZE HOUSEHOLD ELECTRICAL CURRENT!

 

We were trying to figure out if a Raspberry Pi could be made to control a device that is powererd by 120 volts AC. Our first idea was to build a device based on this wiring diagram, which would have let us control two electric outlets individually, using GPIO pins 17 and 18 on the Raspberry Pi. We tried using a Solid State Relay rated at 25 Amps rather than the 40 Amp one shown in the diagram, but the wiring of the two devices is the same (click on the diagram to enlarge):

What we found was that the above method worked IF you get good Solid State Relays. Unfortunately, we purchased two of them, and only one of them would work with the Raspberry Pi. While the rated trigger voltage on the label is 3 – 32 volts DC, we found that one of the relays we purchased required something closer to four volts to trigger. We then found other reports where other purchasers of those types of relays have encountered similar issues. Some people could buy several and find they all worked, others were not so fortunate and would have issues like we did.

We returned the bad one for a refund, and searched for another solution. But we do note that some of those Solid State Relays such as the ones shown in the diagram can handle up to 40 Amps, assuming you use a good heat sink, and even the less expensive units we purchased can handle up to 25 Amps with a heat sink, so had we been trying to control a heavy load we might have been willing to order another one and try it. It’s quite possible that these relays would work better with a proper transistor and resistor added to the control circuit (and connecting one leg of the transistor to the +5 Volt supply on the Raspberry Pi), but since we aren’t electronic engineers we did not attempt to design such a circuit.

We then tried this:

The above circuit uses a SainSmart 2 Channel 5V Solid State Relay Module Board. Unlike the Solid State Relays mentioned earlier, these devices do require a +5 volt power connection, but they only require a trigger voltage of 2.5V – 20V. We purchased one of those and tried it, and had no problem triggering it from the Raspberry Pi.

The only downside is that these devices are only rated to handle 2 Amps (Volts x Amps = Watts, so if your voltage is 110 volts then that means you could in theory use up to a 220 watt load. Remember that motors and some other devices can draw many times their rated load for a second or two when starting up). We only wanted to switch power to devices that draw perhaps a couple dozen Watts at most, so the 2 Amp limitation wasn’t an issue.

EDIT August 26, 2013: Someone (not us) posted a link to this article on Reddit, where in the comments it was criticized for showing a regular outlet in a circuit that can only draw 2 Amps. One commenter labeled this “a pretty irresponsible article” and noted that “using a 2 amp rated, but normal looking outlet with nothing to interrupt the circuit if it is overloaded sounds like a terrible idea and risks starting a fire. It is not hard to exceed 2 amps.” But as someone else noted, you can’t readily purchase a special outlet that is intended for use only with smaller loads. We only show the outlet to illustrate how a small device, for example a device powered by a typical “wall wart” power supply that only draws a few watts, could be controlled. The diagrams above are for conceptual and illustrative purposes only – we are NOT actually advising you to build any such circuit or to use that type of outlet, but if you nevertheless attempt to build such a thing and thereby assume all the risks of doing so, we at the very least suggest you affix a conspicuous label showing the maximum amperage and wattage ratings (for example: MAXIMUM RATING 2 AMPS / 220 WATTS PER OUTLET or whatever is appropriate for the circuit you build). It might also be a good idea to put some type of protection, such as a 2 amp fuse or circuit breaker, in each leg of the circuit between the hot side of the outlet and the Solid State Relay Module Board. But again, this is not and was never intended to be a hardware construction article, so if you attempt to build anything such as this, please be sure you know what you are doing!

By the way, SainSmart also makes these Solid State Relay Module Boards in 4-channel and 8-channel models.

We were a little concerned that the power supply used to power the Raspberry might not support the additional load. According to the description on the SainSmart site, the board we used requires 5 Volts DC at 160mA. According to this page on the Raspberry Pi web site, “Model B owners using networking and high-current USB peripherals will require a supply which can source 700mA (many phone chargers meet this requirement). Model A owners with powered USB devices will be able to get away with a much lower current capacity (300mA feels like a reasonable safety margin).” We aren’t using any USB peripherals at all, let alone high-current ones, and the power supply we are using produces 1000mA (1 Amp), so it would appear that the power consumption is well within the power supply’s capacity. And, we haven’t had any power issues.

NOTE: Devices shown in the diagrams are NOT exactly to scale.

Controlling the device:

Controlling the device is a matter of turning the GPIO pins on or off. We came up with a couple of simple bash scripts to do this:

For GPIO17 (filename /root/gpio17.sh):

#!/bin/bash

if [ -z $1 ]
then
    opt="toggle"
elif [ -n $1 ]
then
    opt=$1
fi

let "sleep = $RANDOM + 10000"
sleep "0.$sleep"

if [ $(pgrep gpio17.sh|wc -w) -gt "2" ] ; then
    exit
fi

if [ ! -e "/sys/class/gpio/gpio17/value" ]
then
    echo "17" > /sys/class/gpio/export
    echo "out" > /sys/class/gpio/gpio17/direction
fi

case $opt in
    on)
        echo 1 > /sys/class/gpio/gpio17/value
        ;;
    off)
        echo 0 > /sys/class/gpio/gpio17/value
        ;;
    toggle)
        value=`cat /sys/class/gpio/gpio17/value`
        if [ $value -ne 0 ]
        then
            echo 0 > /sys/class/gpio/gpio17/value
        else
            echo 1 > /sys/class/gpio/gpio17/value
        fi
        ;;
    reboot)
        echo 0 > /sys/class/gpio/gpio17/value
        sleep 30
        echo 1 > /sys/class/gpio/gpio17/value
        ;;
    status)
        exit
        ;;
    *)
        echo "Invalid option - use on, off, toggle, or reboot (toggle is the default)."
        exit
        ;;
esac

sleep 3

And for GPIO18 (filename /root/gpio18.sh):

#!/bin/bash

if [ -z $1 ]
then
    opt="toggle"
elif [ -n $1 ]
then
    opt=$1
fi

let "sleep = $RANDOM + 10000"
sleep "0.$sleep"

if [ $(pgrep gpio18.sh|wc -w) -gt "2" ] ; then
    exit
fi

if [ ! -e "/sys/class/gpio/gpio18/value" ]
then
    echo "18" > /sys/class/gpio/export
    echo "out" > /sys/class/gpio/gpio18/direction
fi

case $opt in
    on)
        echo 1 > /sys/class/gpio/gpio18/value
        ;;
    off)
        echo 0 > /sys/class/gpio/gpio18/value
        ;;
    toggle)
        value=`cat /sys/class/gpio/gpio18/value`
        if [ $value -ne 0 ]
        then
            echo 0 > /sys/class/gpio/gpio18/value
        else
            echo 1 > /sys/class/gpio/gpio18/value
        fi
        ;;
    reboot)
        echo 0 > /sys/class/gpio/gpio18/value
        sleep 30
        echo 1 > /sys/class/gpio/gpio18/value
        ;;
    status)
        exit
        ;;
    *)
        echo "Invalid option - use on, off, toggle, or reboot (toggle is the default)."
        exit
        ;;
esac

sleep 3

Be sure to make the scripts executable. If you want to control any of the other GPIO pins, just copy the text of either of the above scripts into a text editor (one that will not change the line endings) and then do a global search and replace for the pin number – for example, if you use the script for pin 18, and you want to control pin 4, just replace all occurrences of “18″ with “4″, and then save the script as /root/gpio4.sh and make it executable. Besides 4, 17, and 18, other GPIO pin numbers you can use are 22, 23, 24, 25, and 27 – that’s on a Revision 2 board, but if you happen to have a Revision 1 board then substitute 21 for 27. And if you think the GPIO pin numbering on a Raspberry Pi makes absolutely no sense at all, you are not alone, since the GPIO pin numbers are not in any logical sequence, and bear no relationship whatsoever to the actual positions of the pins on the board. The best thing you can do is look at a chart such as this one to help you keep track of which pin is which.

(Strictly speaking, you may be able to control certain other pins on the Raspberry Pi as well, but the eight GPIO pins mentioned in the previous paragraph should be your first choices for any projects of this type, and controlling any of the others is beyond the scope of this article. Also, note that the wiring diagrams above show Revision 2 boards, which have slightly different pinouts from Revision 1 boards, although none of the pins actually used for wiring in those diagrams are different between revisions. This article assumes you have a Revision 2 board, so if you have Revision 1, substitute any references to pin 27 with pin 21).

The scripts accept four arguments – on, off, toggle, or reboot (actually it also accepts a fifth, status, but that is for a special purpose and it not really intended for use directly from a command prompt). toggle is the default if no argument is specified.

There is a small section at the top of each script that may confuse some readers:

let "sleep = $RANDOM + 10000"
sleep "0.$sleep"

if [ $(pgrep gpio17.sh|wc -w) -gt "2" ] ; then
    exit
fi

That section delays a random amount of time (0.1 to 0.42767 seconds), then checks to see if the script is already running, without writing anything to the Raspberry Pi’s SD card (which could shorten the card’s lifespan if done too frequently). If the script is already running it bails out. While it is unlikely that a user would attempt to start the same script twice, this pretty much guards against it. That’s also the point of the “sleep 3″ at the end of the script, which keeps devices from being toggled on or off too quickly. If you know of a better way to do this, feel free to modify the script, and also to post your suggested modifications in the comments (but, please do not suggest reading man pages. If that is all you can be bothered to offer, please just move along).

The reason we used two scripts for the two pins, rather than just one with an added option to specify which pin to control, is so that you could control the two different pins simultaneously without being denied access to one because the script is already running on the other.

The options should be pretty obvious:

on – turn the pin on
off – turn the pin off
toggle – if the pin is currently off, turn it on, and vice versa (this is the default if no option is specified)
reboot – turn the pin off for 30 seconds, then turn it on

The hidden one, status, is a do-nothing command that only makes sure that the pin is initialized, but does not change its state. It is meant to be used when you are calling this script from Asterisk. Yes, you can do that, once the above scripts are enabled and you know they are working properly!

Controlling the device from Asterisk/FreePBX:

This section assumes that the Raspberry Pi is running Asterisk for Raspberry Pi (RasPBX), though it should work with any other FreePBX-based distribution that runs on the Raspberry Pi. To allow Asterisk to control the GPIO pins, you could add the following lines to /etc/asterisk/extensions-custom.conf (note this requires a few custom recordings and a bit of additional configuration, which will be discussed in a moment):

[custom-picontrol]
exten => s,1,Set(TIMEOUT_LOOPCOUNT=0)
exten => s,n,Set(INVALID_LOOPCOUNT=0)
exten => s,n,Set(_IVR_CONTEXT_${CONTEXT}=${IVR_CONTEXT})
exten => s,n,Set(_IVR_CONTEXT=${CONTEXT})
exten => s,n,Set(__IVR_RETVM=)
exten => s,n,GotoIf($ ["${CDR(disposition)}" = "ANSWERED"] ?skip)
exten => s,n,Answer
exten => s,n,Wait(1)
exten => s,n(skip),Set(IVR_MSG=custom/please-enter-rpi-pin)
exten => s,n(start),Set(TIMEOUT(digit)=3)
exten => s,n,Set(PIGPIOPIN=Background(${IVR_MSG}))
exten => s,n,ExecIf($ ["${IVR_MSG}" != ""] ?Background(${IVR_MSG}))
exten => s,n,WaitExten(5,)
exten => 4,1(ivrsel-4),Set(PIGPIOPIN=4)
exten => 4,n,Goto(custom-validpipin,s,1)
exten => 17,1(ivrsel-17),Set(PIGPIOPIN=17)
exten => 17,n,Goto(custom-validpipin,s,1)
exten => 18,1(ivrsel-18),Set(PIGPIOPIN=18)
exten => 18,n,Goto(custom-validpipin,s,1)
exten => 22,1(ivrsel-22),Set(PIGPIOPIN=22)
exten => 22,n,Goto(custom-validpipin,s,1)
exten => 23,1(ivrsel-23),Set(PIGPIOPIN=23)
exten => 23,n,Goto(custom-validpipin,s,1)
exten => 24,1(ivrsel-24),Set(PIGPIOPIN=24)
exten => 24,n,Goto(custom-validpipin,s,1)
exten => 25,1(ivrsel-25),Set(PIGPIOPIN=25)
exten => 25,n,Goto(custom-validpipin,s,1)
exten => 27,1(ivrsel-27),Set(PIGPIOPIN=27)
exten => 27,n,Goto(custom-validpipin,s,1)
exten => i,1,Set(INVALID_LOOPCOUNT=$ [${INVALID_LOOPCOUNT}+1] )
exten => i,n,GotoIf($ [${INVALID_LOOPCOUNT} > 3] ?final)
exten => i,n,Set(IVR_MSG=custom/sorry-not-valid-rpi-gpio)
exten => i,n,Goto(s,start)
exten => i,n(final),Playback(no-valid-responce-transfering)
exten => i,n,Goto(app-blackhole,hangup,1)
exten => t,1,Set(TIMEOUT_LOOPCOUNT=$ [${TIMEOUT_LOOPCOUNT}+1] )
exten => t,n,GotoIf($ [${TIMEOUT_LOOPCOUNT} > 3] ?final)
exten => t,n,Set(IVR_MSG=no-valid-responce-pls-try-again&custom/please-enter-rpi-pin)
exten => t,n,Goto(s,start)
exten => t,n(final),Playback(no-valid-responce-transfering)
exten => t,n,Goto(app-blackhole,hangup,1)
exten => return,1,Set(_IVR_CONTEXT=${CONTEXT})
exten => return,n,Set(_IVR_CONTEXT_${CONTEXT}=${IVR_CONTEXT_${CONTEXT}})
exten => return,n,Set(IVR_MSG=custom/please-enter-rpi-pin)
exten => return,n,Goto(s,start)
exten => h,1,Hangup
exten => hang,1,Playback(vm-goodbye)
exten => hang,n,Hangup

[custom-validpipin]
exten => s,1,Set(TIMEOUT_LOOPCOUNT=0)
exten => s,n,Set(INVALID_LOOPCOUNT=0)
exten => s,n,Set(_IVR_CONTEXT_${CONTEXT}=${IVR_CONTEXT})
exten => s,n,Set(_IVR_CONTEXT=${CONTEXT})
exten => s,n,Set(__IVR_RETVM=)
exten => s,n,GotoIf($ ["${CDR(disposition)}" = "ANSWERED"] ?skip)
exten => s,n,Answer
exten => s,n,Wait(1)
exten => s,n(skip),Set(IVR_MSG=custom/rpi-gpio-pin-state-selection)
exten => s,n(start),Set(TIMEOUT(digit)=3)
exten => s,n,ExecIf($ ["${IVR_MSG}" != ""] ?Background(${IVR_MSG}))
exten => s,n,WaitExten(5,)
exten => 0,1(ivrsel-0),Set(PIPINSTATE=off)
exten => 0,n,Goto(custom-pincontrol,s,1)
exten => 1,1(ivrsel-1),Set(PIPINSTATE=on)
exten => 1,n,Goto(custom-pincontrol,s,1)
exten => 2,1(ivrsel-1),Set(PIPINSTATE=toggle)
exten => 2,n,Goto(custom-pincontrol,s,1)
exten => 3,1(ivrsel-1),Set(PIPINSTATE=reboot)
exten => 3,n,Goto(custom-pincontrol,s,1)
exten => 4,1(ivrsel-1),Set(PIPINSTATE=status)
exten => 4,n,Goto(custom-pincontrol,s,1)
exten => i,1,Set(INVALID_LOOPCOUNT=$ [${INVALID_LOOPCOUNT}+1] )
exten => i,n,GotoIf($ [${INVALID_LOOPCOUNT} > 3] ?final)
exten => i,n,Set(IVR_MSG=no-valid-responce-pls-try-again)
exten => i,n,Goto(s,start)
exten => i,n(final),Playback(no-valid-responce-transfering)
exten => i,n,Goto(app-blackhole,hangup,1)
exten => t,1,Set(TIMEOUT_LOOPCOUNT=$ [${TIMEOUT_LOOPCOUNT}+1] )
exten => t,n,GotoIf($ [${TIMEOUT_LOOPCOUNT} > 3] ?final)
exten => t,n,Set(IVR_MSG=no-valid-responce-pls-try-again&custom/rpi-gpio-pin-state-selection)
exten => t,n,Goto(s,start)
exten => t,n(final),Playback(no-valid-responce-transfering)
exten => t,n,Goto(app-blackhole,hangup,1)
exten => return,1,Set(_IVR_CONTEXT=${CONTEXT})
exten => return,n,Set(_IVR_CONTEXT_${CONTEXT}=${IVR_CONTEXT_${CONTEXT}})
exten => return,n,Set(IVR_MSG=custom/rpi-gpio-pin-state-selection)
exten => return,n,Goto(s,start)
exten => h,1,Hangup
exten => hang,1,Playback(vm-goodbye)
exten => hang,n,Hangup

[custom-pincontrol]
exten => s,1,Noop(GPIO pin selected is ${PIGPIOPIN} and pin state is ${PIPINSTATE})
exten => s,n,System(sudo /root/gpio${PIGPIOPIN}.sh ${PIPINSTATE})
exten => s,n,Playback(custom/pin)
exten => s,n,SayNumber(${PIGPIOPIN})
exten => s,n,Playback(en/ha/is)
exten => s,n,ExecIf($ ["${SHELL(cat /sys/class/gpio/gpio${PIGPIOPIN}/value):0:1}" = "0"] ?Playback(en/ha/off):Playback(en/ha/on))
exten => s,n,Goto(app-blackhole,hangup,1)
exten => h,1,Hangup

The [custom-picontrol] context above actually allows you to enter any of the possible GPIO pins (available to the user) on the Raspberry Pi, but in our example we’re only using pins 17 and 18, so if you don’t plan on using any other GPIO pins, you may want to remove all the lines that start with exten => N, where N is any number other than 17 or 18. The first two contexts are based on FreePBX IVR logic, except that instead of using the selection to transfer to an extension or other destination, the choices made are stored in variables for use by the [custom-pincontrol] context.

In order for this to work, there are a few extra steps you must take. First, you must give Asterisk permission to run the scripts. Add these lines to the end of /etc/sudoers:

asterisk ALL = NOPASSWD: /root/gpio17.sh
asterisk ALL = NOPASSWD: /root/gpio18.sh

If you want to allow Asterisk to control all the GPIO pins rather than just 17 and 18, here’s a complete list of the lines to add to /etc/sudoers. Note that you must create the additional scripts and make them executable:

asterisk ALL = NOPASSWD: /root/gpio4.sh
asterisk ALL = NOPASSWD: /root/gpio17.sh
asterisk ALL = NOPASSWD: /root/gpio18.sh
asterisk ALL = NOPASSWD: /root/gpio22.sh
asterisk ALL = NOPASSWD: /root/gpio23.sh
asterisk ALL = NOPASSWD: /root/gpio24.sh
asterisk ALL = NOPASSWD: /root/gpio25.sh
asterisk ALL = NOPASSWD: /root/gpio27.sh

You must also create some system recordings and place them in the /var/lib/asterisk/sounds/custom directory. These can be added using the FreePBX System Recordings module, which is actually the preferred way because it will make sure all the permissions are set correctly. These are the four new recordings you need, and the suggested scripts:

please-enter-rpi-pin.wav: “Please enter the Raspberry Pi GPIO pin number you wish to control:”
rpi-gpio-pin-state-selection.wav: “Press zero to turn the GPIO pin off, one to turn it on, two to set it to the opposite of its current state, three to initiate a reboot sequence, or four to hear the current status of the pin.”
sorry-not-valid-rpi-gpio.wav: “I’m sorry, that is not a valid Raspberry Pi GPIO pin number. Please try again.”
pin.wav: “Pin”

If you are handy with an audio editor such as Audacity, you can probably slice the word Pin off of one of the existing recordings that start with the word “Pin” (found in /var/lib/asterisk/sounds/en for English-language speakers). For the others, you can record them yourself, or you could perhaps use text-to-speech software to generate them. There’s also the option of using flite text-to-speech synthesis if it is installed on your system, but you’ll need to modify the contexts accordingly (we don’t care for the sound of flite, so you are on your own with that).

Finally, you must go into FreePBX and under Admin/Custom Destinations, add a Custom Destination called “Raspberry Pi control”, then give it this destination:

custom-picontrol,s,1

Then go into Applications/Misc Applications and add a new Misc Application called “Raspberry Pi Control” or whatever you want. Give it an extension number you want to use when controlling pins on your Raspberry Pi – we used 774 (“RPI” on a standard touch-tone keypad). For the destination, select “Custom Destinations” and “Raspberry Pi control”, then apply the changes and you should be all set.

Other ways to control the pins:

There are many other ways you can control the pins. You can even do it from another computer on your network. For example, we have set up SSH Passwordless Logins from another computer on our local network. Having done that, we can create scripts on that computer that look like this (note you do NOT put these on the Raspberry Pi!):

In ~/gpio17.sh place these two lines:

#!/bin/bash
ssh root@IP_address_of_Raspberry_Pi "/root/gpio17.sh $1"

In ~/gpio18.sh place these two lines:

#!/bin/bash
ssh root@IP_address_of_Raspberry_Pi "/root/gpio18.sh $1"

Replace IP_address_of_Raspberry_Pi with the IP address of your Raspberry Pi – of course, this works best if the Raspberry Pi is always on the same IP address.

Now we can run the bash scripts from a another system, just as if we were logged into the Raspberry Pi. But the beauty of this is that you can call those scripts from other software on the system. Let’s say the other system is a home theater PC, and it has a ~/.lircrc file to accept commands from the remote. You could possibly add lines like this (note these are dependent on the remote you are using, and are just examples):

begin
 prog = irexec
 button = KEY_RED
 config = ~/gpio17.sh &
 repeat = 0
end

begin
 prog = irexec
 button = KEY_BLUE
 config = ~/gpio18.sh &
 repeat = 0
end

Now every time you press the red button on the remote, GPIO17 on the Raspberry Pi would toggle to the opposite of its current state (since no option is specified). And if you pressed the blue button, GPIO18 would toggle. So you could use a remote on a different system to send commands to the Raspberry Pi to power a device on or off. Or you could create a script or program that switches things on or off at certain times. We’ve actually done this so we know it works! It seems that the possibilities are endless. I could envision building something like an automatic lawn watering system controller that could go out and download the weather report, and then decide to water or not water based on the probability of significant amounts of rain. That’s just one example of the sort of thing you could do if you have the ability and the expertise.

Again, should you foolishly ignore our advice to not try building anything like this unless you know how to protect yourself when working with household electrical current, PLEASE remember to use extreme caution! We would really hate to see anyone kill or injure themselves, or set their house on fire (but we will not be responsible if you do that, because we’ve warned you every way we can think of!).


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