Solid State Relays

I find solid state relays (SSR) very convenient, especially for controlling AC power lines.  They typically include opto-isolation and zero crossing detection (explained later.)  Also, they can be controlled with as little as 3V.  This means that you can connect them directly to a GPIO pin. However, I still prefer to put a transistor in between to prevent drawing too much current from the GPIO.

All these features do come at a price.  The typical SSR that I use costs about $10.  PC board mounted SSRs that handle small current can be as little as $1 each.  The 40amp SSRs that I used to control the heater for my hot tub cost about $25.  (I will describe that project in a later post.)

There is an important point you need to know about when using SSRs - the AC and DC versions are not interchangeable.  When switching AC current, the relay should only turn on or off when the voltage is at zero during the AC cycle.  This is what "zero crossing detection" does and it prevents a large surge from entering the device you are controlling.  I have used a DC SSR to control a lamp and blown the bulb due to the lack of zero crossing detection.  Also, if you use an AC SSR to control a DC load, it will turn on just fine, but it will never turn off since the SSR never detects the load voltage crossing zero.  This has confused many a hobbyist and now you won't be one of them.

Running From an External Hard Drive

The SD card of the Raspberry Pi is required for booting.  While you may not be able to boot from an external hard drive, moving the root partition there will significantly speed up the Raspberry Pi.  There are many uses where this configuration particularly useful.  File servers and media servers are good examples.  I use a system like this for a MySQL database server.


There are a number of variations to the process, but basically, the steps are:
o  Install OS on SD Card and boot 
o  Plug in external hard drive
o  Partition and Format hard drive
o  Copy system to hard drive
o  Mount the new system and modify configuration to use the hard drive
I decided not to re-invent the wheel.  The following instructions are just my variations on the instructions provided by Rattus here: 
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=26&t=10914&p=129474&hilit=resizefs#p122476  

Note about the sudo command:  All of the following must be done as root.  You must put sudo before each of the commands.  Or, you can follow the (not recommended, bad practice) habit I have and just enter sudo bash to run a shell as root and skip saying sudo all the time.

Load SD Card with Raspbian in the usual way. Update the system software and firmware with the following three commands. This can take quite a while to complete.
apt-get update
apt-get upgrade
rpi-update
reboot

List the partitions This should show only the /boot and / partitions on the SD card.
fdisk –l

Plug external hard drive into USB and list the partitions again. This should show another HD probably at /dev/sda

Modify the partition table for the external drive.
fdisk /dev/sda
Delete partitions by entering “d” and then the number of the partition to delete. Delete all the partitions. Entering “p” will list the partitions.

At this point, you have a few options.
    a) Create a single partition and move the root there. This will leave the swap file on the SD card.
    b) Create a root partition that fills most of the drive and a small swap partition.
    c) Create a partition for the OS, a small swap partition, and a large partition for application data.

I am using the third option.  The following instructions are fairly easy to change to use one of the other options.  While still in the fdisk program, do these steps.

- Create the root partition:  I made mine 16GB.  Enter “n” to create a partition, “p” to select primary partition and then “1” to select partition number one.  Select the default start sector.  Enter +16G to specify the size.  
- Create the swap partition:  Enter “n” to create a partition, “p” to select primary partition and then “2” to select partition number two.  Select the default start sector.  Enter +2G to specify the size.  Enter “t” to set the type of a partition.  Enter “2” and then “82” to make this one a swap partition.
- Create the data partition:  Enter “n” to create a partition, “p” to select primary partition and then “3” to select partition number three.  Select the default start sector and default end sector to fill up the rest of the disk.  
- Enter “p” to list the partitions and verify things are like you expect.
- Enter “w” to write the new partition table and exit.

Create (format) the data partition with an EXTv4 file system. This command takes a few minutes to finish. There is no need to format the first partition since we are going to do a raw copy of an existing file system over it anyway.
mkfs.ext4 /dev/sda3

Initialize the swap partition
mkswap /dev/sda2

Mount the new root partition.
mount /dev/sda1 /mnt

Use rsync to clone the root partition from the sdcard to the new partition on your hard disk, Note the exclude of /mnt. This takes a while.
rsync -avz --exclude '/mnt' / /mnt

Change the mount configuration file on the new root.
vi /mnt/etc/fstab

Change the root device /dev/mmcblk0p2 to be /dev/sda1

Add /dev/sda2 as a swap partition by adding this line.
/dev/sda2 none swap sw 0 0

Finally, stop the system from using the swap file that it normally uses.
rm /mnt/etc/rc?.d/*dphys-swapfile

And delete the old swap file.
rm /mnt/var/swap

Use your editor of choice (I use vi) to modify the boot configuration to use the new root partition. First make a backup copy.
cp /boot/cmdline.txt /boot/cmdline.orig
vi /boot/cmdline.txt Change /dev/mmcblk0p2 to be /dev/sda1

Now the system is ready to reboot. It should come up normally with the external hard drive mounted as root.
reboot

If the system fails, use another machine to copy /boot/cmdline.orig to /boot/cmdline.txt and then reboot again.

One problem that some people have run into is controllers that take too long to complete the USB discovery. This can be compensated for by adding delays in the /boot/cmdline.txt file with the bootdelay and rootdelay options.

Pulse Width Motor Control

My last project had me digging through old robotics parts to find an H-Bridge Motor Controller.  I found one that had several TTL chips on the circuit as well and it reminded me of a trick some of you may find useful.

First, lets define pulse width modulation (PWM.)  It refers to the duty cycle (how long the line is high vs how long it is low) of a pulse stream.

A duty cycle of 50% looks like this:


A duty cycle of 25% looks like this:


And 75% looks like this:


What use is this for controlling motors?  If you have ever tried controlling a motor by simply turning it on and off, you quickly realize how little control you have.  Your robot goes from stopped to zooming at full speed with nothing in between.  If, however, you use PWM to power your motor, then it will be on at full power very briefly and then off, repeatedly.  With PWM you can provide much finer motor control and have your robot move at whatever speed you like.  The higher the duty cycle of the pulse stream, the more power your motor will output.

There are several ways to produce PWM signals from the Raspberry Pi.  The simplest is to turn a GPIO pin on and off in a loop with delays controlling the pulse width.  This is very CPU intensive and not always consistent.  You can use libraries or device drivers to produce PWM from the GPIO that will be consistent and will use virtually no CPU.  Or, you can use external hardware to produce PWM.

Here is how I have done it in the past. (I haven't built a robot using a Pi yet, but did it with simpler microcontrollers years ago.)  Use six GPIO pins:

• Enable (motor is on or off)
• Direction (forward or reverse)
• Power 0
• Power 1
• Power 2
• Power 3

The four power bits control the duty cycle of a pulse stream and allow 16 different power levels.  Below is a block diagram of the circuit I used to do this.
I used a 555 chip for the pulse generator, a 74161 counter, and a 7485 comparator.  Other chips can be used to achieve similar results.  I built my own H bridge circuit back then, but I recommend checking out some of the motor controller chips available now, such as the SN754410 chip.

Gotta Know When to Fold 'em

For the past few months, I have burned up a lot of hobby time trying to build an interface to control the Orbit 58874 sprinkler control valve.  These simply connect in-line with standard water hose connectors and cost about $20.  However, they turned out to be quite complicated to control.


The valve connects with two wires.  Sending a 24V pulse in one polarity will cause the valve to latch open.  Sending it in the reserse polarity will cause it to latch closed.  At least in theory.  And, as a wise professor once told me:  "In theory, practice and theory are the same.  But in practice, they sure as hell aren't."

I had a 24V power supply, but it was a little too powerful.  If you leave the valve powered for more than a second, you can burn out the solenoid coil.  After melting one of my H-Bridge motor controllers, burning out several valves, and losing a couple of months, this project was officially no longer fun.

Fortunately, someone else has already gotten this working.  If you are interested in this, check out http://rayshobby.net/ where you can find interfaces to these valves for Raspberry Pi and Arduino.  I may just break down and buy his OpenSprinklerPi interface.

But now it's time to throw in the towel and move on to other projects.

UPDATE:  I was mistaken about the OpenSprinklerPi.  It will only support standard valves.  That's easy - apply voltage and they turn on.  Stop and they turn off.

So I have returned to this and will be building an entirely new interface board for the sprinklers.  It will probably include an H-Bridge to control output polarity and relays for each sprinkler.

Problems and Distractions

I haven't been able to post in quite a while - far too many things in real life distracting me.  But now that I have gotten back to hobby time, I am expanding my system with an I2C interfaced GPIO extender.  Everything worked great on the workbench, but refused to work at all once installed.  It appeared that the bi-directional buffer chip that I used to connect the Pi to I2C wasn't working, which made no sense.

I broke down and pulled the interface board down and brought it to the workbench.  Under the magnifier, the problem was immediately obvious.

Soldering is an art.  It takes practice, and I was terribly out of practice when I started this blog (and made this board.)  Bottom line, the SDA and SCL lines were shorted by some over-zealous soldering.  Since I hadn't used I2C, it was never a problem.

I have a feeling I am going to replace this board with a completely re-designed version before too long.

USB Hub Modified to Back Feed Power

Most powered USB hubs will not feed power back into the host system.  This is normally a good thing.  However, it is possible to power the Raspberry Pi by back feeding power from a powered USB hub.
Note:  Doing this bypasses the fuse on the Pi, so it is possible to damage it if too much power is drawn into it.  Since all my peripherals will be connected to the hub and GPIO will not be used, then there is little risk of this occurring.  Another note:  The rev 1 Raspberry Pi boards have fuses on the USB ports.  These will provide some protection but will also reduce the power.  I expect that a typical powered USB hub will have adequate power such that this will not be a problem.

Below is a simple modification to make a USB hub backfeed.  The power connector on the hub I used has +5V on the outside of the barrel, and ground on the inside.  There are three connectors on the power socket - one ground and two power connectors.  When the power plug is inserted then power is drawn from the power supply instead of from the host.  This can be overcome by simply connecting the two power connections on the socket.

Now the 2A power supply for the USB hub will power all of the periperals as well as the Raspberry Pi itself.

I need this much power in order to connect an external hard drive.  The purpose for this system will be described in my next blog post.

Low Memory Automatic Reboot

Below is my solution to the memory leak problem that I discovered in the frame buffer interface used by my digital picture frame.  My first thought was to simply schedule a reboot at midnight, but the Raspberry Pi has no internal clock.  My picture frame is not connected to the network, so it never really knows what time it is. Also, the low memory system hang may occur in less than 24 hours.

The better solution is to monitor the amount of free system memory and reboot when it gets too low.  I use the /proc file system to access the system memory status, the awk command to pull out the value that I need, and a bash script to tie it all together.

lowmemreboot.sh

#!/bin/bash
declare -i fmem
while [ true ]
do
        fmem=` awk '/MemFree/ { print $2 }' /proc/meminfo`
        if [ "$fmem" -lt "10000" ]
        then
                echo FREE MEMORY IS LOW----FORCING REBOOT
                reboot
                exit
        fi
        sleep 60
done

Note that the command that awk runs is surrounded with single quotes.  The entire command is surrounded with back quotes.  This causes the output of the command to be saved in the fmem variable.

This script is run from /etc/rc.local the same way that wait-for-gpio-trigger.sh is run.  When the free system memory drops below 10K, the system will reboot and all is well again for a while.  Now I can just let the picture frame run forever and not worry about it.

I would like to see someone write a real slideshow program that does multiple transition types and supports controls such as pause and reverse.  I have too many more interesting projects on my mind, but maybe someone will find the time to do this.