Wednesday, November 11, 2015

Improved Alarm/Automation System

A Little History

I have been fascinated (some would say obsessed) with home automation for many years. I even had a paper published on the subject in 1978, so I experimented with many options before the Raspberry Pi came along. My first working system was based on an Atari 400. I then went through several versions based on microprocessors (like an Arduino, but much older and more primitive) but was never happy with their limitations. Eventually I was able to obtain an old PC and use parallel printer ports for digital I/O. I experimented with Linux and Windows versions and found Linux to be much better suited for what is essentially an embedded system.

The Pi Comes Along

Of course, the first project that I took on when I started working with the Raspberry Pi was a replacement for my PC based home alarm and automation system. The requirements were straight-forward:
    ● several inputs for motion detectors and door/window sensors
    ● several relay outputs for control of miscellaneous devices 
    ● serial (RS-232) port for connecting to an X10 interface
    ● socket interface for control from other computers
    ● logic to provide some limited intelligence
I describe this first version in a series of my early blog posts. Look at Sept. and Oct. 2012 to see these.

Time for an Update

Updated Alarm/Automation System
In the several years that I have worked with the Raspberry Pi I have learned a lot about the best way to do some things and applied these lessons to my updated alarm/automation system. I think the most important lesson that I learned was to make things as modular as possible. It is inevitable that I will screw up something and if the project is one large circuit board, then I may well have to scrap it and start over. However, if I break that into several smaller boards, then a screw-up will, at most, cause the rebuild of a single small circuit. Some systems fit this designs method very well while some just can not be broken up. The alarm/automation system fits this method well and consists of the following parts:
    ● Raspberry Pi (of course)
    ● Simple "hat" for the Pi to expose all the buses needed
    ● a relay board (bought, not built)
    ● control board for the relay
    ● control board for digital inputs
Each of these is described below.

The "Bus Hat"

The Bus Hat
In order to connect the various pieces to the Pi, I needed a way to access some of the buses available. I created what I call a "bus hat" to do this. This mounts on top of the Pi and provides access to the following:
    ● the I2C bus
    ● the SPI bus
    ● the 1-Wire bus
    ● RS-232 converted to proper levels
    ● 5V input to power it all
I knew that I would be using 3.3V devices, so I did not include any level converters on the I2C or SPI buses.  I did need proper voltage levels for the RS-232 serial line and that was provided by a MAX3232 chip. 

This is a very simple device since all it really does is connect specific GPIO pins to connectors for the required buses.

Bus Hat Circuit Design
My design for this bus hat is shown here. You may notice that this is a little different than most circuit diagrams you will see. I am a little embarrassed to admit that I use Power Point to create these drawings. I am mostly concerned with how the components will be laid out on the perf board, so I make a perf board grid the background image and place the components onto that and add the wiring. Power Point makes this quick and easy. If you need to make a simple and better looking wiring diagram I recommend the Fritzing app. For real PCB drawings I have used Eagle and PCB Artist.

Relay Board

Relay Board

I decided to buy a relay board online. There are quite a few of these available and the price is hard to beat. You should be able to find an 8 channel board like this for well under $10. There is no way I could even buy the components for that cheap.

Relay Control
Relay Control Circuit

To control the relays I needed 8 GPIO lines. The perfect job for the MCP23008 GPIO extender. This chip connects via I2C (there is also an SPI version) and provides 8 GPIO lines. I set them all as output and connect one to each relay control pin. One thing to be aware of - these relays require more power to operate than the Pi can generally provide. They will appear to work at first but then you try to turn on all 8 relays and the Pi will reboot due to the drop in power. The relay board will provide a pin for connecting external power to prevent this from happening. Since I was using 3.3V on the I2C bus, I added a 5V input to the relay controller to provide this power.

Input Board

Input Board Circuit
The input board provides 16 "contact closure" inputs. In other words, a switch of some kind connects two pins. This is typical of the way that motion detectors and door switches work. The input board has the GPIO pins pulled high and the external device will close a switch to connect the pin to ground.

The circuit uses an opto-isolator (sometimes called an opto-coupler) to provide protection to the GPIO pins. This is prudent since the inputs are connected to wires that run all over my house and anything could happen to put bad voltage onto those lines. I even went as far as having the opto-isolators in sockets so they could easily be replaced if they are blown. That's probably excessive, but it was easy enough to include, so I did.

Again, I used a GPIO extender. This time a MCP23017, which provides 16 GPIO pins with an I2C bus connection. Both of the extender chips I used will default to I2C bus address 0x20, so I set the address pins on this one so that it will appear on the bus at address 0x21. Note that A0 is tied high while A1 and A2 are tied low.

X10 Interface

X10 Interface

I control the X10 devices using a CM11A interface from ActiveHome. These haven't been made for a while now, but are still available on eBay. It connects via a serial port. NOTE: I had to swap send and receive from the original design of the Bus Hat to connect directly to the CM11A.

I have several X10 controllers for lighting as well as several wireless motion detectors that connect to X10 through a bridge device. These are convenient since they don't require running wires. The downside is that they use batteries. Fortunately, they are fairly efficient and only require replacing a few times a year.

Smart Phone Interface

Smart Phone User Interface

What home automation system would be complete without a smart phone app? A web interface is much more universally usable, so I went that route rather than writing a phone app. Besides, I'm an Android guy, but my other half uses an iPhone. I had no desire to go down that rabbit hole.

The web interface was created using Apache, PHP, and the socket interface to the alarm/automation system. The details on that will have to be a post of its own.


  1. Hello Ted,
    I have followed your blog for some time now, and find it very informative, thank you. Some time ago I tried to implement an alarm system with the software for your old alarm system, but could not get it to work.
    Would you consider releasing the software for your improved alarm system? I would realy like to try again.
    Thank You

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  4. ​Great tutorial!
    You can use easily DHT11 or DHT22 sensor with only two (2) wires. Here is a useful link:
    Here is one tech video with DS18S20 and DHT11 sensors designing an industrial temperature monitoring system, Raspberry Pi based:

  5. Hi,
    Ted would you be interested in a new paid project? Essentially taking a keyboard like entry (RFID keyboard emulation), looking up a SQL database and then activating a GPO? (A boom gate application)

    1. Sorry, but I don't have the time. That's also the reason I have made so few posts in the last year. I suggest you try somewhere like freelancer or rent-a-coder.