I am new to all this, but it all looks very interesting.
I would like to run this as an alarm system , but also take advantage of using the PIRs etc in something like Hubitat or Smartthings is that possible as I would not have. MQTT broker.
If I wanted to expand using I2C, does it matter what board I use as I am not sure I see one on your site.
gateway is reporting many of its states trough MQTT. So it is easy to use its states on many home automation platforms. I personally use MQTT app on my Android phone to manually switch On/Off some devices connected to gateway and to have overview of sensors. I do not use any home automation platform, as there is build some functionality into gateway itself. Like do action on PIR. Do action based on time or day. Do action when some sensor value raises or fall ..., enough for my purposes.
Gateway has 8 balanced and 4 unbalanced zones. But ofcourse you can have some PIRs connected is series. But expansion through I2C is possible, I have in past started development of expansion board, but I did not finish. As now, there is, I hope, more flexible solution to use RS485(wired) or wireless expansion. Gateway knows how to handle such expansion boards, and they can vary in number of zones. So it can be one single battery powered PIR or stripped down version of gateway with 8 analog zones. I can help you with that, in fact I will post some examples soon on blog how it is done.
What l would like to do is have a self contained alarm system, but also have the ability to use the PIRs and contact sensors in a relatively simple hub such as Hubitat or Smartthings, I could then use them to control lights etc.
The reason why l was interested in I2C was that if the GW has the ability to add more sensors to it I would prefer that, as I thought the Small and simple I2C chip would use much less power and if right next to the Gw the simplest to implement as it needs no firmware.
I would also prefer to not also have a raspberry pi or something similar running all the time to have an MQTT broker running, so could your gateway communicate without broker.
I'm not sure about Hubitat or Smartthings, I do not know them. OHS gateway can communicate via MQTT as for now, but it is open source, you can add any desired protocol. Only thing what is not possible is SSL/TLS.
I2C was my option on beginning as well, but personally use only 7 balanced and 2 digital zones, and no one shown to much interest into expansion board. 8 channel ADC connected via I2C was my plan. Consumption is not critical as PIRs will consume more then expansion board anyway.
Current solution is more flexible, it allows you to add expansion board far far away then over I2C and even trough wireless.
MQTT is not really required, but gateway uses MQTT to check Ethernet connection. If not found gateway periodically reset Ethernet driver.
I'm now concentrating on this new version :). That is why I do not blog too much.
I have first hardware on my desk. 10 balanced zones, 32bit ARM running at 180Mhz. I'm fixing few HW problems, I did not wire some signals, or wired them incorrectly. But I can say I have successfully tested all peripherals, but wireless radio.
I plan to fabricate new PCB with all corrections, then concentrate on SW. I pan to do some blog post with pictures too.
It will not be easy and fast. For example create new web interface will be very challenging tasks, as it uses different techniques. So far I was only able to render static page. Some functionality, however, can be ported easily, as it uses similar RTOS. I hope to have some extra developers to join :)
But HW can be ready maybe end of year. And SW I will release my progress step by step.