This weekend i spent some time to make the experimental PIR sensor setup i made some time ago, a bit more permanent. Ingredients:
- Gray enclosure;
- Battery holder;
- XBee breakout board;
- Digi XBee;
- Red LED;
- PIR enclosure;
- 100 kOhm resistor;
- Wire, double sided tape, hot glue.
The enclosure i used this time is a bit different from the enclosure i used on a previous occasion. This one lacks the inner ribs and opening and closing is a bit harder: for opening the enclosure you need to put a screwdriver between the 2 halves and twist it to get the enclosure open. The first thing i did was making my life easier by removing the ribs that keep the 2 halves together on one side of the enclosure. Some more modifications here and there now make it possible to open the enclosure without any additional tools, but still will not open by accident.
I drilled a 10 mm hole into the PIR enclosure so that the PIR fitted nicely through the top plate; a drop of super glue on the inside did the rest. There was plenty of room for the 100k pull-down resistor inside the PIR enclosure so i soldered it to the PIR there. An extra hole of 4 mm and a cable tie for the wire completed the PIR box.
This being my first motion sensor built this way, i added a LED to be able to see what the XBee radio is doing; is it still transmitting or not, when, how often, etc. ? Maybe this is overkill, but when this sensor is placed somewhere in the house with no computer screen nearby, it can be very handy to see if the sensor is still functioning instead of running to and from the nearest screen…
If the flickering of the LED is to visible or annoying in the future, i can always disable it or make it invisible by a piece of tape over it; but for now, this LED seemed like a good idea to have. The LED lights up when the XBee is on and from the duration that the LED stays on i can see what’s happening; whether it’s transmitting (very short durations) or having PAN trouble (long).
The software part of this sensor will be the most time consuming of it all i guess. Since i want it to be battery powered, i’ll have to consider what is relevant information for my Domotica system, and when it is. In fact, the only thing that really matters is one single thing: fast motion detection! All the rest is less important.
Cause what do we Domoticans use this motion detection for? To turn things ON… not OFF; at least, i don’t.
Further more, a quick look in my events history tells me that from 01-04-2010 till today i missed (my 433 MHz RFXCOM receivers, i mean) a total 869 ‘OFF’ transmissions (meaning “no motion”) from my 10 MS13 motion detectors. That’s way to much to be called reliable! So my system already has the capability to deal with this situation of missing ‘OFF’ transmissions; when a MS13 hasn’t reported neither an ‘ON’ nor an ‘OFF’ for a specific time interval, i set the motion sensor back to ‘OFF’ in software.
So.. i could as well do without this ‘OFF’ completely; i don’t use it as a trigger, so why send this non-information anyway?
What i do want to have is a heartbeat, every 90 seconds or so. And i would like to have an ‘ON’ being sent with a specific interval when motion is detected continuously. That’s what I’ll be working on in the coming week.
To be continued.