Wednesday, November 19, 2014

Short wires II / New PCB arrived

After taking some time to figure out what components died after the bead burned, I found the only one is the Ethernet module. One of the most expensive. This led me to adding extra protection to 3V3 power rail, a Zener diode which limit the voltage to 3.9V, I hope :)

But luckily all other components can be taken to new board that I have just received from factory. Next week I will get busy :)

Tuesday, November 11, 2014

How to wire up PIR

Main board is equipped with 7 analogue inputs that are able to measure different states on the sensors by using just one wire. Well actually you need three together with power and ground. On the right you can see the wiring schema.  It is a simple voltage divider with 22k and 10k resistor that take the input voltage and create different voltage levels based on state of PIR outputs. In this way we can use any commercially available PIR sensor module that is able to operate with 12V power supply.

There can be more sensors on one input, but that would require more than just three wires as you need to daisy chain the signal and signal inputs from PIR and TMP. Not problem for someone who is using CAT5e cable to wire the sensors, it is actually cheaper than original 4-wire cable.

To sum up possibilities with such setting.  We are able to see these states:
  • normal operation
  • trip
  • tamper
  • unknown state / damaged

In software we measure the voltage level in zone thread every 250 millisecond. 4 times a second is safe way as the PIR modules usually keep the outputs on or off for one whole second. Also the thread has highest priority. In order to compensate for different wire lengths, there are voltage  rages feedback specified for each state, rather than a single value. And because input voltage can change when switching to battery and back, the whole measurement is compensated to the input voltage as well.

Saturday, November 1, 2014

New PCB for main board

After not very successful testing of main board in real life, I have put some effort and created a new version of it.

It I hope solves the problem of over current on power lines. I have put five 1206 polymer fuses on it as well as input one and over voltage protection. Also I have re-mapped the digital input pins so now it should be faster to read them in software. I have added one extra digital input and corrected the tamper. Now we have 7 analog input pins and 6 digital input pins. Another fix is reset watchdog circuit that was needed to properly start Wiz820io Ethernet module after power up. Others changes are minor, rearranged the connectors and battery. And I have deleted the 5V rail needed for the old GSM module, because now I use 12V one.

I've send the board for fabrication, fingers crossed :). New schematics are available in download area.