Saturday 6 August 2016

Wireless 433Mhz home alarm system from ebay

Following on with my faulty alarm problems at home (The X10 system) I've been looking for an alternative, waiting for when the current one acts up enough to warrant buying a replacement.
This week it did just that, making strange noises from the sounder and forgetting the remote fobs. So I decided to look around.
I had a few requirements:

  • wireless (to avoid the need for long wiring runs to the sensors)
  • gsm/remote alerting (to sms or ring me during alarm conditions or let me remotely check status, arm/disarm, etc)
  • keychain/remote arm/disarm (Wife and daughter prefer these to tapping in a code)
  • Interface with my home automation systems (More tricky, but basically have some I/O I can tap into)
  • "nice" appearance. It'll be in a prime location in the house so don't want it to look terrible.
  • Cheap (ish) as I'm a cheap-skate so don't want to spend a lot of money on a solution that I'll never be 100% happy with (I also have a high expectation for electronics, so I'm never happy!)
So after a lot of searching, I found a system that might fit the majority of my requirements, and have the potential for expansion and tinkering. It's also not mega-expensive which gives me more incentive to take a look inside and tinker with it!
The ebay listing I found was this one, so the unit came with the control box (and PSU), 3 door sensors, 3 motion/pir sensors, 1 smoke sensor, two keychain remotes, one additional sounder and an outside bell/sounder box. Not a bad package for the price I feel.

Delivery was in a couple of days, so now I have the unit I've started to play with it. So here are my findings on the unit. Firstly it's not the best plastic casing I've ever encountered. It's distinctly cheap, but rounded corners and smooth edges so it's not going to win a fashion award, but it's also not too hideous to put in your hallway!

This is the main control unit, front. At the front you have the two speaker grills either side (As I found out, one is a microphone and one is a speaker). Behind the clip down door is the push button keypad. At the top you have the LCD screen and a series of LED indicators either side for constant system status.

This is the rear view. You can see the 12v power cable going in, the SIM slot and the various I/O screw terminals. To the left you can see an on/off switch. This worried me slightly immediately when I found it, assuming it would turn the whole system on and off. However as it turns out this is only the battery master switch, normal power was not controlled by this switch!
The screw terminals provide 8 wired inputs (and can take normally open or normally closed with the use of a set of resistors supplied), it also has a sounder +/- terminal, a constant 9v output for supplying alarm boxes, etc. Two GND terminals for the I/O ports, then the final two are COM and NO which I've yet to work out their function or use.

Powering it up for the first time, I found the speaker to be VERY loud, even normal arm/disarm and voice prompts were at a volume unbearable for setting it up. The volume level I suspect was fixed so it always ran at that level, no configuration to set it either which was disappointing. So the first job I'd have to do was change this, as it was almost unusable in this state as soon as it was put up it would annoy me!
So, the back came off and I discovered some very recognisable components inside:

Looking at the board, top left (blue square) is a relay, which I think will be the alarm output switched contact.
Below it is a ZTE cellular modem which supports SMS and telephone calls. The coil to the left of it I think is the internal antenna for it (There is also the on-chip connector for external antenna if you need it). To the right of it is the SIM card (with mine inserted). NOTE: This is 2G only (EDGE/GPRS) so you need to find a mobile company that supports this (So O2, T-Mobile/EE I believe should be OK. Three is NOT an option - I tested this). So the potential here is that this won't work very well, and not for long as mobile networks phase out their 2G networks, so something to be aware of here. However chances are you can just swap the board for a newer equivalent that uses the same AT command set.

Over to the top right (with the yellow outline) is a small backup battery, this probably keeps power to the EEPROM for storage of data, time, system state, etc.
The to the very right of the board (which has the black wire coming out of it) is a very recognisable part! It is a 477Mhz receiver that is commonly used in Arduino and hobby electronics, soldered on as a daughterboard. The black wire is the antenna.
If you then look to the bottom left, you'll see the matching 477Mhz transmitter board with it's own black antenna wire running along the left towards the top. So these two are standard boards that you can buy for Arduino development.
Here are similar modules you can buy easily for Arduino development:

Above the transmitter board is a two-pin connector, this connects to the speaker round the front.
Here is the front of the board:

Here you can see the microphone to the top right (I've flipped the board) and the LCD screen and various status LEDs. You can also see the copper contacts for the buttons.
These may need to be altered as the key button presses aren't very accurate, so probably need some better contacts here which i'll work on another time.

This is the front casing, showing the 8 Ohm 5W speaker and plastic casing and buttons.
So, first job is to reduce the volume output of the speaker. Since it's not used much and doesn't carry high current I'm going to simply wire a resistor in series with one of the leads.

The final soldering and resistor work looks like this:

That was a larger value than I'd wanted to apply (120 ohm) but it should reduce the volume appropriately. I powered it up and it reduced the volume pretty well, perhaps a little low (as it had distorted the spoken voice slightly) but that should do for now (It was the lowest resistor value I had!)

Now onto the 433Mhz sensors. These attached to the doors/windows and had a magnet one side and a reed switch a the other, very simple typical design really, so taking a look inside it:

Top left is the red LED which flashes when triggered. Top right is the 433Mhz transmitter crystal, the spring to the right is the antenna.
The push button is brought to the front of the unit as a press button (This is for setting, to trigger a signal). At the bottom is a 12v battery, and above it is a set of 4 jumper pin positions. I'm assuming these set a unique code for the unit so you can change it if so desired, adding a little more security to your setup. Naturally, no instructions on this! To the left you can see the reed switch soldered in there (and without trimming the excess wire!), so quite a simple design once again.

On the other side of the board is all of the transmitter electronics:

Which I was actually surprised it was so complex, as again this is just a simple 477Mh transmitter when the reed switch was activated. I'll dig into the wiring at some future date.

So there it is, all in all there are good points and bad points. For example, 477Mhz is hobby electronics, so you CAN sniff the traffic and protocols, and no doubt spend enough time to watch the data and decode/decrypt them. But for the average user and average building security this unit will do the job nicely.

I've noted down all the relevant chips I've discovered and looked up so far:

Follow-up to this is HERE:


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