433MHz Wireless for Alarm Remotes

2016-08-18 | By Alex Iannuzzi

License: None

Create your own Ebay home alarm system and make it even better for a very low cost. Alarm systems can be found pretty much anywhere, even online. This project takes advantage of an already working basic alarm system.  From the basic system, you can add additional features and functionalities to create a low-cost integrated alarm system with SMS capability and remote access.

The alarm system for this particular project is made up of several wireless PIRs and door sensors, a wireless siren, a central server and two remotes built on nRF24L01. The range of the remotes is a very short distance, which is why I rebuilt them using the RF 433MHz radio module.

I’ve inspected the current remote’s data and recorded a 120-138μs high pulse for 0 bit and 400-440μs pulse for 1 bit, with a rise/fall time of 4μs. They were transmitting 24 bits, but I need them to transmit 168 bits (160 bits for SHA hash and 8 bits for system command).

I started to work on the receiver and went with my own libraries. First, I listened for a blanking period for about 400μs.  If I received a high signal I reset the blanking counter, and once I saw the blanking period I waited for the first high signal.

There are 21 bytes of data to read in so I read 9 bits at a time – 8 bits checksum bit. I then waited for the high transition. If it’s longer than 400μs at a low signal then I abort. Once I reached the high signal, I started a timer to see how long the signal stays high, if more than ~600μs then I exit.

433MHz Transmitter Code

I can wait for a pin interrupt and transmit the SHA hash as long as the button is pressed. I can then transmit the blanking period, loop the 21 bits, transmit high or low, and finally - the checksum.

Modifying existing remotes and building the receiver

I had a SC2260 chip in SMD in one remote and DIP in the other one. After de-soldering it, I found that Pin 15 was the RF transmitter pin, then wired everything up. Now it’s powered with 3V CR1220 coin cell. For the receiver I’m using the 433MHz receiver module with an LM7805 to get 5V. On the alarm system server itself, I have a pin interrupt and wait until the pin goes low, then process an action based on how long it was high.

Download AlarmSM_Remote&Server_433MHz_v1.0

It works well and the range has now extended to the driveway so I should be able to switch it on or off once I’m in the car, like I used to do with the old alarm system.

Check out the entire project development, schematics, videos and code with more detailed explanation.

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