insideGadgets

I picked up a cheap alarm system from Ebay (the “99zone” alarm) which came with a few wireless IR sensors and magnetic sensors that run off batteries and it all seems to work well and I’m looking to add more functionality to it.

The IR sensors have a switch to turn them on or off and are powered by a 9V battery. The only problem is that this switch is more of an inconvenience as you have to walk around to each sensor to turn them on or off. You could leave them always on but eventually the battery will run out in maybe a few weeks/months – the batteries were supplied with the alarm system so they might be pretty cheap batteries. When the IR is activated, it draws about 8mA for 2 seconds.

I won’t be focusing much on magnetic sensors because they would rarely be activated as we don’t open or close doors as many times as we would activate the IR sensors.

The modifications which I am thinking of making to the alarm system include:

• Turning the IR sensors on/off wirelessly depending on whether the alarm is on or off
• Adding an external siren wirelessly and an MCU to detect if the alarm sent an unlock command before it was switched off – in case the house is broken into and the alarm was removed/broken (as it’s not wall mounted)
• Adding on SMS sending capability (you can also buy the alarm system with a GSM module built in)

Let’s take a look at the alarm system in more detail – by which I mean let’s crack it open.

It seems to have a backup battery unit consisting of 6x AAA batteries which is at about 7.4 volts. The alarm system draws 40mA when the front panel lights are on and 26mA when in idle mode. On the top right you can see the wireless module soldered on the board.

Flip it over and we can see the MCU (a STC 10F08XE – unknown chip) and a HT9170D chip which is a DTMF Receiver and I’m guessing is used to process the wireless signal being received.

Inside the IR sensor, you can see the PIR sensor, the BISS0001 chip to control the PIR and a SCT1527 wireless transmitter chip.

Turning on/off IR sensors wirelessly

When you turn the alarm on or off, there is an LED that starts blinking every second or so. The way I’m thinking of doing this is using an ATtiny to monitor that LED and if a voltage is detected, it will change a variable. The wireless device I’ll be using will be the nRF24L01.

The IR sensors will also have an ATtiny and will check in with the server ATtiny every x seconds to save the battery and will ask the server what state it should be in which the server will respond with. I can use a Mosfet to turn the power on or off. I would also like some form of security on the wireless communication (which I have done some research on already).

Adding an external wireless siren / SMS Sending

This will be the same as above, except now we monitor the siren that’s attached to the alarm system. For the SMS side, I can re-use the Nokia Keypad SMS Sender project.

For the next part we’ll look into two way communication with the nRF24L01.

Part 1
Part 2: Two way communication for PIR sensors
Part 3: Secure communication
Part 4: Adding on sirens and SMS sending
Part 5: Modifying the PIR sensor
Part 6: PIR PCB
Part 6.5: PCBs arrived
Part 7: See which sensors check-in
Part 8: Building our own alarm system
Part 9: Remote control and attempted improvements
Part 10: Prototype PCBs