Mems sensors have come to replace these and are much more affordable while providing accuracy and efficiency hence the modern-day fire alarm’s small size. A lot of these sensors can be purchased online at reasonable prices but special sensors such as O2 and special gas sensors are still uncommon. Most gas sensors traditionally employed a sensor with a heating element inside and with great heat comes great power loss. The gas sensor we have is a wall plug type and is the largest of all the sensors in the lot and understandably so. ![]() The batteries leaked in one of the modules killing the PCB which adds to the list of lessons learned in my humble opinion.Īn R433 SAW resonator powers the RF end of the device in the same fashion as the door sensor and typically it is also the most power hungry piece of the puzzle. I was surprised at how low power the module was since it kept working for around 4 years on three AA batteries. There are variants available in the market with multiple sensor elements and other bells and whistles. The sensing element is a Pyroelectric IR sensor common in intrusion detection systems. Personally, I’ve never used that part but a quick google search reveals that a bunch of companies in China use it and its relatives for lower power applications. The motion sensor is more interesting, with a Toshiba TMP87P808 Microcontroller in it. There are no regulators so the two AAA cells provide 3.0V and below to power the system. ![]() There is a write-up on The Mechanics of a Reed Switch that you may find useful. The R433A TO39 metal-can seen in the image is a SAW Resonator commonly found in AM Transmitter modules and the sensor of choice is a reed switch that provides the requisite interrupt to the microcontroller. There is no receive module which means that getting an acknowledgement is out of the question and there would also be a code for sending a low battery alert. A crystal suggests a smaller 8051 variant that supports lower power consumption waiting for a GPIO interrupt which would trigger the transmission. There is evidence of a small SOIC microcontroller on the door sensors, but the part number is not visible. Since everything is 433MHz, the sensor nodes are simple little creatures that I guess, chirp every now and then. ![]() The last part is the vertically soldered PCB with a custom chip inside. My guess is that the Timer/counter peripherals are used to count pulse widths from the RF modules and then pushed into a common buffer which is then read by the main routine and such. The firmware would have to be pretty tightly knit. That puts the core at 250kHz in the worst case. ![]() The funny thing is that the microcontroller is running at around 3 MHz and with a 6T/12T clock machine cycle. The RF part is interesting and uses what looks to be a hobby grade module which means that in order to snoop on the system, all you would need is an Arduino and a similar module. The analog Mux-Demux could be used to switch the Decoder and dialing system to the phone lines, though relays are traditionally used for that task. You don’t want your neighbors throwing rocks to silence a blaring alarm in your absence. There is a DTMF decoder(7) which I assume is how a user would deactivate the alarm remotely by entering a code and then doing some other single digit commands.
0 Comments
Leave a Reply. |