I have actually experimented with connecting into the innards of a low cost smoke detector for the purposes of extracting a remote monitoring signal. I used a unit purchased at a local hardware store for about $12 that runs on a 9V Alkaline battery. I probed around on the circuit board of the device and found that the device electronics consisted mainly of a single chip that is on the PC board inside the screened detector chamber. There is also a large diameter Piezo element in a resonator that has three connections to the PC board. See the picture below for what the module looks like.



I found that two of the connections to the Piezo element come directly from the detector chip. Using an oscilloscope and a bit of smoke to set off the detector I found that the chip produces a 3.3 KHz square wave frequency on these outputs when the unit is activated. The unit I looked at activated the outputs in a 0.3 second ON at 3.3 KHz and then 0.7 second OFF period. During the OFF time both of the chip outputs are at a voltage near to the GND (neg of the 9V battery). The square wave output has a swing from GND all the way to +9V.

I devised the following circuit to detect the ON / OFF cadence of the the alarm output without disturbing the normal function of the alarm device. (Of course the circuit does add a miniscule additional load on the 9V battery during the time the Piezo element is screeching)!! The capacitor in the base circuit of the NPN transistor filters out the 3.3KHz part of the output signal leaving the trasistor to turn ON and OFF in step with the 30% / 70% duty cycle of the output signal. The transistor can be wired out to a remote monitoring device such as an 89C52 type circuit. I show a very simple connection assuming you are experimenting as I was. For a serious application I would recommend some type of EMI/ESD filter or buffer between the board input and the 89C52 input.



I found that the 9V battery clips were attached to the detector circuit board using large through-hole leads. This made a very convenient location to attach the small circuit into the smoke detector. All of the components were soldered right to the bottom of the PC board and then the external wires (a twisted pair) were anchored in place using a bit of Devcon 5-Minute Epoxy. By keeping all of the components flat to the bottom of the circuit board as shown in the picture below I ensured that it was possible to re-install the detector circuit board back into its original housing.



Note: I need to tell you that it is not recommended that you do any disassembly of the smoke detector assembly itself. I show this circuit simply as an experimental procedure and do not imply in any way that it is a legitimate modification of smoke detector. Any modification you make to a smoke detector you are making at your own risk and responsibility. Any liability that may occur due to modifications that you make are yours alone. All that said, if you are attempting to make a remote monitoring and reporting system for the smoke detectors in some building then I strongly recommend that you install additional unmodified smoke detectors in the same areas so that you have and provide full protection.

Michael Karas