Most likely a transistor will provide the buffering that you will require. Get your hands on an oscilloscope and look into the alarm at the voltage levels on the pins of the chip therein. Then design a circuit similar to that I showed a previous thread on this subject.

You are probably using a smoke alarm that has an output that transitions from 0 to 9 volts. If you are using a typical microcontroller with a VCC of 5V then it is not permissable to connect the smoke alarm circuit directly to the microcontroller if such input would make the microcontroller pin go above its Vcc or below its ground. Driving an input to 9V on a 5volt controller will surely cause latchup and the part will indeed get very hot.

Here is a link to the previously posted schematic.

http://www.8052.com/users/mkaras/SmokeCkt.gif

In this circuit the horn driver outputs of the smoke detector chip were signals that had a swing from 0 volts to 9 volts in a square wave of about 3 KHz in bursts of some part of a second long. This signal is applied to a 10K resistor which then begins to charge a 10 uF capacitor as a tone burst starts. After a cycle or two or the 3 KHz in the burst the capacitor attains a charge state up to just above the threshold Vbe of the NPN trasistor of about 0.7 volts. This makes the transistor turn on. Continued cycles of the 0-9v signal during the tone burst continue to charge the capacitor and keep the transistor on. The purpose of the capacitor is to keep the transistor on even as the tone signal goes up and down during the tone burst. Eventually the tone output will stop and stay low. The capacitor will discharge through the parallel 10K to ground and the input 10K resistor. As the capacitor discharges voltage across it soon falls below the Vbe threshold of the transistor and then the transistor turns off. During the tone burst the transistor being ON pulls the 89C52 input to ground. Then the transistor goes back off after the burst the microcontroller input will go back to a 5 volt level as provided by the pullup shown.

Note what when connecting the circuit as shown the "ground" of the smoke detector must be connected to the ground of the microcontroller circuit.

I want to repeat again that unless you use an oscilloscope to "look" at the circuits it will be rather hard to get circuits designed successfully.


Michael Karas