Triggering a Smoke Detector as an alarm

I’m using an eTape http://www.adafruit.com/products/463 sensor and want to trigger an alarm when the water in my sump pump hole gets too high. I have it working with an LED but it doesn’t seem that bright. When I hook the smoke detector up, it doesn’t set it off when its in the range I specify.

// the value of the 'other' resistor
#define SERIESRESISTOR 560    

// What pin to connect the sensor to
#define SENSORPIN A0 

void setup(void) {
  Serial.begin(9600);
}

void loop(void) {
  float reading;

  reading = analogRead(SENSORPIN);

  float voltage = (reading *  5.0) / 1024.0;  
  Serial.print("Voltage "); 
  Serial.println(voltage);

  Serial.print("Analog reading "); 
  Serial.println(reading);

  // convert the value to resistance
  reading = (1024 / reading)  - 1;
  reading = SERIESRESISTOR / reading;
  Serial.print("Sensor resistance "); 
  Serial.println(reading);
  
 
   if(reading < 700 ){    
        digitalWrite(12, HIGH);
        Serial.println("high"); 
    }else{
        digitalWrite(12, LOW);
        Serial.println("low"); 
    }

  delay(1000);
}

I’m sure I’m missing something pretty basic. Thanks for any assist on this.

I am assuming that your smoke alarm is connected to pin 12 somehow? I think your trouble is in your wiring, not your code. What is your wiring to trigger the smoke alarm?

It is easier and more reliable if you connect a piezo to the Arduino.
Use a flat disc that is in toys, cards, clocks and so on.
Use a series resistor of 100 ohms.
You could use the toneAC() library for double the volume, Arduino Playground - toneAC Library
Make a box around the piezo for more volume.

Put an npn transistor across the the smoke detector test button.

Thanks zoomkat - now I just need to figure out which one. Its a 9V smoke detector, I'd need to find out the mA draw of the alarm correct?

Your LED might glow brighter if you set the pin to be an OUTPUT:

void setup(void) {
  pinMode(12, OUTPUT);
  Serial.begin(9600);
}

-br