it is normal to get a negative response when you connect your device to +ve and the output of the last circuit. For a positive response, change the +ve to -ve. in other words, connect + on your voltmeter to the output of the alarm and - on your voltmeter to common ground.
The only way I can see it being a constant value is if it is a DC output. It looks like your circuit does not come with an output coupling capacitor.
Check your voltmeter against a good power source. If you see a weird result, your voltmeter is screwed.
and if your circuit IS an alarm, you might want to connect a capacitor and a computer speaker in series and use that as your voltmeter. If you hear noise, you know your alarm works.
Once you get a true output value, look for a relay that can operate with the voltage.