I purchased a battery operated smoke/fire alarm few days ago and it showed up today. It runs on 9V and will make a loud sound if smoke is detected. My intention was to hook it up with my home automation system so that I would receive alert if it would go off including SMS, pushbullet notification to my phone, email etc.
The Funky v1 is ideal for the purpose because it is really flat/tiny and would fit inside the alarm. It will tap into the piezo siren and sleep until the siren is activated. Upon activation, it will make a wireless transmission to my home automation system (Raspberry Pi running Node-Red) for further processing and alerting me on my phone.
The piezo siren is activated with a 200ms series of 6.5Khz 50% PWM at 9V so I had to create a small voltage divider + a capacitor to smoothen the signal out and bring it to 3V so the Funky v1 can handle it. A 4.7K:10K + 1 uF ceramic capacitor works well:
I then created a miniature PCB using 0603 sized SMD components for the voltage divider and low pass filter so that all can fit in the fire alarm sensor:
The Funky v1 is equipped with a MCP1703 LDO that will supply 3.3V from the available 9V.
For more detail: DIY Internet of Things Fire Alarm