DIY Internet of Things Fire Alarm

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, push bullet 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:
DIY Internet of Things Fire Alarm
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.
Everything is fit inside the original case.
The Funky is running the following code, basically it sleeps waiting for a pin-change interrupt from the piezo siren to kick in. It will wake every 30 min to provide heartbeat transmission and let the home automation system know it is alive and well.


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