Smart Home Connected Smoke Sensor

The simply objective of this project is to take attractive, low-cost, 'dumb' smoke sensors and make them really smart and extremely reliable. Our approach means we can use deploy four sensors for less than the cost of one Nest Protect device. Each is powered via a protected 12V dc power supply which eliminates the needs for batteries and this also provides extremely secure and reliable networking/signalling. Each one added becomes much smarter and fully integrated within our smart home, adopting all of its capabilities. This includes local and 'whole house' voice announcements, full alarm system integration, alerts and remote notifications, including remote reset.

Smoke detector
We have installed networked smoke detectors in our home in the past but, for this project we wanted a more compact and better looking device, that also met our technical requirements.

We opted to use the ESP. Fire line 12V optical smoke detector (PAD-212) because it is a low-cost (~£20), compact, low-profile device that looks really good.

It also meets another of our requirements in that it has no built-in sounder (though we also use some that do). It also has a visual indicator when activated in the form of a red LED exposed via two transparent plastic 'blades'. In standby mode these flash briefly every 30 seconds or so.

Design & Connectivity

The smoke detector comes in two parts, a mounting base and the main detector unit. This means it is possible to wire it up and just replace the detector unit, which is simply pushed into the base and rotated slightly to lock it in place. There are also some guide markers to ensure it is aligned correctly.

Smoke detector parts

These types of detector are designed to work with an alarm control panel and signal an alarm by increasing the current drawn when smoke is detected. It exposes three terminals and we have connected wires to them for testing: +12V (red), ground (black) and signal (blue).

4N26 opt-isolator
Our testing showed that when it is activated there is approximately a 3V drop on the alarm 'output'. We are using this voltage drop to drive the LED in an opt-isolator IC, with the output transistor switching a 12V signal back to our our optically isolated input board. We are using a 4N26 opt-isolator. To limit the current into the 4N26 (to about 6mA), we are using a 220Ω resistor.

Once activated these devices do not reset and the power needs to be removed to perform a device reset. To address this we have included a momentary push-to-break switch in the power line. This could be automated using digital I/O if required.

Smoke detector circuit

active fire alarm
This is what the sensor looks like once it has been activated. The red LED parts are continuously on. Power needs to be disconnected to the sensor for about 3 seconds to reset it and extinguish the lights.

Optically isolated input board
These sensors are interfaced to our NOT DEFINED using our generic optically isolated input board. This approach also provides a 12V dc power feed to the sensor.


The whole of our Home Control System (HCS) is powered via our 12V dc Uninterruptible Power Supply (UPS), so we don't have to worry about power outages :-)


Once we connect a smoke sensor like this to our Home Control System (HCS), we have a given it the ability to generate both sound (tones) and voice notifications. As we know the location (zone) of the sensor, we can make the voice announcements specific to the region of our house or have them cover the whole house.

Each can also now send notifications via SMS, email, etc. and all activity is logged, to enable full diagnostics analysis.

This £20 'dumb' sensor is now extremely smart! It is now an integral part of our smart home and requires no batteries. It inherits all of the capabilities of our smart home, so it has a voice, it can send SMS messages, etc.

Smoke sensors have have a limited lifetime and ours can easily be replaced when they do expire. Because they are cheap, we can install them where ever it makes sense to do so. We have one in our smart conservatory, garage, loft, landing, hallway and kitchen.

We are currently considering intelligently deactivating individual smoke sensors based our smart home's understanding of what we are doing in our home.

Above all else, we now have simple, reliable, low-cost smoke sensors distributed around our home that require minimal maintenance (no batteries) and that we can rely on to just work when needed.

Related Projects