12V DC Safety Lighting

Safety lighting over our stairs
Safety lighting is designed to come on when it is dark outside (as determined by our twilight sensor) and provides a low-level ambient lighting in places where it is needed. It provides just enough light to see by and helps negotiate hazards such as stairs, corridors and doorways. It is also designed to come on only when our home is occupied.

We have several types of lighting in our home. Our safety lighting is designed to provide just enough light to see by and complements our convenience lighting, which is much brighter. Safety lighting avoids having bright lights switch on at night time, which then dazzle you. We also have automated security lighting.

Design

All of the safety lighting in our home is powered via our 12V DC Uninterruptible Power Supply (UPS) and consumes less than 0.2W of power.

Control

The safety lighting power is controlled and switched by the Home Control System (HCS), which uses an Ethernet IO board to switch the power via a power transistor.

Wiring

We have defined a colour coding convention for wiring and ducting in our smart home. This approach makes the installation easier, much tidier and also means it could be removed very quickly from our home if required. It also ensures clear separation of low voltage wiring and keeps it away from the mains wiring in our home.

Automotive pink wire
Colour coded (pink) wiring is used to distribute the (switched) power to all of the safety lights. Because it is readily available in 100m rolls, we have used a 16/0.20mm, 0.5mm², 11amp 'thinwall' cable. At some points, the distribution network splits into a 'star network' and this is achieved using a 6-way blade terminals and insulated blade connectors.

A common earth is distributed from the the battery bank, through the ducting and runs to each light. This reduces the amount of wiring required and this common earth then works regardless of the functions each light is used for. All earth (-ve battery terminal) wiring is done using higher current black wire.

Lighting

All of our safety lighting is using very low power 5mm LEDs to reduce the current and thus the voltage drop across the wiring. The total lighting load is less than 0.2W. For example, the lighting over the stairs in our current house is a single warm-white LED that provides just enough light to see and draws just 4mA current (at 12Vdc). We have used warm-white LEDs as they can be easily hidden with larger MR16 LED lamps and can server multiple purposes.

Software

The 'Safety Lighting' is an object modelled in our Home Control System (HCS) as a static Java class. It maintains state in a local database.

Conclusions

This project has been operational in our home for since 2010. Whilst it is not a major feature of our home, the safety lighting is a worthwhile and very useful feature. It provides the confidence to negotiate doorways and stairs before any convenience lighting is activated or required. It is also used in our ensuite bathrooms and prevents the need for a bright light being switched on, possibly waking up someone else in the bedroom. Safety lighting is also a subset of our emergency lighting.

We have noticed that products like the Nest Protect feature 'Pathway Lighting', which is described as safety lighting and activated by movement. The problem with this approach is that it is not always on at night and the alarm may be too far away to activate usefully. The lights are also quite bright and would be better compared with our convenience lighting. There is also often a conflict in where best to place a smoke alarm, safety lighting and convenience lighting though, so we have not linked these functions.

Further Reading

Updates

November 2014

Finally got around to adding more safety lighting in two more rooms in our house. We have taken a slightly different approach this time, using the LM317L IC to provide a fixed current driver for our white LEDs. We are using a 220ohm resistor to limit the current to 5.7mA. This avoids wasted energy by using a resistors in series with the LEDs and ensures a consistent light output over a very wide range of supply voltages.

Our safety lighting is all 12V driven and our circuit allows three LEDs (in series) to be driven from 11.3V or more, two LEDs from 8.4V or more and one LED from 5.6V or more. Our circuit handles input voltages as high as 37V.

3D printed LED mount
We have 3D-printed some LED mounts to enable invisible safety lighting to be added to existing MR16 (12V) down-lighters. We are experimenting by adding safety lighting to many more rooms in our homes. This includes all corridors, stairs, landing and bathrooms.

current limit circuit
This is the mount with the LEDs fixed and the current limiting IC wired up.

fitted to halogen MR16
When fixed to a normal halogen MR16 bulb, the light shining through is just enough to provide light to see by, without making it look like someone has left the lights on.

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