Kitchen Convenience Lighting

This is not a new project but, we thought it was about time we wrote it up, as there was some interesting learning from it. The objective was to automate some convenience lighting under the wall-mounted cabinets, above the kitchen work tops. When we had our kitchen re-built from scratch in late 2010, we added some cables to provide 12V dc power to a manual switch and we also ran a cable up into the loft to enable automation.

This is an example of automation in our current home that uses our software controller project.

Components

12V LED lighting strip
We have used 12V LED lighting strip modules that can be linked together in series. Each one is about 50cm long. They are cool white in colour. In retrospect we should have gone for a warm white colour, match the other lighting and general colour scheme in our kitchen.

We used high-quality automotive cable to provide the power to the lights. The cable run from loft to kitchen is approaching 20m so we expected some voltage drop along the cable and decided to use low current LED lighting strips. When powered from a regulated 12V DC supply we measured just 7.4V across the lighting strips. Despite this, they were perfectly bright enough for the intended use. We decided we could quite easily power them directly from our battery bank and not use a regulated power supply.

Installation & Configuration

The key thing with this project was to install the cables when the kitchen was rebuilt. It would pretty much impossible to retro-fit this later given our granite work tops and fitted cabinets. As it is, the whole installation is totally invisible. There are three cables; a permanent 12V power feed, a switched (automated) 12V power feed and an earth. All go via a single pole, two way switch mounted under the end of the cabinets.

The LED lighting strips appear reflected on our granite work tops, so it was essential that we aligned them in a regular and tidy fashion. They also run from edge to edge. One thing we also had to do was to seal any gaps between the valances and the cabinets as any light leakage looks untidy.

It also requires a PIR sensor to be installed in a suitable location in the kitchen. This is something we also installed when the kitchen was redone. It is a wired sensor and standard 6-core alarm cable runs up to our Home Control System (HCS).

Our Home Control System (HCS) provides the intelligence to control the lights and they come on when the twilight sensor detects it is dark outside. The automated power feed runs via a light switch so that the lights can be turned on manually if need be.

In Use

Our data visualisation project showed the optimum time for the kitchen worktop lights to stay on for once triggered, is about 6 minutes. This ensures they don't switch off when we are in the kitchen.

We thought it would be nice if the lights ramped up gently when you walked into the room but, the reality of it is that you need them to come on quickly. We tried adding a capacitor across the LED strips but they take about 1A of current and a very large capacitor would be required to get any decent ramp effect. The in-rush current would also likely blow the 2A fuse in the switched output line. The only way to realistically achieve this is to use a ramp circuit in the control line (e.g. a transistor and RC network) with a local power source.

Despite having the PIR sensor set to maximum sensitivity, the kitchen lights don't come on immediately when you walk in. It takes a bit of movement for the PIR to activate, despite it being configured to activate on 1 pulse detected. To improve this situation, we now also use the kitchen door contact sensor to trigger the lights when the door is opened (but not closed).

Summary

LED lighting is fine for convenience lighting but, this kind of installation would not work if more significant lighting is required. A local power source is required and the wired cables should simply be used for control only. In our next home, this would be achieved using mains powered lighting (12V with a local transformer) and a solid state relay for control. This also means we could use lower voltages (e.g. 5V control lines) and would not require relay boards on our output controllers.

This project reinforced our view that whilst there is a place for 12V convenience lighting, it is not realistic to distribute 12V DC for high current lighting applications over any distance.

As with all of the automation we have installed in our home, it is over-laid on top of the manual controls that would be expected. There is a light switch to also switch the lights on if required. We did consider a 3-way (on-off-on) switch so that the lights could be manually switched off as well but, we decided this was not a required capability.

We did consider dimmable lights and technologies to enable this. There is a time and a place for dimmable lighting but, the kitchen is not one of them in our view. It is better to have a lighting types, suited to the way the kitchen is being used.

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