Convenience Lighting Timer

This project is currently defined and designed. Depending on how our intial testing goes, we may decide that it is not worth using in our home.

In our home we have some convenience lighting, which is controlled by our Home Control System (HCS). These use PIR sensors and door contact sensors to turn on certain lights after dusk. These lights are switched on for a set duration whilst occupancy or movement is detected, with the timer being restarted by new movement detected. This is a software implementation of a retriggerable monostable.

The downsides of using this approach are that:

  • It uses a digital output for each convenience light controlled this way.
  • It typically requires a separate switched power feed from an I/O board mounted in our 19" rack cabinet.
  • There is a short delay in the PIR detecting a person and the light coming on.

Because of these limitations we are looking at an alternative hardware solution with this project.

Design

A hardware implementation of this function would be very compact and cheap to implement. It would also overcome many of the disadvantages encountered but, it might also introduces some new limitations. This design aims to minimise these though.

It would be very easy to use the minimum amount of electronics to realise this piece of hardware but, as the costs are very low we plan to use full optical isolation on the inputs and not to cut any other corners.

Note that this project is purely about the courtesy or convenience lighting. We have stayed in houses where PIR sensors have also triggered bathroom fans and was incredibly annoying!

Manual Control

We may also want to have a manual switch as an override, to ensure the light is always on or off. This fits better with our philosophy on home automation.

Wiring

The wiring is simplified with the the PIR sensor (and associated twilight feed) passed down to the timer circuit and on to the PIR sensor (along with power). It would also be possible to use the same 6-core alarm cable to carry the door contact sensor wires (just two of them).

From a simpler wiring perspective, it makes a lot of sense to locate the timer next to the light.

In those installations where the courtesy light is a low current LED device (most of them), it would also be possible to run power down the 6-core alarm cable to the timer.

Inputs

We know we require a circuit that can handle multiple inputs, so that we can connect both a door contact sensor and a PIR sensor to drive this lighting. Both of these would be passed through on to our Home Control System (HCS) I/O boards as they are also used for the security system and other functions. We haven't yet thought of a situation where we need more than two triggering inputs but, it is quite likely that our design would support more using diodes or logic gates.

We can't just connect a door contact sensor to our circuit as this provides a continuous 12V signal when the door is open. We would need to trigger on a 'rising edge' or convert this to a pulse. Typically this is achieved by using a rising edge detector.

Lighting

Typically the convenience lighting is an existing lamp that is also switched manually. In our kitchen though, the convenience lighting is separate LED strip lighting and mounted under the kitchen wall cupboards (out of sight, behind plinths).

Power

In all cases, we envisage this circuit being used with a 12V PIR sensor and thus being fed power in the wire leading back to the I/O board in our 19" rack cabinet. This will thus be used to power this circuit.

The actual lighting power will be electrically isolated and separate. This means this circuit could be used with both 12V or mains powered lighting. The circuit will do this by switching a low-current relay, which will be rated for mains voltages.

PCB blade fuse holder
We have standardised on automotive blade fuses in our home and will use one in the lighting power circuit, to protect the the relay contacts from excessive currents.

Summary

At the moment we are still thinking long and hard about whether this is a good idea. Some of the advantages of full control by our Home Control System (HCS) are lost with this hardware solution. It is currently very easy to decide what triggers the light coming on and it can use all of the environmental factors (dusk, dawn, house status, etc.) available. It is also just a simple configuration change to alter the timer delay.

We are currently building a prototype to test out in one of our rooms though. We will then test it out and see how well it works before we decide on whether we use this approach more widely. We can see a situation where we end up using both approaches to controlling convenience lighting though.

Our initial test case, is going to be to replace the software controlled convenience light in our ensuite bathroom with this hardware-based equivalent. The convenience light will be triggered by both a PIR sensor and the door contact sensor.

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