Mixing low and high voltage control

[JonSenior]

Hi there forum, first post and it risks being a long one... sorry!

I'm currently in the process of building my house and while right now it has only 2/3rds of the timber frame and no walls or roof, it's time to work out the wiring.

The plan had always been to wire the house for monitoring (temperature & humidity) mostly to keep tabs on the effectiveness of the insulation and the ventilation system. The monitoring system will be based around the Raspberry Pi linked to a 1-wire network. We're not planning for house-wide entertainment systems, and any shutters will be traditional wooden ones that you close yourself so the requirements for automation are fairly limited. However, two of the rooms have multiple entry points and the local (French) wiring regs dictate that each entry point has a light switch. The "normal" solution would be to wire up a 3-way switching set-up, but as my current appartment has a such a set-up and a certain combination of switches will cause one switch not to operate, I'd like to avoid this (not to mention the rats nest of wiring that it generates)! The next solution is to replace the switches with push-buttons, and put a DIN-rail relay in the switch box. This solves the problem but is expensive and I feel I'm missing an opportunity.

So having thought about it, I decided that the way foward would be to use a latching relay (I don't like the idea of using electricity to keep the switch on) with a simple driver circuit. This can be built at relatively low cost and adds the possibility of computerised monitoring and / or control. The plan is to use a T flip-flop to drive the two coils of the relay (via a pair of monostable timers to generate the pulses). This would allow a single toggle input which means that any number of switches can be connected to it and they simple need to provide a debounced pulse to toggle the light on / off. The paired outputs from the T flip-flop allow for monitoring of the current state of the light and eventual computer control.

The problem is with the switches. The idea is that the control system (drive circuit and relay) is simple logic and doesn't require a computer (this is going to be a work in progress and I'd like to be able to use the house before the control software is written), but it's going to have to run at TTL / CMOS levels and I'm not sure that the voltage drop on a 5v control system won't be too great. I'd have confidence in a 12v system, but that adds another level of voltage conversion to the mix (5vdc, 12dvc, 240vac). The only other solution that springs to mind would be to use another set of non-latching relays with 240vac coils to provide a method for generating 5v pulses, but this seems overkill to say the least.

So... to ask a real question.

1) Would a 5v control system run over standard mains wiring (1.5mm²) suffer too great a voltage drop to be usable?
2) If so, can anyone suggest a circuit that allows a 240vac pulse to generate a 5vdc pulse reliably?

Thanks in advance and sorry for the long and rambling post.

Jon

[JonSenior]

400 views and no answer?

A little bit of maths showed that for control circuitry (<1A for certain, probably <0.1A for most cases) the expected voltage drop on 1.5mm² cable is going to be around 0.1V over 40 metres. Most wiring runs in my house will be significantly less than that and 4.9v will trigger TTL just as well as 5v.

So to answer my question 1 (in the hope that this information is useful for someone else in the future); No. The voltage drop will not be too great.

As a consequence my house will probably end up with 5v wiring to the switches and latching relays controlling the lights. Thankyou for listening!