3-way light switching using SPST switches

[nothanks]

I want to re-purpose a couple of existing SPST switches and wiring to provide 2-way switching of a light. At the moment the only way I can think of doing this is to have the switches operate relays and configure their contacts as SPDT and DPDT (intermediate). Is there a nice packaged gizzmo that will do this or have I got to build one? Is there a better way?

(I know I could cut chases and replace the cables but this is in a (very) recently decorated area and management would not be amused if I did that).

[alan_m]

alternative solution -- kinetic wireless switch -- no relays needed

[alan_m]

And if you need to extend the depth of the back box - something like

[Jeff Layman]

Yep. You can pair up to 10 Quinetic switches with a receiver. Only problem is that for two switches and one receiver, it'll cost over £60.

[nothanks]

I've standardised on the MK Logic plus range of switches and sockets so it would be odd to have a few switches that looks different - I looked at the MK Echo range of kinetic switches but the price made me cough.

[Theo]

I was trying to think about something with toggle contactors, but I think your relay was is likely best. The only thing is it will take some static power consumption as it's likely a relay coil will be energised most of the time.

Another alternative might be a latching relay/contactor and momentary switches (maybe available in the MK range?) which would avoid having the static power.

I can't think of a packaged gizmo beyond a DIN rail setup. Where do you want to fit it?

Theo

[nothanks]

Whatever it is would need to be in the ground floor ceiling. I've replaced most of the boards on the landing so access is easy.

I had wondered about using momentary switches and wiring them in parallel to operate something that changes state when there's a pulse on its input - like a D-type bistable. A D-type is easy enough with a few NAND gates but then needs a low voltage supply. I can see how to implement the function with 3 relays, but that's no better than the original idea - it feels like there should be a way of doing it with 2 relays but it's eluding me at the moment.

[nothanks]

Aha! It seems that an "impulse relay" like this

will do what I want.

[Andy Burns]

Apparently common for corridor light switching in germany, why are they better than 2way or 3way switching?

[Theo]

One thing you can do is use the latching relay with momentary switches for the 'on'/'off' action, and have a timer operate the 'reset' condition. This is useful in corridors in say blocks of flats - the lights go on when you press any switch and off after a few minutes.

It's also probably easier to wire if you have a lot of switches - eg a switch outside every flat so you don't have to fumble around a dark corridor to find the switch. All these switches can be run in parallel, while the

3-way switch would need a series loop through all the switches.

It can also be done with buttons where the timing is done with a pneumatic action - the button presses in to engage and slowly pops out over 5 minutes or so. But that's more to go wrong.

Maybe you might do this inside a flat too - the corridor lights allow you to leave and then switch off automatically.

Theo

[nothanks]

After a bit more thought and googling I can see how to implement the multi-input momentary toggle function with 3 transistors, a few discretes and either a relay or a FET (which should all be more robust than CMOS NAND gates) BUT two of the switch positions are in multi-gang plates and that rules-out momentary switches. It also rules-out kinetic switches. I seem to be back to either 3 relays to reproduce a standard multi-way switching situation, risking wrath by digging-out chases, (or sleeving and reallocating the earth wire - I'm not serious about this!)

[Rob Morley]

similar.

[Theo]

I thought about doing it with DC, maybe even repurposing the earth wire as ground (could use a transformer and earth the ground rail). But you'd still need to switch AC for the light with a relay (or solid-state relay), I presume? And the transformer plus extra circuit would make things more complicated.

I suppose grid-style configurable switch plates aren't an option? (not sure if momentary switches are availabe in grid format)

You could do it if the switch was isolated low voltage DC, no need for an earth there :-) Although running that in the same box as mains might be less good (I presume you would still have an earth via the mains side anyway).

Theo

[chop]

That doesnt reuse his existing spst switches. If you are going to get new switches, you might as well just use the normal spdt new switches.

[alan_m]

Kinetic have single grid switches that could be installed in a grid plate with conventional switches.

The receiver doesn't have to be in switch back-box.

[alan_m]

The problem may be with the absence of suitable wires.

[nothanks]

Changing the switch isn't the problem - it's the lack of a third wire!

[Animal]

You just need a bit of electronics that converts each switch operation into an output state change. Or I suppose you could do it the 1920s way, making a rotary sawblade shaped thing that presses on a switch, with each button press moving it half a tooth on.

[nothanks]

That's why I mentioned a D-type bistable earlier ...

[John Rumm]

Is there anything particularly special about the current switches that makes them difficult to substitute?

Otherwise there is a danger of spending lots on a complicated fix which could circumvented with some cheap replacement switches.