switching problem

[Bill Wright]

The cupboard has two doors, which open independently. The plan is to fit two push switches of the type sold for this purpose, one for each door. The switches will control a light that will shine into the cupboard, which is rather deep and gloomy. These switches are 'on' when the plunger is out, so the light comes on when the door is open. I also intend to use a timer so that if the door or doors are left open the light will eventually go out. The timer will commence timing when the supply is applied (the door(s) are opened) and if the door or doors are left open the light will eventually go out. If the doors are closed during the timing period the light will go out immediately. In order to initiate a timing period and turn the light on the door must be closed, then the timing period starts when the door opens. I've sorted the timer out. It's a multifunction one, and it does what I want. But ideally I'd like the light to come on if either or both doors are opened, and stay on for the full duration of the timing period even if one of the two doors is shut. With one door it's simple. The one switch controls the timer perfectly. But with two doors and two switches the best I've done is to have the switches in series, so that both doors have to be open to initiate the timing period. With the switches in parallel, if one door has been left open the timing period will not commence when the other door is opened. It is necesssary for both doors to be closed, then either to be opened. If the switches are in series, for the timing period to start one or both doors must be shut, then both must be open.

Suggestions anyone?

Bill

[Harry Bloomfield]

Bill Wright formulated the question :

I can only think of using two such timers in parallel each fed from a door switch, or a pair of instantaneous switches - ones which makes briefly as a door is opened, then relying entirely on the timer.

[Dave Liquorice]

Patently it doesn't or you wouldn't be asking the question... B-)

What does your multifunction timer switch need to trigger and retrigger the timing period? I suspect it needs a rising edge, generating that with just two SPST switches is going to be tricky. Some devious arrangement of two DTDT switches *might* do it utilising the break when a switch changes over to produce the rising edge. SPDT switches won't work in the normal lighting two way switching arrangement.

I suspect your easiest solution will be to find a timer module with two trigger inputs. You may have to go as far as a small PLC.

[Harry Bloomfield]

Dave Liquorice has brought this to us :

To turn a light on 8-o

[Dave Plowman (News)]

There's probably a way of doing it with logic - but since you're using a timer to switch the load I'd fit two reed burglar alarm switches per side

- the type with changeover contacts. Some clever wiring of those should give you the logic you need. They'll be more reliable in operation too than those pukka door switches which are quite critical in how they're installed.

[Bill Wright]

Ha! Well, it worked fine til the person building the cupboard decided to fit two doors instead of one big one!

In the mode that I'm using it needs power on. This starts the timing period.

I see what you mean.

I suppose I could use two timers, but it's a faff and an expense.

Bill

[Bill Wright]

It's funny that, because I've done this job in motorhomes (a simple one switch-one light arrangement with no timer) and I've had problems with reed switches sticking (this was on 12V DC), so this time I decided to use mechanical ones! I must say these mechanical switches don't seem very good. The push force needed is quite high for one thing.

Re reed switches, I've used them for gate contacts and found that they fail. This is with big butch ones. Someone advised a cap (can't remember the value) across the switch and that helped a lot.

Bill

[John Rumm]

So for system inputs we have:

SW1 SW2

For Timer inputs we need:

Activate Reset

For timer output we need:

Timer Elapsed

And for system output we need

Light On

The equations are therefore:

Activate = SW1 | SW2 Reset = !( SW1 | SW2 ) Light On = ( SW1 | SW2 ) & !Timer Elapsed

OK, we now need to make a raft load of assumptions since we don't know much about the timer. I will assume it has a run, and a reset capability

- lets also assume they are mains level and active high.

A couple of relays should do it:

Theory:

Your activate signal is simply the wire or of the two switch demands. That is also used as a level sensitive input[1] to cause the timer to run, and also lights the lamp via RL2.

RL1 inverts the Activate to provide a reset signal for the timer. The timer is held in reset all the time both doors are shut. (if the timer has a combined reset / run line, even better - you don't need RL1 at all then ;-)

Once a door opens, the light comes on and the timer runs. One the timer elapses, its elapsed signal switches RL2 which turns the lamp off. You will need to shut both doors to get the light to come on again.

Note, take much of the above only as an idea in principle. If your timer has a "not triggered" output you can do away with the second relay.

[1] If its edge triggered, then you have a race hazard since it won't be release from reset until slightly after the edge on Activate.

[Dave Liquorice]

don't think that will reset the timer if one door is already open and the other is opened.

Ah it doesn't, unfortunately that breaks the spec...

Have you got the logic/wiring the right way round for "elapsed" and RL2? Or is your design to keep the elasped high and the relay energised to keep the lamp off with the doors shut. Wouldn't it be better to have the RL2 energised only when the lamp is supposed to be on?

We really need to know the make/model/version of Bill's timer before thinking much harder about the logic/switching arrangements. Perhaps it does have a "reset/retrigger" input perhaps it doesn't just being a simple delayed off non-retriggerable device.

[Dave Plowman (News)]

They also tend to push a near closed door open. Switching on the light.

I've got them on near every window and door here on my home installed alarm and haven't had one failure. Decent reed switches have a very long service life - assuming their rating isn't exceeded. If you're using an electronic timer the trigger current should be tiny. I dunno how they cope with vibration though - but this won't be a problem indoors.

If you're not familiar with alarm types there are two suitable - either flush fitting or surface mount. Switch and magnet come in similar housings.

[Toby]

Have the original switches paralleled, then install a second switch on each door that is normally closed, but on opening the door, it is pressed and released (Like a bell switch on a shop door, but normally closed), wire these in series with the paralleled original switches.

Toby...

[John Rumm]

Does it? Depends on how you read it I guess. Either door opens and the light comes on and the timer starts. Close both and it goes out. Leave one or both open and eventually the timer turns the light out. Once this happens you need to close both before opening a door will relight the lamp.

The supply to the lamp is derived from the Activate signal (i.e. switch output) not a permanent live. It passes through a normally closed contact on the relay. So the relay is unenergised in the door shut state, and only gets to turn the power *off* when energised by the timer.

From one of his other posts it could simply be a apply power, and it triggers some time later device. If that is the case we can lose RL1 altogether - the switches then turn the light one and run the timer. The timer is reset by removal of power (i.e both switches closed) and on expiry trips RL2 to open the supply to the lamp.

[Recyclist]

I'd use a PIR sensor and a timer, or even a PIR sensor with a timer built in.

T

[Bolted]

Or a photo-sensor, if it doesn't matter that the cupboard light won't come on if it is dark and there are no other lights on nearby (which doesn't seem too unreasonable).

[Recyclist]

A photo-sensor to switch a light on?

T

[Dave Liquorice]

Aye, there is one in the anemomter that is currently whizzing round in an F7 near gale outside, producing a pulse every rotation. That reed switch has failed in the past but this one has been in service for at least a couple of years.

So lets say 60 rpm average, we do get calm days and days like now. 1 pulse/sec or 63,072,000 operations over two years...

[John Rumm]

Prolly better off with a hall effect switch rather than a reed with that many cycles...

[Bolted]

Yes, it's a cupboard which is a bit gloomy with a door open and very much more gloomy with the doors shut. As long as you have daylight or other lights on nearby, with suitable tweaking of the sensitivity there is no reason it won't work.

[Recyclist]

:

So, how do you get the light to switch off?

T

[Harry Bloomfield]

Bill Wright wrote on 08/09/2009 :

They are not very good at handling inductive loads which would cause the contacts to weld.