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Old 17th Sep 2019, 5:20 pm   #1
PsychMan
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Default Mains One Shot Timer?

Im looking for a one shot timer that can turn on a small extractor fan for a defined time, say 3 hours for example. Once set up it can be activated by a button.

I had lashed up something but I wonder about how sensible I am deploying this, and wonder if there's a simpler and possibly safer way of achieving what I want.

My lash up currently consists of a small timer module, SMPS, relay w/ snubber, push button, neon, and inline fuse holder with 1A fuse. I had bodged it into a double metal back box, including mounting some of the boards with epoxy and standoffs.

The idea was to install it in a garage / utility area for activation when a tumble drier is on. But as I look at my somewhat hideous creation, I wonder about how safe it would be long term. The box is earthed, it has a 1 AMP inline fuse on the live wire, we have decent RDCs, but still. It looks a bit....err....crap?
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Old 17th Sep 2019, 5:33 pm   #2
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Default Re: Mains One Shot Timer?

Can you not just get a cheap plug-in timeswitch then open it up so the motor drive supply is taken from the OUTPUT side?

Then, set it to come on for 3 hours (or whatever you want, up to 23 hours 45 min). Switch on by rotating it till it comes on. When the time has passed, it will switch OFF and then stop!
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Old 17th Sep 2019, 5:43 pm   #3
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Default Re: Mains One Shot Timer?

I suppose so....I do have some timer plugs knocking about. But it would stick out in what is already a pretty narrow space. It just doesn't seem as....cool!

That said, my time is pretty limited these days, and perhaps im over engineering here.

Its funny, I hadn't considered this, but am actually doing the EXACT same thing with 1 hour timer plug powering a nightlight in my little boys room
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Old 17th Sep 2019, 6:10 pm   #4
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Default Re: Mains One Shot Timer?

Plenty of industrial units will do exactly what you want, I use one for the extract fan in my garage. Widely available but most fit DIN rail so stick out a way from the rail.
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Old 17th Sep 2019, 8:22 pm   #5
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Default Re: Mains One Shot Timer?

There are main switching heater controllers that give an option of 2/4/6 hours and then switch off, some cycle the 2 hour option though.
I believe horstmann and electrisave make them amongst others.

There is also this https://www.screwfix.com/p/elkay-3-w...on-white/34572 that will do up to two hours.

Or this https://www.screwfix.com/p/greenbroo...ails_container which has a customisable 'Boost' mode that could work.
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Old 17th Sep 2019, 8:37 pm   #6
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Default Re: Mains One Shot Timer?

There are loads of commercial-off-the-shelf things that deliver a timed "on" output in response to a button-press/instantaneous-contact-closure.

The simple stuff - as used to illuminate a stairwell when the button's pushed and give you a few minutes of light - are often pneumatically-based. Better ones use a crystal-oscillator and a divide-by-N counter for the actual timing.

In my experience, a frequent 'issue' is if you only have a 2-wire supply to the timer (like if you're replacing a simple on/off switch so the switch is in series with the load) - if the load doesn't present a hard open/short-circuit to the timer it can be tricky to get sensible results. Which is why the pneumatic-type are popular in the stairwells of rental properties!
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Old 18th Sep 2019, 12:15 am   #7
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Default Re: Mains One Shot Timer?

Assuming all you want is to extract what is coming from the drier to the outside world, would it not just be easier to plumb the drier exhaust to the outside?

Most driers used to have a kit available for this.
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Old 18th Sep 2019, 9:42 am   #8
PsychMan
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Default Re: Mains One Shot Timer?

Thanks folks, some nice ideas to look at there. I may well just to as Kalee suggested, I'll have to think some more.

And yes Terry I do have that arrangement already, though not quite plumbed into the drains yet (another to-do list item, including said drain to run it into!) , it just goes outside right now. But in spite of this it does still create a little moisture in the room, so I'd like to run a fan in addition to this.
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Old 18th Sep 2019, 10:34 am   #9
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Default Re: Mains One Shot Timer?

Can you just connect the fan to the tumble drier timer output itself?

Shouldn't be too difficult to get at.
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Old 18th Sep 2019, 11:20 am   #10
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Default Re: Mains One Shot Timer?

When I had a similar problem I used a humidity sensor on a fan. That’ll work if the space gets damp for any reason.
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Old 18th Sep 2019, 11:24 am   #11
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Default Re: Mains One Shot Timer?

Quote:
Originally Posted by Herald1360 View Post
Can you just connect the fan to the tumble drier timer output itself?

Shouldn't be too difficult to get at.
Yes, I like this idea. I would fit a trailing socket from the dryer into which the fan could be plugged. You'd need some kind of small inline fuse (1A should be ample) to protect whichever bit of the dryer you'd connected it to from overload.

Or better still, couldn't you arrange for the dryer to vent through a wall (or window) so that no forced ventilation of the room itself is required?
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Old 18th Sep 2019, 11:48 am   #12
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Smile Re: Mains One Shot Timer?

Hi,
A while ago I was looking for a device to stop car batteries being over charged through my forgetfulness, and came up with a mains relay, a push button and a plug in timer.
The relay was wired to keep itself energised via the normally open contacts that were in parallel with the push button. The timer was set for the required time and turned to the start of the sequence. (The time of day was unimportant).
When the timed period expired, the relay dropped out, losing its own feed, and the charger turned off. If, as often with me, I forgot about it, the relay couldn't re-energise unless the button was pressed again.
Crude, cheap, but it worked.
Cheers, Pete.
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Old 18th Sep 2019, 1:37 pm   #13
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Default Re: Mains One Shot Timer?

I don't know if they are still around, but at one stage, you could get a power board that had one continuously powered socket and the others wouldn't turn on unless the master socket had something plugged in and drawing power..

If they are still around, plug the drier into the master socket and the fan into one of the slave sockets.
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Old 19th Sep 2019, 12:04 am   #14
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Default Re: Mains One Shot Timer?

Ready made units are available from ebay and elsewhere.
Suggested search term "run back timer"
Many types have a very short time, but up to two hours are listed by several suppliers, and longer times exist but are less common.

They are intended for mains voltage and merely need mounting in a suitable box of earthed metal or of non flammable insulating material.
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Old 19th Sep 2019, 8:44 am   #15
vidjoman
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Default Re: Mains One Shot Timer?

I still think my solution in post #10 is the simplest as once installed it need no setting or pushing a button etc. It's mechanical and just works when the humidity gets above a certain level. I bought 5 for a £ a few years ago, fitted 2 in my previous house and gave the others away. Works by a micro switch sensing the tightness/slackness of a membrane. They kept the bathroom and utility room completely free of damp for years with no maintenance, batteries, power or user input required. Power is obviously required for the fan and wired via the micro-switch on the sensor.
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Old 21st Sep 2019, 5:54 pm   #16
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Smile Re: Mains One Shot Timer?

Hi,
Yesterday I had to scrap an oscillating fan in which the two motors had burnt out, However, it had a simple clockwork timer that could run for up to two hours.
It's small enough to fit into a standard enclosure and is rated at about 1.5 amps.
Just an idea.
Cheers, Pete.
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Old 22nd Sep 2019, 10:02 pm   #17
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Default Re: Mains One Shot Timer?

Another alternative suggestion - https://www.timeguard.com/products/t...ic-boost-timer

David
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Old 24th Sep 2019, 10:05 am   #18
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Smile Re: Mains One Shot Timer?

Hi,
Re: Vidjoman's post at no. 10 suggesting a humidity detector. Just a little note of caution.
We had one in our kitchen when we lived in the UK. It would (sometimes) start up when we were cooking, and carry on with a high pitched whine for hours thereafter.
In the end, I got so fed up with it that I took out the sensor and fitted a small pull cord switch. We didn't need a timer, as the whine was a constant reminder that it was running!
Cheers, Pete.
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Old 24th Sep 2019, 10:23 am   #19
vidjoman
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Default Re: Mains One Shot Timer?

The noise level will always be there whatever motor/fan used but selecting a quiet or slower running fan will reduce the noise level. My ones were fitted into a wooden trunking through a brick wall and were fairly quiet.
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Old 24th Sep 2019, 11:25 am   #20
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Default Re: Mains One Shot Timer?

Clas Ohlson used to sell a very nice time-out unit that would go up to 50 mins. I was building one for my wife at the time we saw them, so we bought a couple.

I wish we had bought more because they are very useful.
Sadly they no longer stock them and I have have not seen anything like it sold elsewhere.
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