Fan POV question

I have two ceiling fans in my house that never get used (I live in the north of England, it's seldom warm) that I'd like to turn into a POV.

Now, I've had a think about the code and hardware and I'm pretty sure I can do it all except measuring fan speed. I'd like to be able to measure fan speed at startup (so it'll work on other fans or if the speed is changed) but I don't really know the best/cheapest way to do this. Any suggestions?

Single magnet and a Hall effect switch.

Do you mean a hall effect sensor, or a reed switch? (a hall effect sensor measures change in magnet field, a reed switch is basically a type of button that is 'pressed' when a magnet comes near it)

I presume a reed switch is going to be cheaper and more readily available?

Would the reaction time be swift enough? I know the fan is only going to be going at a few hertz but it still seems quick to me.

I certainly didn't mean a reed switch - even if it could switch fast enough (which I doubt, unless you had a massive magnet), the things bounce badly, upsetting your timing. Hall effect sensors however, can switch extremely cleanly at quite high rates.

Cool, thanks Groove.

Any suggestions on a UK supplier who'll sell me one? I find myself out of my depth if I can't get it at Maplin :(

RS will certainly have them, as will Dodgykey, CPC/Farnell probably. The ones I've used were packaged like transistors. I'll see if I can find some part numbers.

Part numbers would be appreciated, I've had a browse of the CPC and farnell online stores and the range is bewildering.

I think I've used Honeywell in the past, but I've forgotten the exact device:

http://cpc.farnell.com/jsp/search/productcompare.jsp?N=411&Ntk=gensearch_003&Ntt=Hall+effect&Ntx=mode+matchallpartial&suggestions=false&ref=globalsearch&_requestid=1146199&isGoback=false&isRedirect=false

Thanks!

You do realize what a fan is for, don’t you? It has nothing to do with direct heating or cooling…

You might find that, on cold days in the winter, turning the fan on allows you to lower the temperature of your thermostat (saving you money) while making the house feel less stuffy (by circulating the air); provided you have it pushing the air in the right direction (down from the ceiling when heating, up from the floor when cooling).

Ta for the advice, but I don't have central heating. I've a gas fire in my lounge and none of the rest of the house gets heated. I might live in Wigan now, but I'm a tight fisted yorkshireman through and through ;)

There's a fan in each of the bedrooms. I don't think the previous tennant used them either - they're caked in dust.

Do you mean a hall effect sensor, or a reed switch? (a hall effect sensor measures change in magnet field, a reed switch is basically a type of button that is 'pressed' when a magnet comes near it)

Actually hall effect sensors are available as either a analog output (measuring a magnetic field's strength) or as a switch (magnetic field more or less then a preset setpoint).

Here is an example of a Hall effect switch:

http://www.allegromicro.com/en/Products/Part_Numbers/3213/

Lefty

Interesting, never even heard about them. They appear to be reed switch simulating hall effect sensors. I have some reed switches laying around that would easily be able to switch in time, but they wouldn't work on a fan I suppose.. they're not supposed to be triggered that quickly. The hall effect switch looks interesting, definitly going to remember it! might come in use for a future project =)