Reading some type of magnetic sensor

Hi Folks,

I got my hands on a second hand weather station (no controller, just the parts) which I am trying to get working with the Arduino. The anemometer is my first port of call. Opening it up revealed far more than I was expecting.

The item of interest is the anemometer sensor. One of the blades in the little "fan" is magnetic. The pickup though is sealed in the plastic case which I cannot get to. It simply has two wires hanging from it. Sticking a voltmeter to it and bringing a strong magnetic past it gives me a voltage of up to 7mv. Connecting this to the Arduino though picks up so much noise and not enough of a change to make a valid measurement.

I simply have one end of the "thing" into ground, the other into analogin. I have tried pulling it up with a 47k resistor but the same happens.

How can I accurately read these tiny pulses? I cannot average them as this thing spins pretty quickly.

Alternatively, anyone know what the sensor might be? It's tiny (like a small cap). It has a resistance of 328 ohms.

TIA

Cheers, Crispin

It sounds like it is a small coil, the magnet is inducing a voltage in it. It needs to be amplified with an operational amplifier before you can read it with the Arduino.

The other possibility is that it is a hall effect sensor but you need to supply that with power. It might be easer using a hall sensor switch in place of the coil as this has a built in amplifier and an output suitable for direct connection to the Arduino.

Thanks Grumpy Mike,

I had thought it was a coil. I have a failed pc fan which I believe have a hall effect switch sensor in them. I’ll rip it open and see.
I’ve been looking for a decent source for components in London. The only two I know, Maplin (Almost laughable) and RS (So long as you don’t mind buying 20 at a time) and neither of these stock and small sensors.
As an aside, anyone know of decent retailers in the UK?

Cheers,
Crispin

I use Farnell. There is a minimum order of about £25.00 nowadays (I used to have a credit account with them when I was spending about £800 a month and there was no minimum order). While they are not the cheapest they do have a good range of stock and orders normally arrive next day plus P&P is included in the price (but not VAT).

Thanks - will try them.

I just foudn out there is an RS down the road from me so I'll try them later and see if they have stock (I found the sensors in the catalog. Who knew Hall Effect Switches and Hall Effect sensors are not the same :) )