Magnetic Switches

[Sketchy]

Hi, firstly I shall have to make an apology for I know nothing of electronics - hence my post.  However, I would be very grateful if someone could spare the time to read the rest of my post and offer some advice.

I am wanting a switch which will be capable of switching a stopwatch on and off as it passes through a magnetic field.  I am aware of reed switches, although after a bit of trial and error I have decided that these lack the sensitivity I require.  I am looking for something that would be sensitive enough to switch at a distance of approximately 5 meters.

Unfortunately I haven't a clue what the strength of my magnetic field is.  It is part of a timing system that I bought a while ago which uses a watch which is activated by a set of poles which give off a magnetic field.

Any help you could give me will be much appreciated.

Thanks again

Sam

[pyrohaz]

Hi Sam!

Have you heard of Hall effect sensors? I'm not sure whether they would work with the 5m distance you are proposing but I do know that the output voltage varies with the level of magnetic field. Maybe a bit of research into these? I know that they are used within the twisting throttles of electric scooters and such products but whether they would would work for your job, i'm unsure about.

Hall effect sensors can be bought off Rapid. :)

Edit: For it to switch a stopwatch in your case, the output of the hall effect sensor would need to be amplified for a change (The magnetic field around it changing) and that small change being amplified, compared to its normal state. This can be achieved with a differential amplifier. The output of the amplifier could then be used in conjunction with a relay to switch your stopwatch.

[Sketchy]

Cheers for the info.  I was aware of Hall Effect sensors although I wasn't sure if these would be sensitive enough.  I'll look into what you've said and hopefully I'll be able to report some success! 

Thanks again

Sam