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Question regarding magnetic pickups

  1. Jul 20, 2010 #1
    Im working on a bit of a science project, my knowledge of electronics is limited...for this project i need some type of pickup on a rotating object to connect a circuit to open a solenoid valve whenever the object is at a given point on its rotation...and it has to fast enough to do this a few hundred times a second

    my first thought was a magnetic pickup, because it doesnt need any contact with the object to get a reading...but to my understanding most magnetic pickups, like the ones used on engines and such only change the resistance between two terminals...in order to use this I would think I need some type of control module to measure that change in resistance, and turn it into an on/off switch, seems like a little too much for me

    I have never seen one that simply goes from open circuit to closed when the pickup mark goes by, does something like this even exist? ive been searching everywhere for it, and im finding nothing

    can someone point me in the right direction? not sure what to even call said switch, if it even exists, thats probably why i cant find one

    are there any other ways im unaware of to getting a non-contact pickup switch other than magnetically?
  2. jcsd
  3. Jul 20, 2010 #2
    http://en.wikipedia.org/wiki/Reed_switch" [Broken] and a magnetic marker, I am not sure about the speed, but your application doesn't sound that fast...
    Last edited by a moderator: May 4, 2017
  4. Jul 20, 2010 #3
    awsome, exactly what I was looking for...thank you

    did some quick searching around, it appears that most of the reed contacts are for alarm system applications which are not intended to be cycled quickly...im worried i may wear one of those door and window switches out with just a few minutes of usage on my application...but if i can pick one up for a $5, its worth a try

    the wiki page said reeds are used on bicycle tachometers, so theres definitely some high speed reeds, just gotta find one
    Last edited by a moderator: May 4, 2017
  5. Jul 20, 2010 #4


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    I'm surprised the wiki page says that reeds are used on some bike computer pickups -- I think the speed and wearout limitations would preclude them from that application.

    If the speed of the rotating object can be slow sometimes, then you should use a Hall effect sensor to detect the passing magnet. Yes, you will need a small amount of circuitry to convert that signal into whatever you need to drive your solenoid.

    If the speed is always pretty fast, then all you need is a small coil to pick up the voltage from the passing magnet. Again, you will need a small amount of circuitry to convert this voltage pulse into the signal to drive your solenoid.
  6. Jul 20, 2010 #5


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    Welcome to PF suprant0010. Have you considered doing it optically? Here's a little DIY kit that may give you some ideas for your project.
    http://www.quasarelectronics.com/velleman/k8092-optical-proximity-switch-kit.htm" [Broken]
    Last edited by a moderator: May 4, 2017
  7. Jul 20, 2010 #6
    speed is going to remain a constant, at least at this stage of the project...so no need to worry about changes in pulse width

    i was hoping to keep things as simple as possible, seeing as how my knowledge of circuitry is very limited...if possible i wanted the pickup, whether it be a reed or some other type of switch, to close the circuit that activates solenoid directly

    the solenoid im using only takes 0.8 amps at 12v DC, im seeing plenty of reed switches that can handle 1 amp...it seems to be a good route to take, my only concern being how long they will last under these conditions

    interesting...the only problem i see with it is it says that its not intended for dark areas, my project is enclosed in a housing and will be completely dark inside
    Last edited by a moderator: May 4, 2017
  8. Jul 20, 2010 #7


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    On the reed relay (or any relay), keep in mind that their contacts bounce for several ms (this should be spec'd in their datasheet). At 200rpm, the bounce time of the relay is almost equal to the rep rate...

    The optical pickup is a great idea. You can use a reflective photopickup device that has its own light source, which would work great inside a dark enclosure.

    What kind of solenoid valve are you using that has that kind of response time?
  9. Jul 20, 2010 #8
    well, its not really a solenoid valve in the general sense... its an electronic fuel injector

    figured id just call it a solenoid, because i didnt want to get into detail about the actual project
  10. Jul 21, 2010 #9


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    Oh, cool. Are you planning on tuning the delay between the sensor and the injector by moving the magnet/optical reflector on the rotating mechanism, or would it be better to put that adjustment in the circuit that detects the pickup and fires the injector...?
  11. Jul 21, 2010 #10
    yeah, trial and error until i get it where it needs to be
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