Question regarding magnetic pickups

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Discussion Overview

The discussion revolves around the design of a non-contact pickup switch for a rotating object, intended to activate a solenoid valve or electronic fuel injector at specific points in its rotation. Participants explore various technologies, including magnetic pickups, reed switches, Hall effect sensors, and optical sensors, while addressing the challenges of speed and durability in their applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using a magnetic pickup for its non-contact nature but expresses uncertainty about the need for a control module to convert resistance changes into a switch action.
  • Another participant recommends reed switches, noting their common use in alarm systems, but raises concerns about their durability under high cycling rates.
  • Some participants mention that reed switches are used in bicycle tachometers, indicating the existence of high-speed variants, but express skepticism about their suitability for the project.
  • A suggestion is made to consider Hall effect sensors for detecting a passing magnet, with the acknowledgment that some circuitry would be necessary to drive the solenoid.
  • One participant proposes an optical pickup as an alternative, providing a link to a DIY kit, while another raises concerns about its performance in a dark enclosure.
  • Concerns are raised about contact bounce in relays, particularly at high rotation speeds, which could affect the timing of the solenoid activation.
  • Participants discuss the nature of the solenoid being used, clarifying that it is an electronic fuel injector rather than a traditional solenoid valve.
  • There is a mention of adjusting the timing between the sensor and the injector, with one participant suggesting trial and error as a method for tuning the system.

Areas of Agreement / Disagreement

Participants express a variety of views on the best approach to achieve the desired functionality, with no consensus on a single solution. Different technologies are proposed, each with its own advantages and limitations, and concerns about durability and performance remain unresolved.

Contextual Notes

Participants note limitations regarding the speed of operation, the potential for wear on components, and the need for additional circuitry to interface with the solenoid. The discussion reflects a range of assumptions about the operational environment and the specific requirements of the project.

Who May Find This Useful

This discussion may be useful for individuals interested in electronics, particularly those working on projects involving non-contact sensing technologies and solenoid activation in mechanical systems.

suprant0010
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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 doesn't 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? I've been searching everywhere for it, and I am finding nothing

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

are there any other ways I am unaware of to getting a non-contact pickup switch other than magnetically?
 
Engineering news on Phys.org
http://en.wikipedia.org/wiki/Reed_switch" and a magnetic marker, I am not sure about the speed, but your application doesn't sound that fast...
 
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0xDEADBEEF said:
http://en.wikipedia.org/wiki/Reed_switch" and a magnetic marker, I am not sure about the speed, but your application doesn't sound that fast...

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 there's definitely some high speed reeds, just got to find one
 
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suprant0010 said:
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 there's definitely some high speed reeds, just got to find one

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.
 
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"
 
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berkeman said:
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.

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 I am using only takes 0.8 amps at 12v DC, I am 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

dlgoff said:
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"

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
 
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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?
 
berkeman said:
What kind of solenoid valve are you using that has that kind of response time?

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
 
suprant0010 said:
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

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...?
 
  • #10
berkeman said:
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...?

yeah, trial and error until i get it where it needs to be
 

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