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Oscillating reed switch

Discussion in 'General Electronics Discussion' started by electronictony, Jan 9, 2018.

  1. electronictony

    electronictony

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    Jan 9, 2018
    I would be grateful for any ideas for a ready made device containing an isolated reed swich in a relay and oscillating device (magnet?) that will switch the reed switch on/of with a control between 5 and 50 Hz. I have tried a pulse generator instead of a reed switch but this will not work properly owing to a barrier.

    The reed switch will carry 1 volt and uA and the oscillating device will need to be operated by a 5vDC, 9vDC or 12vDC power supply.
    Tony
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Try a grandfather clock with adjustable length of pendulum for adjustable frequency and a magnet at the end of the pendulum to activate the reed switch ;)

    In all honesty: depending on your requiremenents with respect to accuracy anything from a quartz oscillator down to a NE555 based astable multivibrator can be used to generate the timing signal.
    The timing signal ( 5 Hz ... 50 Hz) can be used to drive current through a coil which in turn generates a magnetic field to actuate the reed switch. Depending on the actual reed switch you are using lifetime may be limited. Assuming a lifetime of 1 million switching cycles and a rate of 50 Hz, this amounts to a lifetime of t = 5.5 hours only.
    No mechanical switch is suitable for lots of switching cycles at a comparatively high rate of switching. Dependion your application you might consider an electronic switch instead. 1V at 1 µ is well suited to electronic switching in general. What kind of switch should be used depends, as stated, on the application.
     
    hevans1944 likes this.
  3. hevans1944

    hevans1944 Hop - AC8NS

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    One million cycles is way too limited for a low-current, low-voltage, magnetic reed switch. For your application, 1 V and a few microamperes, expect upwards of one billion cycles, or a thousand times longer than what a measly million cycles lifetime would provide. This is still not good enough for extended operation. Investigate a solid-state switch approach instead.

    Early in the previous century, when car radios were new and used vacuum tubes, a vibrating, magnetically driven, reed switch was used to produce a form of AC from the 6 V car battery and, with separate contacts, also provided synchronous rectification of the stepped-up AC after its application to a transformer. All this just to get a few hundred volts DC to drive the plates of the vacuum tubes. Most folks called the contraption a vibrator and vibrators were notorious for failing early and often. BUT IT DID WORK! Led to the development of vacuum tubes that could operate with very low plate voltages, typically 12 V, supplied for the then-new 12 V ignition systems, but that didn't last long as transistors came on the scene and completely replaced tubes for automotive radios.

    Later, the vibrator design was refined to a simple SPDT vibrating reed and used to modulate low-level (microvolt) DC signals so the signals could be amplified by AC-coupled amplifiers. This had a major advantage at the time of totally eliminating DC drift in the amplified DC output. It was mainly used in sensitive strip-chart recorders and was called a chopper. The technique is still used today, in solid-state switch form, to achieve very low offset and very low drift in precision operational amplifiers.

    What does this even mean? What barrier? What pulse generator? Provide us with a sketch of what you are trying to DO, please. Or is this project covered by the Official Secrets Act, and you can say no more?
     
    Harald Kapp likes this.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Still only 5500 hours, approx. 1 1/2 year.
    I totally agree.
     
    hevans1944 likes this.
  5. electronictony

    electronictony

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    Jan 9, 2018
    Thanks for the advice.

    It is simply a flowmeter operated reed switch that I wish to replace with a test instrument. The barrier is a Pepperl-Fuchs device to prevent 25VDC getting to the reed switch. The 25 VDC is driving various pumps and signal devices. I can only get to the flowmeter reed switch input on the barrier.

    The square wave pulse generators I have tried will put something through the barrier but lasts only 2 or 3 seconds before stopping.

    The only thoughts I had were to use a reed relay but not sure if pulse generator(NE555) would drive the 5 VDC coil.

    I am not skilled in electronics and therefor trying to find a ready made device. For someone who can only think of putting a magnet on electric drill near a magnetic reed switch any help would be appreciated.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Do you have a circuit diagram for the setup of flow meter, Pepperl-Fuchs device and power supply? Or a link to an application note for said device?
    The "barrier" could be anything. We cannot help if we don't know what this means in electronic terms. Possibly a simple photocoupler can solve your task.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Maybe the solution lies here. What did you try? Why did they "stop"?

    ak
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Hmmm. So you want to disconnect the flow meter, which contains a reed switch that closes and opens in response to flow, and replace it with "something" that will simulate the actual flow meter, perhaps for test or troubleshooting purposes? Yet, you "are not skilled in electronics" by your own admission. Why are you even allowed to work on this without any understanding of how the flow meter interfaces to the Pepperl-Fuchs isolation barrier? Which isolation barrier device (model or part number, please) does the flow meter connect to? Who is the manufacturer of the flow meter and what is its model or part number? How is the flow meter supplied with a voltage (1 V?) that will allow a minuscule current (μA ?) to flow when the reed switch contacts are closed? How is this information communicated through the isolation barrier to someplace that it can be used?

    Your suggestion of mounting a magnet in an electric drill and bringing it close to a magnetic reed switch is an excellent idea. However, I would use a small, dedicated, permanent magnet DC motor (many are available on eBay) with a PWM (pulse-width modulated) driver (also available on eBay) to provide a variable rotation speed instead using of a commercial, off-the-shelf (COTS) electric drill, which is likely to be too big and unwieldy for field use.

    Attach a small magnet to the end of the PMDC motor shaft, with the line between the N and S poles perpendicular to the shaft. Mount both the motor and a miniature reed switch to a small wood or plastic board in a fashion that allows you to position and re-position the reed switch relative to the magnet until you achieve at least one reliable contact closure per revolution of the motor shaft. The reed switch will respond to both the N and S poles of the magnet, so you will probably obtain two contact closures per revolution. Whether one or two, make sure the contact closures are reliable.
     
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