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Thread: looking for solenoid advice for woodblock metronome project

  1. #1

    looking for solenoid advice for woodblock metronome project

    Im looking to build a metronome that can do some odd time signatures. I envision the project will reside in a wood shell with a metal panel that has controls mounted on it and two solenoids that hit different faces of the wood shell to get 2 different sounds. The whole thing should be about the size of a beer can.

    Id love some advice about push solenoids. I want something that thumps hard and resets in no more then 300 milliseconds. Should I be looking at 5V board? Is there a way I could use a capacitor or two so I can get a better wack? I understand capacitors in theory but have no idea on how to design with them.

    I am seeing plenty of solenoid percussion projects but not many where the solenoid itself is the beater. I am sure other people with more knowledge then myself have worked this out before- feel free to push me at existing threads or projects.
    Thanks!

  2. #2
    Senior Member oddson's Avatar
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    You don't need 5v as you do need to power the solenoid with a much higher voltage anyway and at a very high current as it starts to move so you need to sink the curent though a FET transister as a switch and a diode to prevent back currents as it settles mechanically after firing.

    Something like this showing a Pi as the signal source.

    Click image for larger version. 

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    caveat lector - I'm not an EE so you may need qualified help with the details.

    As for solenoids, any chance you can locate an old door chime? I recall playing with one as a child... I think they ran at 24 volts and should have the usable mechanical characteristics.
    Last edited by oddson; 10-02-2019 at 06:07 AM.

  3. #3
    Senior Member oddson's Avatar
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    Looks like the pulldown resistor is usually omitted and a 1 kOhm limits the signal current. Lots of examples with BJT transistors on 12 volt rails.

    The pulldown isn't required if you use the internal pulldown I suspect. If you want velocity control things get a bit more complicated.

  4. #4
    So it is probably unrealistic to fire solenoids just using the power lead on an arduino board? I cant seem to find anyone that has done it before

  5. #5
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    To get a feel for what a solenoid can do, have a look at a relays. This https://www.sparkfun.com/products/100 will draw 10mA at 5V (right at it's lower limit) and pull a very light arm a mm or so. The USB power from your computer to the arduino maxs out around one amp, so while you could power a reasonable number of relays from that 1000mA you would still have a total pull measured in grams. To get reasonable forces in sensible currents both relays and solenoids use higher voltages and more turns of (thinner) wire to get the drive power needed.

    Also do not underestimate the back EMF from turning off an inductive coil, which will easily reach 1000V if not tamed with diodes or similar components.

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    Save yourself piles of pain and use a proper solenoid driver chip for this. There are some in DIP packages but you can get them nice and small with SMD packages too. TI has some really nice ones that have spike control, current limiting and short circuit detection and things like that. I like the NCV8460A. It is small, handles a couple of amps no problem and takes a simple logic-level input. It does have an open-drain status output so you can see if it has tripped its overcurrent limit etc.

  7. #7
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    Just a quick theory note. Current is what you want for speed and power. The solenoid is just a coil of wire so there are not voltage "limits" like with semiconductors. You can push 100V through a 1V solenoid as long as the wire inside doesn't melt. So if your duty cycle is low enough you can use a much higher voltage than the coils (continuous) rating. Some solenoids have both continuous ratings and at various duty cycles.

    So in general to get a hard fast whack you will want higher voltage. You could look at a boost converter say to 12V and then a large capacitor to store the voltage and release it quickly through the solenoid on command to the driver chip. A lot of solenoids have a return spring so they just push or pull under current and then the spring pulls them back to the rest position when the current is released. This is usually really fast from a human perspective but you will have to read the specs on the solenoid to get the actual parameters.

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