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Basic Electronics » Building a coil??

August 09, 2011
by Ralphxyz
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I "think" I am going to have to build my own needle valve system for my Water Curtain project.

I purchase 10 solenoid valves off ebay at $5.00 apiece and they turned out to be excellant valves.

The problem is the seller only has 11 of them left and now wants $8.00 so it is going to cost me $100.00

and I will only have 21 values. I need 32 at the lest and preferable 64 or 128.

The only 1/8" solenoid valves I can find are starting at $25.00 and the ones I can find do not match the specs of the ones I have.

So I am thinking possible I could take whatever money I do have and buy a lathe instead of buying more valves and build my own valve system.

It would definitely be a value add to use my own valving system.

So far it seems that I do not want to use a pressurized system so I am picturing a tray with a couple of inches of water with a bunch of needles poking down (I'll make a drawing).

So I need to make a solenoid.

Let me say that I have never done such a thing or anything even remotely related.

So it seems I need a lathe, lets say I have $500.00 to $750.00 for a lathe.

First Question: Anybody have a recommendation about what I should get?

Then how to build a coil?

The Nerdkit guys made some coils for the Robotic Xylophone using their lathe.

They turned their own spools from what looks like 1" plastic stock is that what I should do?

Then how do you determine the operating voltage of a homemade coil?

How do you determine the stroke?

Would a flatter spool have a smaller stroke?

I only will need to move a 1/8" so how much coil would I need?

How do you determine the direction of the stroke?

What makes it go up or down or do you just flip the coil to change direction.

Or is it + on the top outside windings with - on the bottom (inside windings).

With a reversal of the poles to go in a different direction?

Then how about the solenoid rod material could I use stainless steel? (I have some stainless steel rods already).

Like I said I have absolutely no idea what I'm doing.

So I can really use your help even if you do not have expertise, chances are you know more than I do.


August 09, 2011
JKITSON's Avatar

I have a Smithy lathe/mill combination unit. For what you want to do this would work well. I think you will find the mill (which can be used a an excelent drill press) would be very usefull. Check Ebay for Smithy items..


August 10, 2011
by Ralphxyz
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Darn there is a Smithy CM 1114 combo lathe/mill/drill press on eBay for $1695.00.

Now how to make a solenoid??

How is the voltage determined?

It appears from the limited technical info I have found on the web that voltage determines strength and speed.

I do not believe I am concerned with much strength but speed is critical.

So if I wanted to use a 24v solenoid how would I make it 24v?

Also throw length? Does the length of the coil determine it's throw?

So if I wanted a 1/4" or 1/8" throw would I use a 1/4" or 1/8" spool?

If I had a lathe I could get various sized PVC bar sock to turn various diameter spools then I could answer some of my own questions.

I know the Nerdkit guys made their coils what parameters did they use?

What does the rod need to be made of?

There are lots of examples of making a solenoid on the web using a soda straw but very little detail.

Technically I suppose a soda straw and nail might actually work but I "think" I need to limit the throw length.


August 10, 2011
JKITSON's Avatar

Go to SMITHY.COM and look at the model 1220xl. Brand new with shipping about 2K. This will give you some good info also..


August 10, 2011
by Robotnik
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Okay, just to make sure i am following you... You are going to poke needles through a tray and use solenoids to create a magnetic field that will pull the needles up 1/8" - 1/4" which will allow water to flow through the tiny holes in the tray?

I don't think there should be too much math involved in this, just know the more times you coil the wire the stronger the magnetic field, and the more current flowing through the wire, the stronger the magnetic field. And the needles should move to the center of the coils of wire when you run current through them. If needles are magnetic that is. I think I remember making a compass with a needle in grade school, so i think they are.

August 10, 2011
by 6ofhalfdozen
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A couple bits of info for you, hopefully helpfull ones.

  • most common stainless steels (304,316, most 18-8's) are NON-magnetic and most likely won't work(or well enough for your needs). I work with SS solenoids and most are made of 330 or 430 stainless because of their modest magnetic behavior. From what I know of them they are somewhat expensive and might not work for you because of cost..

  • the coil dimensions are kinda funny because your main goal is magnetic field strength acting on the plug with a time component. So a "short, fat" coil and a "long skinny" coil can have the same effective solenoid "kick" depending on the rod size and power input. In a lot of ways this is a "how big of a resistor do I need to keep from frying my fancy component " type question, because it depends on a lot of external variables that you need/will set in order to know. I have several equations for this at home, which if no one has posted tonight, I can post. Though, for my personal rule of thumb, if I want a 1/4" throw, I would want at least 1/2" tall coils to make sure I have enough pulling power and pulling clearance. Perhaps the NK guys will chime in soon with some details on how they sized their coils for their tutorial.

  • As for the voltage, I think you just set your desired values to 24V and size things around that. If I remember my physics right, it is the current change in a coil not the voltage that causes the magnetic field to expand/collapse around a coil and vice-versa. So I would therefore think that speed of the solenoid movement is based on speed of current change as well as physical factors such as any frictions and hesitations on the plunger's part.

  • you mention using PVC as a base for winding your coils around. Personally, I would be hesistant to use PVC. I know it is nice, cheap, and very easy to get, but it is horrible at building/holding static as well as I have read some articles talking about PVC being somewhat conductive at higher frequencies (10's Khz and up). While PVC might work fine for your solenoids, it might also cause all kinds of havoc with your coils (like a rogue capacitance/inductance) and possibly fry something. I think this is why the NK guys used something like HDPE instead of PVC. Just my take on this, and wanted to pass it along.

August 10, 2011
by Ralphxyz
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Wow the Smithy 120XL is on sale at $1769.00 with the tool pak and extas thrown in. drooll drool slober slober

I am finding some stuff on line like:

Solenoid strength, (power), is calculated simply by the current through the coil times the number of turns, or known as Ampere-Turns.
To change the voltage of the coil the diameter of the wire must change in order to change the resistance and have the power remain the same.

There are a lot of how to make a solenoid using a straw and a nail but not a lot on the specifics.

Yeah Robotnik you got the picture.

I was picturing probable the "needle" as 1/4" - 3/8". Currently my nozzle is about 1/8".

I found some relatively cheap solenoids on line. The 36133 24 VDC SOLENOID

which are basically what I was picturing except the throw is 1-3/16". So I ordered a bunch of them.

I do not know how much throw motion there will be in 5ms maybe I could add a capacitor to slow the current down so that I would not get that big of movement before it is turned off. With the flyback diode I am using on my solenoids I should be able to discharge the capacitor fairly quickly.

My mind is just wondering trying to see all of the possible scenarios.

So how would I shorten the throw?

Thanks 6ofhalfdozen you always come through with the details. That's a good point about using PVC (Poly Vinyl Chloride), HDPE (High Density Polyethylene) would be fine there probable a lots of di-electric materials that can be used.

Well possible the solenoids I found will put off a little longer until I get a lathe, it sure would be nice to have one.

Of course I would be hard pressed for the money but I usually come up with ways to get my tools, I have learnt through life that it is so much cooler to have the correct tools. That using a pair of pliers when you should be using a wrench is not best.

I would still like more information on making a coil/solenoid.


August 10, 2011
by rajabalu21
rajabalu21's Avatar

Please check this. These valve type of solenoid may be useful for you.


August 10, 2011
by Rick_S
Rick_S's Avatar

Ooh Raja, good find. Ralph, you really might want to look at those. They are only $2.50 ea. and look pretty easy to pipe.

August 10, 2011
by Ralphxyz
Ralphxyz's Avatar

Oooh, fantastic I get the All-Electronics mailing every month but had not seen those.

I will definitely try them.

Thanks Raja

August 12, 2011
by 6ofhalfdozen
6ofhalfdozen's Avatar


I found that several of the availible solenoid calculators online are more user friendly than the equations I have. So here are some links to various solenoid calculators and usefull things, some are online while the coil maestro is a freeware downloadable mini-program. hope something helps!

this is the link for coilmaestro, the freeware mini program which helps with wire and coil specs

this one is more antenna based, but looks at higher frequencies with a much more thorough view of the details..

kind of generic but easy to use

this is a wire calculator for solenoids (how many feet, etc)

August 12, 2011
by Ralphxyz
Ralphxyz's Avatar

Thanks 6ofhalfdozen, I am going to try the solenoid valves that Raja found.

They look exactly like what I might need.

They are not pressure units since they still have the low pressure atmospheric tubing pieces from when they were cut of.

That matches my pressure settings.

The question will be there response time, how fast they will open and close and there duty cycle.

I do not have the timings worked out yet but I will be turning them on and off a lot more times then they were designed for.

I imagine they were used for a IV drip and probable were designed for once a day ON/OFF at most so they might not last.

I also ordered some of the solenoids I mentioned above, these are just solenoid I will have to make up the valving.

I am just picturing a tray with some holes and the solenoid mounted over the hole with my nozzles screwed up into the holes.

They have a 3/4" mounting tab so I should be able to mount them.

One problem is the throw is 1-3/16" which I think is huge.

That and they say they pull 8.5 amps to open. That's 204 watts the coil doesn't look big enough to pull that kind of power.

The coil measures 2-1/8" long x 3/4" dia so maybe, but I hope not, as I would be using 32 at a minimum and posible lots more.

That would be a lot of power usage even at 5ms on time. Potentially thats 6528 watts for 32 solenoids turning on at once, geesch.

I will definitely have to test them.

I know 32 solenoids will pull a certain amount of power but that definitely seems high.

The 8 solenoids I am currently using pull 6 watts @ 24v each, so that is manageable.


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