dibolero
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Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 12:15 pm

Me and my friends are working on a single braille cell as part of a competition here in our country.

However based on our research, the PI's GPIO pins can only output a maximum of 16maH or else we'll risk breaking it.
Would it be safe to use the GPIO pins to run a current through our solenoids?

We've tried making our solenoid pull a pin using a battery and it works if we don't use a resistor however using a 100ohms resistor and the pins won't move.
We haven't tried this on the PI as we're afraid we might break our pi and it's really hard to get one here.

We'd really appreciate your help!

jamesh
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 12:19 pm

Pulling more than 16mA is likely to break you Pi, especially the sort of current that would be required to activate a solenoid, unless it's really really tiny.

You will need some sort of powerhing circuitry inbetween.
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dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 12:39 pm

Our solenoid is really small, its around half an inch long and has around 200 turns.
It works with a 3v battery without using any resistor however it doesn't work with a 100 ohm one.

What do you mean by a power circuitry?

klricks
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 1:05 pm

dibolero wrote:
Wed Jul 11, 2018 12:39 pm
Our solenoid is really small, its around half an inch long and has around 200 turns.
It works with a 3v battery without using any resistor however it doesn't work with a 100 ohm one.

What do you mean by a power circuitry?
Not sure why you think adding a resistor would be helpful?
You need a transistor circuit or relay board to switch the 3V battery to the solenoid.
The RPi GPIO is then used to switch the transistor or relay board.
Google for hundreds of tutorials YT etc.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Stretch w/ Desktop OS.

dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 1:19 pm

klricks wrote:
Wed Jul 11, 2018 1:05 pm
dibolero wrote:
Wed Jul 11, 2018 12:39 pm
Our solenoid is really small, its around half an inch long and has around 200 turns.
It works with a 3v battery without using any resistor however it doesn't work with a 100 ohm one.

What do you mean by a power circuitry?
Not sure why you think adding a resistor would be helpful?
You need a transistor circuit or relay board to switch the 3V battery to the solenoid.
The RPi GPIO is then used to switch the transistor or relay board.
Google for hundreds of tutorials YT etc.
We were trying to simulate youtube LED circuits tutorials replacing the LED with our custom solenoid.

I haven't read much on transistor circuits but do you mean that we'll be using a different power source and only use the GPIO pins as a switch?

What we were planning to do was to use the pi powered by a lipoly+powerboost1000 itself as the power source/controller of the solenoids.

jamesh
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 1:53 pm

dibolero wrote:
Wed Jul 11, 2018 1:19 pm
klricks wrote:
Wed Jul 11, 2018 1:05 pm
dibolero wrote:
Wed Jul 11, 2018 12:39 pm
Our solenoid is really small, its around half an inch long and has around 200 turns.
It works with a 3v battery without using any resistor however it doesn't work with a 100 ohm one.

What do you mean by a power circuitry?
Not sure why you think adding a resistor would be helpful?
You need a transistor circuit or relay board to switch the 3V battery to the solenoid.
The RPi GPIO is then used to switch the transistor or relay board.
Google for hundreds of tutorials YT etc.
We were trying to simulate youtube LED circuits tutorials replacing the LED with our custom solenoid.

I haven't read much on transistor circuits but do you mean that we'll be using a different power source and only use the GPIO pins as a switch?

What we were planning to do was to use the pi powered by a lipoly+powerboost1000 itself as the power source/controller of the solenoids.
You need a transistor, which can use the tiny current available from the GPIO's to switch a larger current - your battery. Solenoids take a much larger current than LED's so you cannot directly compare the two projects.
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dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 1:55 pm

jamesh wrote:
Wed Jul 11, 2018 1:53 pm
dibolero wrote:
Wed Jul 11, 2018 1:19 pm
klricks wrote:
Wed Jul 11, 2018 1:05 pm

Not sure why you think adding a resistor would be helpful?
You need a transistor circuit or relay board to switch the 3V battery to the solenoid.
The RPi GPIO is then used to switch the transistor or relay board.
Google for hundreds of tutorials YT etc.
We were trying to simulate youtube LED circuits tutorials replacing the LED with our custom solenoid.

I haven't read much on transistor circuits but do you mean that we'll be using a different power source and only use the GPIO pins as a switch?

What we were planning to do was to use the pi powered by a lipoly+powerboost1000 itself as the power source/controller of the solenoids.
You need a transistor, which can use the tiny current available from the GPIO's to switch a larger current - your battery. Solenoids take a much larger current than LED's so you cannot directly compare the two projects.
So just to clarify, it would not be possible to power solenoids directly from a RPI?

https://www.hackster.io/hitherejoe/brai ... der-e86060 I couldn't see any transistors being used in this project that's why I'm a little bit confused.

jamesh
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 2:06 pm

If they are small they don't need a huge amount of current, but that will depend on the solenoid itself. If you can stay below 16 (well, probably a bit more) then it should work.

But, in your case it sounds like you are not supplying enough current, so the solenoid doesn't switch. In which case you will need the transistor approach. I am not a electronic guy so I cannot help with that.
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dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 2:17 pm

So I guess our only possible options would be to buy ready made mini solenoids right?

I really appreciate your answers thanks alot!

jamesh
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 2:19 pm

dibolero wrote:
Wed Jul 11, 2018 2:17 pm
So I guess our only possible options would be to buy ready made mini solenoids right?

I really appreciate your answers thanks alot!
Well, probably cheaper to use transistors to do the switching - they are very cheap. Certainly cheaper than solenoids!
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rpdom
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 2:56 pm

I owuldn't recommend trying to drive any solenoid directly from a gpio. They are an inductive load and could damage the gpio with reverse EMF when turned off, unless you use a diode to prevent that.

PiGraham
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 3:21 pm

dibolero wrote:
Wed Jul 11, 2018 2:17 pm
So I guess our only possible options would be to buy ready made mini solenoids right?

I really appreciate your answers thanks alot!

No, I think you should be OK with home-made solenoids. What you should do is provide a separately regulated power supply and switch the current through the coils using an appropriate transistor switch.

Do you know what current you need to pull the solenoid in, and how much to hold it?

You might be able to use one of those[ur=https://www.ebay.co.uk/itm/3Sets-28BYJ- ... wLWBbMXoDl] ULN2003 Motor Driver Board that are often supplied with 28BY stepper motors[/url].


Those give you 7 channels on each board all individually controlled. Motor power supply is kept away from the Pi power (only GND pins connected)


Check the datasheets for the chip for current and voltage limits.

You could find the needed voltage and current for your solenoids by using a current and voltage regulated power supply. Set some voltage and current limits then switch power onto the coil. If it pull-in you can try lower voltage and/or less current. Iterate until you find the limits for reliable operation.

If you find you need more power to pull the solenoid in than to hold it you could provide a supply sufficient to pull-in well and to hold driver the signal with PWM at, say, 30%. That way you can hold for as long as needed without verheating the coils or driver.

dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 3:53 pm

So we took a stupid risk and tried connecting the homemade solenoid to the pi without a resistor and surprisingly it didn't do anything.
The 3 volt battery was able to move the pin so isn't the 3.3v gpio pin supposed to be enough?

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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 6:47 pm

dibolero wrote:
Wed Jul 11, 2018 3:53 pm
So we took a stupid risk and tried connecting the homemade solenoid to the pi without a resistor and surprisingly it didn't do anything.
The 3 volt battery was able to move the pin so isn't the 3.3v gpio pin supposed to be enough?
The gpio pin can't supply as much current as a battery can. It can only go up to about 16mA. Even at that level it might not be able to supply the full 3.3V

You may have just burnt out that gpio by overloading it.

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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 7:27 pm

dibolero wrote:
Wed Jul 11, 2018 3:53 pm
So we took a stupid risk and tried connecting the homemade solenoid to the pi without a resistor and surprisingly it didn't do anything.
The 3 volt battery was able to move the pin so isn't the 3.3v gpio pin supposed to be enough?
You shouldn't connect inductive loads to logic circuits, including directly to 5V or 3v3 supplies.
Every time a coil turns off it generates a voltage spike that can be many times the normal supply voltage. That can cause a crash or do some damage. Use separate regulators for Pi and solenoids.

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Re: Controlling homemade solenoids using Rpi Z W

Wed Jul 11, 2018 9:52 pm

dibolero wrote:
Wed Jul 11, 2018 3:53 pm
So we took a stupid risk and tried connecting the homemade solenoid to the pi without a resistor and surprisingly it didn't do anything.
The 3 volt battery was able to move the pin so isn't the 3.3v gpio pin supposed to be enough?
You don't understand enough about what you are trying to do. Every electrical component - including your solenoid - has resistance. The resistance (ignoring inductive effects) determines how much current flows through the component when you connect a voltage across it.

Let's suppose your solenoid has a resistance of 10 ohms. Connect a 3V battery across it and 300mA will flow through it (assuming the battery is able to supply 300mA, which it probably is). However, the GPIO pin can only supply 16mA. That means your solenoid will only get 16mA, and the voltage will be dragged down to 0.16V because you have connected your solenoid to the GPIO pin - in other words it is no longer supplying the 3.3V you expect. Not only that, but this also easily capable of killing your RPi.

Before you can go about designing how to drive your solenoid, you really must measure its resistance, or alternatively the current that flows when you connect it to a known voltage (and I don't mean just assuming a pair of batteries gives you 3V - the voltage will change when you connect the solenoid).

The minimum resistance something can have and be safe connecting directly to a GPIO pin is 330 ohms. The only way a solenoid will have anything near that much resistance is if you wind it with incredibly fine wire. It takes energy to move something, and the energy you are supplying to the solenoid is the volts multiplied by the current. So the less the volts, the greater the current you will need to supply the same energy.

I have written a full explanation of what to do to connect things to GPIO pins, but by the sounds of things you have some learning to do in basic electronics before it will be any use to you. However, for what it's worth:

Beginners Guide to Wiring Things to the GPIO

Note that relays will be no use for your application (too slow). You should probably look at ready-made motor driver HATs.
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dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 1:44 am

Z80 Refugee wrote:
Wed Jul 11, 2018 9:52 pm
dibolero wrote:
Wed Jul 11, 2018 3:53 pm
So we took a stupid risk and tried connecting the homemade solenoid to the pi without a resistor and surprisingly it didn't do anything.
The 3 volt battery was able to move the pin so isn't the 3.3v gpio pin supposed to be enough?
You don't understand enough about what you are trying to do. Every electrical component - including your solenoid - has resistance. The resistance (ignoring inductive effects) determines how much current flows through the component when you connect a voltage across it.

Let's suppose your solenoid has a resistance of 10 ohms. Connect a 3V battery across it and 300mA will flow through it (assuming the battery is able to supply 300mA, which it probably is). However, the GPIO pin can only supply 16mA. That means your solenoid will only get 16mA, and the voltage will be dragged down to 0.16V because you have connected your solenoid to the GPIO pin - in other words it is no longer supplying the 3.3V you expect. Not only that, but this also easily capable of killing your RPi.

Before you can go about designing how to drive your solenoid, you really must measure its resistance, or alternatively the current that flows when you connect it to a known voltage (and I don't mean just assuming a pair of batteries gives you 3V - the voltage will change when you connect the solenoid).

The minimum resistance something can have and be safe connecting directly to a GPIO pin is 330 ohms. The only way a solenoid will have anything near that much resistance is if you wind it with incredibly fine wire. It takes energy to move something, and the energy you are supplying to the solenoid is the volts multiplied by the current. So the less the volts, the greater the current you will need to supply the same energy.

I have written a full explanation of what to do to connect things to GPIO pins, but by the sounds of things you have some learning to do in basic electronics before it will be any use to you. However, for what it's worth:

Beginners Guide to Wiring Things to the GPIO

Note that relays will be no use for your application (too slow). You should probably look at ready-made motor driver HATs.
Luckily the PI and its GPIO pins are fine for now. Yeah it's true most of our background is on programming and this is the first time we've ever even touched a pi and an electric circuit so we're kind of really clueless.

So what I've got from this is our options are either
1. Create a solenoid using thinner wires. If we do this, would creating a solenoid that would have internal resistance of 330ohms mean that the 3.3 gpio pin with its 16maH current would be enough to move a pin?
2.Use transistors. Based on the replies here that would be the most proper thing to do so we'll do a bit more research on that. I don't think ready-made motor driver HATs are an option because they're most likely not available in our country.

Just to clarify, was this system wrongly designed? https://www.hackster.io/hitherejoe/brai ... der-e86060
This is a braille project on hackster, he used 100ohm resistors with 5v solenoids and this was basically our starting point and basis of what we did with our project.

Again, we greatly appreciate everyone's replies here. We're learning bit by bit and luckily nothing broke as of now.

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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 7:44 am

dibolero wrote:
Thu Jul 12, 2018 1:44 am
1. Create a solenoid using thinner wires. If we do this, would creating a solenoid that would have internal resistance of 330ohms mean that the 3.3 gpio pin with its 16maH current would be enough to move a pin?
The question is whether 50mW (the maximum power available from a GPIO pin, = 3.3v x 16mA) is enough to move your solenoid armature. If it isn't (and it probably isn't), no amount of fiddling is going to solve the problem.

If you have a solenoid which works, stick with it - you've solved half the problem.
dibolero wrote:
Thu Jul 12, 2018 1:44 am
2.Use transistors. Based on the replies here that would be the most proper thing to do so we'll do a bit more research on that. I don't think ready-made motor driver HATs are an option because they're most likely not available in our country.
RPi is available but HATs aren't? OK, if you say so. My battle on this forum is to try to get the message across that people can't just connect anything they like to GPIO pins, unlike software where you can do more-or-less anything and get away with it. Yes: a transistor buffer should be EVERYONE'S FIRST THOUGHT, and only connect directly IF YOU ARE SURE the naked GPIO pin is enough. Hell, you need a transistor even for driving a relay.

Do you not have access to a multimeter, so you can measure what's going on? Really, that's the starting point for doing anything like this. Without measurement, you are shooting in the dark and so would I be if I try to advise you.
dibolero wrote:
Thu Jul 12, 2018 1:44 am
Just to clarify, was this system wrongly designed? https://www.hackster.io/hitherejoe/brai ... der-e86060
This is a braille project on hackster, he used 100ohm resistors with 5v solenoids and this was basically our starting point and basis of what we did with our project.
Well, if I hadn't seen the video I wouldn't have believed it could work. "I’m no expert in electronic engineering, but nothing has caught on fire yet so I think it’s doing good enough for a prototype so far" - that just about sums it up. There are lots of things wrong with that write-up, and it doesn't even rate as novice engineering in my book (definitely beginners slopes). The fact that he has got it to work is no guarantee someone else following the instructions would also get it to work, and that is a minimum requirement in engineering.

First, and worst, the push button circuit appears to be running from 5V and applied directly to a GPIO pin. That is definitely forbidden (even if via a resistor), and easily avoided by using 3.3V instead.

The diagram is slightly misleading, at first glance the top row of solenoids is shown connected to 5V (but actually they are connected to 0V).

However, the solenoids are a problem. There is no indication of what they are or where they came from, but a typical 5V solenoid has a resistance of around 10 ohms and is intended to be run from 5V. It has a return spring, so the power in the magnetic coil has to move the armature against the strength of the spring. So as not to overload the GPIO output, he has included a 100 ohm resistor in series, and to protect against inductive spikes he has heard about putting in a diode. The placement of the diode is wrong and will not protect the GPIO output from the negative spike at switch-off, and it is decreasing the output voltage. The resistor is limiting the current, but in doing so cuts the operating voltage available to the solenoid to about 0.25V (assuming the solenoid is 10 ohms - it might not be).

So, in short, it's a wonder this "project" works at all, it's lucky it hasn't destroyed the RPi, and without obtaining exactly the same solenoids it is unlikely to work for anyone else.

Your home-made solenoids cannot possibly have anything like this performance, but if they work stick with them.
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PiGraham
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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 10:06 am

dibolero wrote:
Thu Jul 12, 2018 1:44 am
Just to clarify, was this system wrongly designed? https://www.hackster.io/hitherejoe/brai ... der-e86060
This is a braille project on hackster, he used 100ohm resistors with 5v solenoids and this was basically our starting point and basis of what we did with our project.
I am afraid so. It's not designed well. I the solenoids are from Pimoroni as suggested by the buy link I assume the spec is this:

https://shop.pimoroni.com/products/mini-solenoid

Rated voltage: 5V
Current (at 5V): 0.42A
DC resistance: 12Ω±5%
Stroke: 3.5mm
Connection type: bare wire


They are connecting 5V to one side of the solenoid coils and the switch pull-up. That is not recommended and will mean that current flows intp the gpio pins. Resistance will limit that current and you may get away with it but it is bad practice. Don't do that.

You can safely drive a load from a higher voltage supply (5V, 12V, whatever) simply by using a transistor. A ULN2003 looks ideal for this, given it has 7 Darlington transistors designed for switching small inductive loads (coils).


The braille box circuit also has diodes in the wrong place for suppression of back emf. The diode should be reverse biased across the coil terminals, as close to the coil as possible. The ULN2003 has these diodes built into the chip. This is fine if you keep the solenoid wires short.


It might be better to make the solenoid push the braille buttons up rather than pulling them down

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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 10:09 am

What is the motivation for making your own solenoids?
As a learning experience that's a good thing to do but you can buy them for £1 each and they probably work better.
https://www.ebay.co.uk/itm/DC-4-5V-40g- ... 213f93e414

dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 1:35 pm

PiGraham wrote:
Thu Jul 12, 2018 10:06 am
dibolero wrote:
Thu Jul 12, 2018 1:44 am
Just to clarify, was this system wrongly designed? https://www.hackster.io/hitherejoe/brai ... der-e86060
This is a braille project on hackster, he used 100ohm resistors with 5v solenoids and this was basically our starting point and basis of what we did with our project.
I am afraid so. It's not designed well. I the solenoids are from Pimoroni as suggested by the buy link I assume the spec is this:

https://shop.pimoroni.com/products/mini-solenoid

Rated voltage: 5V
Current (at 5V): 0.42A
DC resistance: 12Ω±5%
Stroke: 3.5mm
Connection type: bare wire


They are connecting 5V to one side of the solenoid coils and the switch pull-up. That is not recommended and will mean that current flows intp the gpio pins. Resistance will limit that current and you may get away with it but it is bad practice. Don't do that.

You can safely drive a load from a higher voltage supply (5V, 12V, whatever) simply by using a transistor. A ULN2003 looks ideal for this, given it has 7 Darlington transistors designed for switching small inductive loads (coils).


The braille box circuit also has diodes in the wrong place for suppression of back emf. The diode should be reverse biased across the coil terminals, as close to the coil as possible. The ULN2003 has these diodes built into the chip. This is fine if you keep the solenoid wires short.


It might be better to make the solenoid push the braille buttons up rather than pulling them down
I'm glad it wasn't too late, we were about to follow his entire system until we saw a post about the pi's maximum output current.
Does his design pull the current from the 5V gpio pins?

That was our plan! Suspend the coils and put the pins at a lower position so the magnet pulls it up.

As of now our homemade solenoid needs around 350 mA to work properly.

dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 1:38 pm

PiGraham wrote:
Thu Jul 12, 2018 10:09 am
What is the motivation for making your own solenoids?
As a learning experience that's a good thing to do but you can buy them for £1 each and they probably work better.
https://www.ebay.co.uk/itm/DC-4-5V-40g- ... 213f93e414
We wanted to make a mini-version of the braille box. Basically what we want to do is use camera OCR to analyze text, send that to our algorithm and convert that into braille patterns.
The goal is to make the system as small as possible so that's why we decided to make our own solenoids. They're really hard to get here as well.
What we use as the coil's core are lollipop sticks with 300 turns each and is only half an inch long.

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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 2:03 pm

How fast do the pins need to move?
I sonder if you could use miniature linear stepper motors.
example on eBay

Advantage of a screw is it will not give under finger pressure and takes no power to hold position.
Possible disadvantage is might be too slow to move.


There are lots of variants larger or smaller than that one.

Note that soldering tiny connections can be very difficult.

dibolero
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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 2:06 pm

PiGraham wrote:
Thu Jul 12, 2018 2:03 pm
How fast do the pins need to move?
I sonder if you could use miniature linear stepper motors.
example on eBay

Advantage of a screw is it will not give under finger pressure and takes no power to hold position.
Possible disadvantage is might be too slow to move.


There are lots of variants larger or smaller than that one.

Note that soldering tiny connections can be very difficult.
Ideally they could go up and down in one second. It's not really a requirement that it can withstand finger pressure,as long as the user feels the pins as they go up.

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Re: Controlling homemade solenoids using Rpi Z W

Thu Jul 12, 2018 2:57 pm

dibolero wrote:
Thu Jul 12, 2018 2:06 pm

Ideally they could go up and down in one second.
That's much slower than I expected. I should think linear steppers would be a good option for a smaller device. I would guess they might move 3mm in 250ms, but that is only a guess.

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