So there is another thing I don’t get.
Let’s say that I want to drive two dc-motors. I can do this with a motor shield that I click on an Arduino UNO or MEGA.
But does the shield uses all the inputs and outputs?
Or is it possible to also connect other stuff ?
Like push buttons, switches, potentiometer,…
With regards
Hans Vertongen
No.
Certainly.
That would depend on the chip/s used on that shield I'd say. If you know the chip (many use the ancient L293 or L298) you could count up how many it needs; or one would hope the shield's documentation would say what pins it uses. Certainly if it's a shield that works on an Uno then if you use it on a Mega it would be easy to see which pins are not physically covered.
edit... I wonder how up-to-date this site is. For the shields it has "decoded" it shows which pins do what.
Oké thanks. But all the shield that have found until now don’t have the ability to connect other inputs or outputs.
How does one do that?
When the shield is clicked on the Arduino, you can’t get to the holes for other inputs or outputs…
Sometimes you can, it depends I guess on how well thought out the shield is. The DFR0009 LCD shield, for example, exposes the unused pins as holes and one can solder a few header pins on.
Most motor shields in any case use the old 293/298 and you would be doing yourself a favour by moving to more modern, more efficient chips which usually come as modules but not actual shields. Then you can use whatever pins you like. Look at Pololu for their huge list of motor modules to suit all manner of V and A requirements.
Thank you!
Do you have a link of an example?
Can they be used for steppermotors?
Here is the list of drivers for dc brushed motors at Pololu.
But on that page you can tick any number of boxes and filter their products as you wish, and that includes stepper drivers.
Is it me or are they expensive?
I would be using it in a classroom… so budget is kind of an issue…
I really don't know if they're expensive or not, and I buy minute 1-2 off quantities. But don't forget you get what you pay for 
That said, i just proposed Pololu as an example of modules that don't use ancient 293/298 chips and which aren't shields.
One thing I love about Pololu is the quality of their connection diagrams, example below:
Thank you!
Do you need to solder them yourself?
I can't remember tbh, I think so yes.
Some have stackable headers, some don't lots to choose from. As octopirate says, a Mega may be a better option for you (yes, the shields are designed to work the same as on Uno for the Mega). You just have to shop for the one that suits your needs. What size motors? What's it for? A separate motor controller instead of a shield may also be a better option.
I didn't actually say that; OP had mentioned Mega as well as Uno in first post.
When you buy cheap, you buy twice.
sir yes sir 
Another option would be to use a potentiometer and transistor setup if it's for educational purposes and you need to keep the budget down.
Now that is interesting! I didn't know that. But indeed, the pins match! Thank you!
What could I use to drive this stepper:
I just bought one, but normally I would be able to use 2 - 4 steppers (with other inputs / outputs)
I just bought one, but normally I would be able to use 2 - 4 steppers (with other inputs / outputs)
Pick a Pololu driver than can do a little more than 1.5A continuous current pull per phase
