What are some paths for a child (7-year-old) to learn electrical engineering (in a safe way)?
I do not want to saturate (or frighten away) the child with lots of concepts, I want that my son learn playing with the basic and core of electrical engineering.
Or is 7 years not an age to learn electrical engineering? Or is it dangerous? I will always be by his side.
This question may be off-topic but I'll take a stab anyways.
I wish I had Lego Mindstorms when I was a kid: http://mindstorms.lego.com/en-us/Default.aspx
The ease of construction with lego and pre-fabricated parts will definitely suit a child - at that age they may not have the manual dexterity to build their own projects using tools like wire strippers or a soldering iron. Mindstorms also has a programming element to it, using a version of LabView - these days, I believe programming is an essential part to learning electronics/electrical engineering. Others may disagree with me on that point but I think it's a good thing to learn anyways as it teaches how to think logically.
After this, your child may decide to move in to different directions, either lower level (how things like transistors and motors work) or higher level (more interested in programming and control). Moving to a platform like the arduino and building things from scratch would be an interesting next step.
EDIT: Lego Mindstorms recommended ages: 10-up, so this may not be suitable for a 7-year old.
At that age you need to keep it very basic and watch for what really interests him. A lot can be done with a few light bulbs, switches, relays, electromagnet, and a small motor.
Get a decent set of alligator clip leads. Wobbly connections can frustrate and discourage, but it also has to be easy.
I got a bell around that age, and I remember it being really cool how it would self-oscillate by breaking it's own connection when the hammer moved too far. Be patient as this may take a little to understand. You can make a buzzer with a relay too, but the result isn't so dramatic.
Many children enjoy putting together simple stuff that emits light and/or sound. An assortment of LEDs and resistors, breadboard, and a battery may be enough to get started. For more difficult things you may want to try kits.
I bought Make: Electronics recently. This is a very hands on book. It explains how to blow things up in an almost Mythbusters style (e.g. fuses and LEDs). It's all safe though - so suitable for a seven year old I would say.
I started my kid off with Snap Circuits when he was quite young. They come in different sizes - get a small one first and if he likes it then you can get one of the bigger ones later.
Get a bunch of random electronic stuff and encourage him to take it apart & generally tinker with it. Teach him to solder and unsolder.
It doesn't take a whole lot, at least not initially. I got started with making 'flashlights' and buzzers from scratch, must have been about 6 or 7 years old. My father showed me how to make a flashlight bulb work with a battery. Not long after that, how to wind a bunch of wire around a nail set in a piece of wood, bend some thin sheet metal into shape so the electromagnet could pull it down, and set another nail or sheet metal piece to act as a normally closed contact. These items could be run satisfactorily from a single 1.5V flashlight battery. The most dangerous thing was probably the sharp edges on the sheet metal. Being a kid, I must have built dozens of variations on these before it got stale. After that it was a 'make your own motor' kit from the local R.S., and then on to the spring-clip 150-in-1 kit type things.
In retrospect, I think this trajectory worked because at least initially, there really wasn't any quantitative stuff to worry about. There was immediate gratification in seeing a light come on, or a buzzer sounding, and it did help establish the idea of a 'circuit', with simple loops of things in series. Plunging in with resistors and capacitors and transistors straight away would probably not have held the interest of the 7-year-old mind.
You can teach/learn some concepts with a simple model railroad lyout. I remember learning something about shorts and opens, switches, solenoids, tranformers, and parallel and series circuits from working on a train layout.
I also remeber making a simple electromagnet using insulated wire and a big bolt, powered by said train transformer.
Though the Lego Mindstorms kits may be too hard for some grade schoolers, Lego has a WeDo robotics construction kit that's targeted at even younger budding robotics hackers; it's rated at 7+. My eight year old daughter liked hers, she asked for it for last Christmas (actually two years ago now that I think of it). Bonus, the software is a simplified version of the Mindstorm software, which is a simplified version of LabVIEW, so you get 'em on an upgrade path that ends with real engineering.
I remember my first experience with electricity in school was when we got a bench power supply, a potentiometer, and a few wires, and were invited to experiment. We learned that for a few configurations nothing happened (very educational!), but if we connected it the right way we saw the current meter go up and down when we turned the potmeter's knob. We didn't understand yet what actually happened, but we felt like magicians. Later the teacher explained about current and resistance.
So that's one way to get started. You need a decent, current limited, power supply, so that the experiments can't do any harm. A 9V battery may be an alternative. Let the boy play with small lamps (LEDs are more vulnerable) and switches, and teach him about parallel and series switches. Let him construct a buzzer with a relay.
He'll be most interested and eager to learn if he can see or hear something happen.
edit
I wouldn't expose the kid to electronic parts, not even a diode, until he fully understands voltage and current. A diode may look simple to explain, but it's no use explaining it if you can see him think: "Yes, current... what was that again?"
Many sources sell hobby kits for children which may be useful to you as a starting point. They generally use batteries instead of mains-connected parts, which can be seen as a risk reduction. (You can still short batteries and make a nice mess, but to me it's still safer than AC!)
As your child gets older, there are more sophisticated kits available.
Most of these kits specify 8 years and up, so supervision and patience will be needed.
After being safe, the most important criterium is that the kid must like it.
My own kids (7,9,11) love to break things apart. When a new PC, CD player, or whatever arrives they immediately ask when (note: not 'if'!) they can take the old one apart. A classmate of one of my kids is totaly wild about LEDs. Bristelbots are also popular. For one of my kids I made a few componets (LED+resistor, switches, battery) on wood pieces, with alligator clip cables to connect them. He also has an old philips-style experiments set. A nephew was wild about the nightrider-display kit I gave him. When I was young I made crystal radio's for all my friends, and I put simple battery/inductor shock generators my room's doorknop (my sisetr did not like that, neither did my mother when she told her).
Roundup: experiment, find out what your kid likes. When it is powered by a few penlight batteries (no rechargeables!), and does not try to generate high voltages stored in a capacitor, it is unlikely to do any harm.
I started with batteries, wires, light bulbs and switches. Building simple logic circuits was endlessly fascinating. I moved on to a Digi Comp plastic toy computer. Later I learned about voltage, current, resistance, and how to solder.
Kits are very valuable, but I think the important thing is to let the child's direction of greatest interest be the guide. You can expose your son to lots of different possibilities but he'll be the one who picks up something and runs with it.
Regarding safety, the list of dangers isn't too long. There are high voltages, chemicals, hot things and sharp things. Later there might be other dangers (lasers can hurt retinas, and large electrolytic caps can explode) but those won't be immediate concerns. To keep voltages low enough to be safe, stick with batteries or wall wart power supplies. You're the best judge of whether he's ready for a soldering iron; if yes, find him some safety goggles.
My start was a crystal radio kit, not much more than winding a coil of wire, a diode, and earpiece. But, listening to AM radio stations got me hooked. Next was a Radio Shack 150:1 electronics kit, followed by a ham radio license.
Kits like this are an excellent introduction to electronics and just as much fun for parents helping their interested children.
Consider my suggested safety and learning guidelines for novice experimenters at:
https://harris1.net/info/sci_tech_health/elec_exp_guide.htm
NOTE: They were developed for independent self-educating adults and older teens, but may be worth considering for techniques of adult supervision of children learning electronics.
They ARE a bit of a fussy, worry-wort "buzz-kill", but are intended to start experimenters with safety-conscious orientation to tinkering with electricity, and provide safe boundaries to protect them, and hopefully thereby reassure them of what they CAN do safely.
I encourage comment and critique of them, knowing they could be better.
While I think the "many-experiments-in-one" kits are a great and exciting tool for learning electronics (whether you're 7 or 70), they often have VERY poor documentation and/or tutorial design. You can do a lot with them, but the printed manuals (or, worse, online manuals) are rarely well-thought-out logically-progressing learning guides.
Consequently, you (or someone more knowledgable) need to analyze the kit's manual, to figure out the best way to learn from it -- often by doing the suggested experiments in a different order than they appear in the manual.
A clear example of this is the wildly chaotic manual that came with the classic (and wonderful) Radio Shack Science Fair 150-in-1 Electronic Project Kit -- which can still be found, used, here and there, online.
It's got great hardware, but the manual illogically mixes basic and simple projects with moderate to complex projects, in no rational order at all. Following the book, in page order, can create more confusion than education.
I've attempted to list the kit's manual's projects -- re-sorted in order, by complexity and subject -- in this online guide: https://harris1.net/info/sci_tech_health/RadioShack_150in1kit.htm
Doing the manual's projects, in the order in this list, will probably yield a more logical, coherent and satisfying learning experience for the electronics novice.
I'm sure it's a flawed list, and I welcome critique & comment, but it's surely better than the factory's approach.
And I encourage those fellow tech instructors, with experience with other kits, to offer similar lists of improved experiment sequences, and publish them online. (If you need an online spot for them, I can accommodate on my site; leave me a message.)