it should be very easy but I can't find away to do it,
have 20 short sections of foil connected to 20 inputs
No it will not be very easy, that is quite an involved project.Along side each LED you will need to have a Hall effect sensor, that is 20 sensors wired up to your Arduino. With a Uno that will mean using port extenders to read that many inputs. It will also cost quite a bit.
is there anything eles i need ?what about pwoer surce ?
also what about porgraming the arbiuno is it hard ?
I am from Germany
So for number 1 you are saying this is not good ?
Number 2 if I'll buy the led you suggest to me I will not need port extender correct ?
Number 5 I will use the led you suggested to me and I am going to use 20 pcs 4-Pin WS2812B WS2812 LED Chip & Heatsink Board DC5V 5050 RGB WS2811 IC Built-in RGBW RGBWW WWA LED Chip
I will have to calculate the voltage output for those and make sure the hall switched is good with them
Number 4 are those good ? A3144E Hall sensor OH44E magnetic sensor switch Hall-Effect IC contactless
Number 6 about the programming .......... (I need to do it by myself without help) and I am not going to cheat.
also you have mantion that i need three resistors for each LED correct ?
2* PCF8574 I2C TO 8-BIT DIGITAL PORT EXPANDER
No you do not need resistors for the WS2812 type of LED with the exception of a resistor in series with the line from the output of the Arduino and the input of the LED strip. This has not got a critical value it can be anything between 330 ohms and 510 ohms.You also need a capacitor of value about 100uF to put between the power and ground terminals of the LED strip.If you like I can show you a way not to use those but use 1K resistors and a serval analogue port pins, in place of these port expanders.