Hi, sorry for the very basic question but I’m a long time programmer with absolutely no experience in electronics.
Hopefully, you’re going to get some guidance from your instructor (and your textbook). Do you have the prerequisites for this class? Maybe some of your classmates have some electronics experience?
I continuously get data from the three sensors and any data is written to a file in a SD until I turn off button (and led).
You’ll need an add-on board (or shield) for the SD card.
The main questions are: can I connect all these sensors? Is there any limit? What should I care about before purchasing any sensor?
A sensor can be almost anything. The simplest sensors will be analog or digital with one connection (plus a common ground connection). Common sensors would be temperature or light sensors, or infra-red motion sensors, or you can get a sound sensor board, etc.
The digital pins are 0 (logic 0 = low) or 5V (logic 1 = high). The analog pins are 0-5V. The digital pins are “normally” binary logic, but they can be configured for basic serial or I2C if you need to communicate more data (or if your sensor uses something like that). So, you’ll want to use sensors that work with 5V or otherwise can “communicate” with the Arduino.
The Arduino has 14 digital input/output pins and 6 analog input pins. You’ll need an output pin for the LED and a few pins for the SD card. The other pins can be used as inputs from your switch and sensors.
Isn’t there any relation between the power needed by the sensor and my batteries? Moreover, are batteries safer than power suppies?
A “wall wart” power supply is safe (as long as it’s from a reputable manufacturer/supplier ). You can also power the Arduino from USB as long as all of our electronics works with 5V and as long as you’re not trying to power motors or LED strips, or anything that takes lots of power. i.e. You can plug your Arduino into the USB port and run [u]Blink Example[/u], and the only additional hardware you need is a USB cable.
It doesn’t help to find shields like this one: https://www.seeedstudio.com/Base-Shield-V2-p-1378.html (I don’t understand if they are required, they are simply useful or should I avoid them).
A “shield” is a plug-in add-on board for the Arduino. There are all kinds of shields. That particular one is a “breakout board” that simply allows you to plug-in some connectors. So for example, your sensors would need mating connectors. It’s really not necessary, but it is a nice way to make connections to your Arduino.
There are “prototyping” shields that you can solder your own components/wires onto if you want to build your own circuit.
It’s also common to built your own custom circuit on a [u]breadboard[/u]. Breadboards are designed for temporary experimentation or prototyping but I’ve used them on permanent projects several time. The big downside is that they are bulky.