So maybe it's a very low class form of question, but please bear with me. I'm working on a project with 7 HC-SR04 ultrasonic sensors connected to one Arduino. I tried to unify the trigger and echo pin, but the sensing isn't good enough, so I dropped the Uno and convert it to Mega. My question is, do I need external power source for those 7 Sensors? And how do I connect all of them? Thanks
Well, according to the datasheet the "working current" is 15mA. Even if you had all 7 working at once, which I doubt you will - you'll be working through them in sequence I guess - that would be just 105mA - way below the 800mA the Arduino's on-board regulator can supply, and only a fifth of what a USB port can provide.
The datasheet doesn't mention the quiescent (idle) current, but it will be a tiny fraction of that 15mA and so can mostly be ignored. Running the sensors in sequence, which is the most likely scenario, would use about the same amount of current as if you had 1 sensor - or 20 sensors. Most of the current consumption would be while you are actively producing an ultrasonic chirp.
So no, you don't need an external power supply to run them all - just take them all from the Arduino's 5V pin.
You will probably not have any kind of problem with the Arduino power source, while in the test bench. When using it for the Drone/Bot you seem to be working, make sure to have a stable power source to the Arduino, and, if possible, let your sensors have their own energy input.
I had previous experiences with the sensors not working properly because the Arduino was not getting enough energy through the Arduino's pins when mounted.