Powering Arduino from 7805 voltage regulator

Question

I am trying to build an Arduino Uno based robot. My questions are the following:

1) The motor driver also has an on-board 78M05 voltage regulator from ST electronics. I used a 9V battery to power the robot, and the 9V was connected to the input of the voltage regulator, and the +5V output I used to power the Arduino. I did so by connecting the 5V from the regulator to the 5V pin on the Arduino and the GND's together. I read that this is not advised, and solutions ranged from making sure the 5V is regulated, to connecting a diode between +5V and Vin to using one of the ICSP pins and so on. So which is actually a good way to do this?

2) I later realized that the DC motors are rated for max 6V dc, and so switched the 9V battery to 4x AA batteries giving a total voltage of 6V. When I connected this to the regulator, the regulator and the Arduino still turned on, even though the data sheet says the minimum input voltage is 7V. So what is going on here? Can anyone help me understand what is happening?

Answer 1

I read that this is not advised, and solutions ranged from making sure the 5V is regulated, to connecting a diode between +5V and Vin to using one of the ICSP pins and so on. So which is actually a good way to do this?

Nothing wrong with that as long as it is regulated 5V (which yours is). You can use either the 5V pin on the header, or the 5V pin on the ICSP connector. They are both the same. It is good practice to connect a diode between 5V and VIN to prevent current flowing backwards through the onboard regulator should something connected to VIN want to draw current.

I later realized that the DC motors are rated for max 6V dc, and so switched the 9V battery to 4x AA batteries giving a total voltage of 6V. When I connected this to the regulator, the regulator and the Arduino still turned on, even though the data sheet says the minimum input voltage is 7V. So what is going on here? Can anyone help me understand what is happening?

A regulator has a "drop-out" voltage. That is often in the order of 1.2V for these kind of regulators (it's certainly right for the Arduino's on-board one.

You are providing less than the regulation voltage plus the drop-out voltage, so the output of the regulator will be the input voltage minus the drop-out voltage.

As long as that is above the lowest voltage the Arduino can run from at 16MHz then it will still work fine. You should be fine down to about 4V output from the regulator, and even lower if you're lucky (you may start getting stability problems though since the ATMega328P isn't rated to run at 16MHz at lower voltages).

It also means that the GPIOs on the Arduino will be running at a lower voltage as well.