Using the same power source for microcontroller (arduino) and rest of the circuit?

Question

I have an Arduino, it controls a 6V DC motor and 12V DC solenoid (which only really needs 7V for what its doing).

Originally I wanted to use 1 rechargeable 7V battery (or 9 or whatever the closest I can get) to power all three of these.

But I've been seeing recommendations from people to use separate power supplies for Arduino and the rest of the circuit, otherwise they describe a lot of "noise" when the motor and solenoid suddenly engage. (not sure what is meant by "noise")

So is there a way to accomplish my task with one 7 - 12V power supply??

Answer 1

You may be able to run the microcontroller (μC), motor, solenoid from a single power source¹. That's probably the first approach that you should try. Make sure to include back-EMF diodes for motor and solenoid.

¹ Of course, you need to generate the appropriate supply voltages from that power source. I assume that you understand this.

Solenoids and motors can in fact generate voltage spikes, which may cause problems for the μC. For example, spikes can cause a brownout. (Such spikes are a type of conducted interference. The term "noise" is often used for such nuisances. More rigorous definition of noise here.) Whether you will encounter these problems or not depends on the size of the coils and the capacitance between the supply rail and ground in your circuit.

Here's a related thread, where someone ran into problems with spikes generated by a solenoid valve.

edit:
Here's a detailed answer in another related thread.
Here's a page on EMI suppression for motors.

Answer 2

When people talk about "power supply" they mean one of two things:

1. power source

2. power regulating circuit

What is meant by having two power supplies is not using two batteries but having two separate independent power regulating circuits.

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Noise refers to voltage that changes rapidly (and usually randomly). The reason motors solenoids make circuits noisy is that they generate back-EMF which creates temporary voltage spikes in the circuits. In the case of motors the spikes are periodic.