How necessary is an external power source?

Hi, I'm making a medium-large project that involves multiple relays, sensors and an LCD display.

The material list for the pilot test goes like this:

  • 4x DHT-11 sensors
  • 1x 4 Relay module
  • 1x 20x4 LCD display with I2C driver
  • 1x 4x4 matrix keyboard
  • 1x DS3231 RTC module

The final project is supposed to operate 17 relays (with the possibility of up to 10 of them being simultaneously active for 10 seconds or so, and 8 simultaneously active for indefinite time) and 8 DHT-11. I'm using an Arduino mega2560. The project is also meant to operate 24/7.

So far with a DHT-11, two relays and the LCD connected to the arduino powered via USB I've noticed a subtle dim in the LCD backlight when the relays are active.

So the question is. How necessary/recommendable is it to connect the relays, sensors and LCD VCCs to an external source? I was planning to use a 9V wall adapter to power the arduino, but since, for example, the sensors are rated at max 5.5V I would be forced to use either a 5V wall adaptor (which I've heard the Arduino's 7905 is not a fan of), a 9V adaptor with and external 7905 or a 9 and a 5V adaptor at once. Also I run on a (very) limited budget, so any budget-wise advice will be appreciated.
Thanks :smiley:

Just get a regulated 5v power supply and power the arduino from its 5v pin

oh, with the Vin pin, right? I've never used that one before so I wasn't sure what it was for.
Is it that simple? :sweat_smile:

Qdeathstar:
Just get a regulated 5v power supply and power the arduino from its 5v pin

No he said the 5V pin, not Vin.

geninchuni:
(which I've heard the Arduino's 7905 is not a fan of), a 9V adaptor with and external 7905 or a 9 and a 5V adaptor at once.

The 7905 is a -5V regulator, you must mean the 7805 +5V regulator - important not to get these mixed up...

Grumpy_Mike:
No he said the 5V pin, not Vin.

I use the 5V pin as a voltage source, I didn't know it also worked as power "input". Thanks for the correction there.

MarkT:
The 7905 is a -5V regulator, you must mean the 7805 +5V regulator - important not to get these mixed up...

Yeah, you're right. I meant 7805, thanks.

Yes 5V regulated supply is the way to go.

Get one with sufficient current rating for your relays - typical is 80-100 mA (check the rating for your relays!) so a 2A power supply will do. That could be a mobile phone charger.

Power the Arduino through its 5V pin; the relays direct from the power supply. Sensors from 5V or the Arduino's 3.3V output as appropriate.

5V works as a power input for 5V supplies, if your supply is greater than 5V then you use the Vin pin to send your higher voltage into a regulator which outputs 5V which is fed to the arduino. Vin needs voltages atleast several volts more than 5V due to voltage drops within it.

due to voltage drops within it.

No, it is due to the drop out voltage of the regulator. That is the excess voltage a regulator NEEDS in order to perform its function of regulating.

Infraviolet:
Vin needs voltages at least several volts more than 5V due to voltage drops within it.

And will not work if you have other devices connected to the 5 V terminal as the regulator on the Arduino is inadequate.

geninchuni:
I use the 5V pin as a voltage source, I didn't know it also worked as power "input". Thanks for the correction there.

think of it as the point "where the 5 Volts exist in the whole setup" - either supplied or tapped.

it is actually used more often as an input than a 'supply' due to the (power)limitations mentioned by the others.

Post #10, that will vary by hw much current the other devices take. If they are a few simple IC chips they can still be ok, the regulator is good to give a bit mroe current than the arduino circuitry alone will ever take, but if you try to power anything of low resistance like motors or even LEDS (even with big resistors in series to limit current) then the onboard regulator won't be able to handle it.