Is it ok to use the 5v pin from uno to power a few small components or would i need a seperate 120 to 5v converter for the 16*12 LCD, a relay, and a CCS118 air quality sensor?
In general - no.
Uno VCC pin is not a power supply.
Use an external 12v-5v voltage converter to power the arduino and all your devices.
You can usually draw 50 to 100 mA from the 5V pin, especially if Vin is kept low, like 7V. Something like an LCD is fine, but probably not a relay.
However, if the 9V comes from a PP3 battery, the battery won't last long at all.
An official Uno R3 utilises a 1A NCP1117 LDO 5v regulator. Under nominal conditions with a 9V input, this regulator can pass 500mA without concern.
Notice I said An official Uno R3. If yours is a clone, we do not know what you have. The NCP1117 will look like below with the 117 marking in some form. If yours doesn't have these markings, all bets are off. Other regulators can be as low as 50mA reliably.
If it has LD50, it's a cheap 117 clone. I wouldn't trust them over 200mA.
As far as your circuit, your power load would be something like:
Uno > 30mA
LCD > 40mA
CCS811 > 30 mA
Relay > depends on relay 10-50mA
This conservative audit results to 150mA
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I respectfully disagree. Since the voltage regulator on the Uno is has very little heat sink it is best to keep the dissipation under 1W. With a 9V into Vin that means that the regulator must drop 4 volts. To keep the dissipation under 1W the current needs to be limited to less than 250mA. The Uno draws around 50mA, that leaves about 200mA for peripherals.
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Nice Schematic, it just needs some annotation such as part values. Use a solid state relay as the schematic shows and you should be ok. I am assuming this will be at room temperature. Two simple rules:
#1. A Power Supply the Arduino is NOT!
#2. Never connect anything inductive to an arduino pin.
And Thank You for doing so.
I configured a circuit to allow me to control the current flowing through the regulator on an official Uno R3 using a 9V ps. Testing the regulator temperature was a very subjective touch test. A 'does it feel right' type test.
I didn't feel comfortable pushing more than a continuous 200mA. 260mA was too hot for my callus covered finger tip. It most likely would have eventually failed.
BTW: My Uno draws 33mA with the Arduino as ISP code loaded.
I did a topic about Arduino power.
See post 4 for some calculations without a heat sink.
Good test, and reasonable conclusion, although I do not recommend drawing even that much (post #3).
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