usb gives 19V i think
I need more voltage than the usual pc voltage
Can I use an AC adapter along side the usb ?
the adapter has 12V output and 1.5A. Is it safe to use it ?
What gave you that idea? USB voltage is 5V.What voltages do you need? What are those sensors? Can you provide data sheets for them? Is it more voltage that you need or is it more current?You can not connect the outputs of 2 power supplies together.Powering through Vin or the power jack means that the Arduino and all peripherals that are on the 5V rail are powered by the onboard 5V regulator. The on board 5V regulator is not heat sinked so will supply limited current before it overheats and shuts down. The recommend max power dissipation for the regulator is 1 Watt. With 12V into the regulator the max current is about 140 mA (1W / (12V - 5V)). The Arduino uses around 50ma of that leaving less than 90mA (max) for everything else. I would use a buck converter to drop the 12V to 5V and connect that to the 5V on the Arduino, bypassing the, weak, 5V regulator.
Your question is a massive non-sequitur.USB is 5V.Arduino's are 5V devices (generally).Sensors are 5V devices (usually).You do not need more voltage. More voltage would only fry your components.You didn't say which Arduino board, but here's a discussion of various ways to power the Uno.You also don't say anything about the sensors, what they are and how many, but they are usually very low-current devices and the 5V (or 3V if the sensors are 3V devices) can normally power several sensors at the same time.
You will need external power for the servos. That power source will need to be able to supply the stall current of the servo motors. That is about 1A per servo for small hobby servos more for the larger servos. A 4 AA cell battery pack is a popular power source for servos. Connect the + of the battery pack to the servo plus and the - (minus) to the - of the servo and to the Arduino ground.The gas sensor heater needs nearly 200mA. You may be able to power that sensor from USB, but not from the internal 5V regulator powered by a 12V wall wart. See reply #1. You could power the heater from a 12V to 5V DC-DC converter as well as the Arduino.The buzzer can be powered by the Arduino or USB.
The best way to run an Arduino is to apply an accurate external 5 volts to the V in pin.