In contrast to the Arduino Uno, which operates at 5 V, all of the ARM boards operate at 3.3 volts, so they will run off a single cell rechargeable Li-Ion or Li-Po battery. Some of the boards, such as the MKR WIFI 1010 and the MKR WAN 1310, include a battery connector and charging circuitry that will charge the battery whenever USB power is available, making these boards ideal for wireless battery operated projects.
Operating at 3.3 V means that you must take this into consideration when connecting external components such as LEDs and sensors. Switches and resistive sensors, such as the LDR we learned about in “Use a Light Sensor Instead of the Pushbutton” will work fine, but active sensors designed for 5 V, such as the sensor from “Testing the Temperature and Humidity Sensor” may not work properly at 3.3 V. Extreme caution must be used when mixing both 3.3 V and 5 V components in a circuit. In particular, voltages greater than 3.3 V must never be present at any pin of a 3.3 V component.
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