Inexpensive and low power transistors such as the 2n2222 can often be used, but they are, unfortunately, limited in their capabilities. Three popular “in-between” devices that allow us to control relatively high voltage and/or current with relatively low voltage and current are relays designed for Arduinos, and two transistors: BJT and MOSFETS. However, this is often not true for other devices, as most “real world” devices use more than 5V and more than 20ma. LEDs, 3mm, 5mm, or 10mm, can usually be controlled directly from the Arduino’s pins. that require more voltage and/or current than can be handled directly by the Arduino pins, we need to place a device between the Arduino and those higher requirement devices. To control devices, such as motors, lamps, coffee makers, toasters, etc.
However, if you use USB power for the UNO, you are limited by the total power provided through the USB port. Thus, it can provide greater current, perhaps up to several 100ma. Keep in mind that the 5V header on the UNO is not connected through the ATmega328P microprocessor. Thirty (30) ma of continuous power should not present a problem for these pins under normal circumstances, although I try to follow the recommendation of 20ma.
However, other sources suggest an Arduino I/O pin can handle 40ma as an absolute maximum (without damage to the Arduino). Most Arduino pins can handle 5VDC, and according to the official Arduino store URL, 20ma of continuous current. When using an Arduino we often need to control more voltage and/or current than can be handled directly from the Arduino pins.
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