Darlington transistors are a low-cost way to drive high powered devices, such as motors, or even relays. They consist of two transistors arranged in a series, where one feeds the other (allowing the gain in the current to be multiplied). That is, if the first transistor has a gain of 20, and the second one also has a gain of 20, together, they will provide an overall gain of 400.
This means that 1 mA on the base pin (1) in the following diagram will allow you to drive up to 400 mA through the Darlington transistor. The Darlington transistor's electrical symbol is shown in the following diagram:
The electrical symbol ...
