You can make this Transistor timer easily for driving a relay for few seconds to few minutes. It is too simple and uses only two transistors and few other passive components. Just press on the switch to start relay and after the time ends, relay will switch off. You can also set the time using a Trimpot.
You can make a simple Variable Power Supply unit with two Transistors and a Pot. Two high power complementary transistors are used in the circuit to handle high current. Just provide 14 volt DC to the input and adjust a Pot to get 1 to 12 volt DC from the output. It is useful to power circuits while testing.