How to operate a 12 volt relay using less than 12 volts?....

By using this connection we can operate a relay with one volt

this is very useful for controlling large powered things,

WORKING: This is working due to transistor switching action, when applying above .7v to the base emitter junction of a transistor makes its collector emitter junction short circuit, a resistor 0f some kilo ohms is used for reducing the current to the base it should in the range of some micro amperes other wise it will damage the transistor, and the diode is used for blocking the inductor voltage, when switching of the inductor it will try to oppose the voltage in some milli seconds so a very big value of self induced emf is generated ie,

E=Ldi/dt, where L is fixed value, but dt is very small so E will be very big value so it will try to damage the transistor so a free wheeling diode is used for protecting the transistor when ever the self induced emf is generated the diode will try to circulate only through the relay coil,