The state latch will override the signal from the PIR, if the 12v-light is wished to be turned on constantly. Make sure you buy a PIR that can hold the signal of detected movement for a while or change the size of the capacitor. Otherwise you have a flickering LED. The 3V signal allows the transistor to conduct, respectively to switch the light on. The same circuit I designed for the case you bought a (composite) 12V switch I.e. without the use of a transistor)
