This is a light switch or light activated switch circuit.The relay on when LDR uncovered and relay off when LDR covered.Adjust VR1 for light sensitive.LED will turn on at the same time with relay.The circuit use LM311 comparator.This circuit need 12Vdc power supply.source

If you need to decorate your car this circuit is simple to build.The circuit will operate as a nice fade-in fade-out visual effects when switching your in-car lights on and off. The fading time can be changed by adjusting the charging and discharging times of the capacitor. To do this, adjust the values of the 10 KOhm and 4.7 MOhm resistors.BUZ74 is a SIPMOS power transistor (Siemens MOSFET) that can handle up to 500V. Instead of BUZ74, you can use any other power MOSFET that has a VDS value greater than 50V.source

If you need to dimming white LEDs array with low voltage source you can accomprish by this dimmer circuits.This is a simple LINEAR CONSTANT CURRENT source such as batteries.Also, this circuit can operate without switching EMI.The circuit provides continuously variable regulated current (~25-400mA) from a 4-6 Volt source.
The battery pack can be 4 AA, C, or D size Alkaline, Ni-Cad, NiMH, 1.5V AA Lithium cells, or a 6V Lead-Acid. This circuit can be scaled for larger or smaller arrays, and should actually handle several Amps with a larger heatsink.source

This circuit designed by Cook.It can switch a fairly high amount of current, an IRFZ34N MOSFET can handle over 35 Amps if connected to a proper heat sink. Higher power FETs, such as the IRFZ48N or IRF1010Z can be substituted if even larger currents are required. It is also possible to connect multiple FETs in parallel for even more current capacity. Always use thermally conductive grease between the FET and the heat sink, and remember that the heat sink is electrically live. source