Dark sensor using relay
Here's a circuit that should give you a few seconds' time-delay (you can adjust the time-delay by changing the value{s} of R2 &amp;/or C1), some hysteresis to reduce cyclic switching, and should consume a little less power, due to the use of a comparator &amp; a mosfet, instead of the 3 BJTs.







simulate this circuit &ndash; Schematic created using CircuitLab

NOTE: As @Trevor pointed out on laptop2d's answer, you could remove the relay &amp; freewheel diode, then simply ground the LEDs through the MOSFET (same as how the relay is grounded through it now) to switch them.