An integrated circuit employing a photo-diode activates an alarm when an intruder causes a change in the ambient light patterns by more than about 5% in an area under surveillance. The circuit includes a voltage regulator for supplying a back bias voltage to the photo-diode which voltage is substantially purged of noise that may be superimposed on the main d.c. supply voltage conductors. The regulator circuit includes a modified current-mirror type current source and a V.sub.BE -multiplier circuit providing a log-log noise transfer function.