Circuit :  Mick Devine, UK

Description
This is a light controlled switch which will latch (stay on) once light has reached a preset level. The latch can be reset using S1. Notes
Variable resistor R1 adjusts the light threshold at which the circuit triggers. R1's value is chosen to match the photocells resistance at darkness. The circuit uses three gates of a CMOS 4001 Quad NAND gate. Gate U1a acts as the trigger, U1b and c form a latch. S1 resets the circuit. At switch on, providing light threshold has not been reached, C1 provides a brief negative pulse at U1a's gate. This will set the latch to off, unless light threshold has been reached, then press S1 to reset the circuit. The output device may be a low power piezo buzzer.