darkroom light

darkroom light it to say that normal lights cannot be used in a darkroom when photo- graphs are being developed — not even if you drop your glasses! The circuit here is a simple, inex- pensive design for a darkroom torch (or light) that can be mounted in a case small enough to fit into your pocket even with a 9 V battery included. It gives enough light for note-taking or finding this or that in a darkroom, but the light is emitted by three special yellow LEDs which can safely be used near black/white or colour paper. Red LEDs are used for orthochromatic material (we had to look it up too, it means "giving correct relative intensity to colours in photography"!). An energy saving circuit is included that auto- matically switches the lamp off when the ambient light is above a certain level. The diagram for the circuit makes it look like a mini power supply. When the circuit is switched "on" with Sl T2 conducts and provides, in turn, a base drive current to transistor T1. This trans...