Working in a darkroom is always fraught with problems. You surely know Murphys Law of. . ., but Let’s not go into that here. Suffice it to say that normal lights cannot be used in a darkroom when photographs are being developed not even if you drop your glasses! The dark room timer circuit here is a simple, inexpensive 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 automatically 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 S1 T2 conducts and provides, in turn, a base drive current to transistor T1. This transistor then supplies the base current for T2 via R5 and P1. Switching S1 off causes C1 to deliver a negative pulse to the base of T2 and this transistor then stops conducting. Tl also stops conducting and the LEDs go out. The energy saving circuitry requires the addition of just one component, the LDR. When enough light falls on it the LDR's resistance causes T2 to switch off and extinguish the LEDs. The light level at which this happens is set by means of preset P1 Q LEDs D3 . . . D5 must be high efficiency types and are either red or yellow depending on what sort of photographic paper is used. There are various high intensity LEDs available, although the light intensity level can also be changed by varying the current flow through T1 (by substituting another value of resistor for R1 ). With the values stated about 20 mA flows through the LEDs and, seeing as the current consumption when the LEDs are off is only a few nA, the 9 V battery should last quite a while. Finally it is important to remember that some types of photographic paper are sensitive to all colours, including red and yellow, so check this before using the lamp.
Dark Room Timer Circuit is shown below