erik fiss | galleries | Yashica Electro 35 Repair Manual

As you start depressing the release button, the shutter release switch connects the red lamp L1 into the circuit. But L1 can turn on only if transistor TR3 conducts. And the triggering of transistor TR3 depends on the resistance of the CdS cell, as established by the light conditions. Consider that the light conditions are so bright that a shutter speed faster than 1/500 second is required. The resistance of the CdS cell is then low. Consequently, enough of the battery voltage is dropped across resistor R4 to trigger transistor TR1. Transistor TR1 triggers transistor TR2. And TR2 triggers TR3 to turn on lamp L1. You can turn off lamp L1 by setting a smaller diaphragm opening -- that changes the resistance in the emitter circuit of TR1.