exposure meter and development timer

12 A great deal has been published about exposure meters and develop- ment timers in electronic magazines and Elektor is no exception. How- ever, it is very rare to find an article covering both devices at the same time. For this reason a combination design is presented here. As usual an LDR (light-dependent resistor) has been included in the bridge circuit of the exposure meter. The amount of light falling on the LDR determines the degree of imbalance in the bridge network. During measurement, relay Ref is activated via S2e and the exposure meter and development timer 1 D0 0 52c Tr1 = 12 V/500 mA 02 R5 OM 2 741 34 enlarger is turned on. Balance is then restored manually by adjusting potentiometer P1. The final value of P1 will correspond to the exposure time required. Indication that the bridge is in balance is given by two LEDs (D7 and D8). Of course, this could also be indicated on a centre-zero meter but this could be difficult to read in the dark and would probably be more ex...