Dark-room exposure meter

EE October 1986 METER DARK-ROOM EXPOSURE No photographer can work properly in his dark-room without some sort of light meter. The Instrument proposed here is not expensive, easy to build, and, apart from the exposure time, it indicates the contrast in relative tight values In spite of its simplicity, the meter is accurate enough for virtually all re- quirements. Moreover, it is con- structed from standard components throughout, with the possible excep- tion of the Type BPW21 photodiode. Operation of the meter is simplicity itself: a push button for normal ex- posure measurement; another push button for constrast measurement; and a microammeter for the read- out. Circuit description The first notable aspect of the circuit diagram in Fig. 1 is that three differ- ent levels of supply voltage are re- quired: +2 V; +5 V; and +9 V. At first sight this may seem extravagant, but it is not really as will be seen later. Moreover, the three levels are ob- tained relatively easily. The +9 V i...