Timer For Fixing Bath

EE JULY/AUGUST 1987 by A Behrens When, after developing, photo- graphs are immersed in the fix- ing bath at irregular intervals, it becomes difficult to observe the correct fixing time for each of these. This problem is solved by the present timer, which is capable of "remembering" up to 32 immersion times, and auto- matically provides a signal when a photograph is to be taken out of the bath. Any time a photograph is immersed in the fixer, the user presses the start key on the timer, which responds by lighting a LED. When the fixing interval has lapsed, the timer provides a short beep. The circuit is composed of a 64-stage shift register which is loaded with zeroes on power up, because it lacks a reset input. Electronic switch ES, connects the frequency deter- mining capacitor to the input of clock oscillator N, . The logic level that exists at the D. ter- minal of IC3 is shifted towards output Q at a speed that is defined by Pi , which enables defining fixing times between rough...