Camera exposure control circuit - has photocell input to Schmitt trigger controlling shutter electromagnet and coupled automatic resistor changeover for flash

摘要

The circuit comprises two sections, a photocell operated delay circuit (1) and a shutter electromagnet (M) switching circuit (2). The switching circuit (2) comprises two transistors (T1, T2) Schmitt trigger input with the shutter electromagnet (M) in the collector of the second (T2). - For daylight the photo-resistive cell (P) has a low value compared to a resistor (R1) and the first transistor (T1) switches on when the shutter release is initially operated. Further operation opens a switch (S2) across a capacitor (C), switches off the first transistor (T1) so that the second (T2) conducts and operates the shutter magnet (M). In the operate condition the magnet holds the shutter open, for a period determined by the time constants (C and P) in the photocell circuit. For low light situations the photocell (P) has a high value compared to the series resistor (R1) and switches the transistor (T1) off. The second (T2) operates the shutter electromagnet (M) which changes over the switch (S4) from the photocell (P) to fixed resistor (R2) flash operation.