GB1347187A - Electronic timer - Google Patents

Abstract

1347187 Timing circuit PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 8 Sept 1972 [11 Sept 1971] 41802/72 Heading H3T [Also in Division G2] A timer comprises a transistor, the collector of which is connected to a capacitor to charge or discharge the capacitor at a constant current, the emitter being connected to one end of a linear potentiometer, the other end of which is connected to the base and the wiper is connected to a supply line, there being a further resistor between the other supply line and the base and a switching device coupled across the capacitor. The ratio of the resistance between the wiper and base and the resistance between the base and said other supply line is limited to no more than 1À3. The capacitor 14 may be previously charged by a supply at 17, 18 and discharged by the transistor 12 or the capacitor may be charged by the transistor (Fig. 2, not shown). The voltage across the capacitor is sensed, such as by placing a relay coil 16a in parallel with the capacitor, until it reaches a level to alter the state of the sensor. The relay may be connected via a threshold switch (61) to the capacitor in the "charging" mode of operation and the capacitor discharged at the end of the period by a shunted resistor. The linear potentiometer 8 varies the rate of charge or discharge in a logarithmic fashion. The relay may operate a lamp in photographic exposure apparatus.