Digital light meter system for a camera

Claim 1

In a digital light meter system for a camera including an oscillator, a counter for registering the pulses produced by said oscillator, a D/A convertor for converting the digital values registered in said counter into an analog value, and a comparator for comparing the output of said D/A convertor with an output from a light measuring and computing circuit, which latter output has a temperature coefficient K and is proportional to the sensed brightness of an object as a function of preset values of film sensitivity and either aperture opening or exposure time, the improvements characterized by said D/A convertor comprising: (a) a constant voltage source providing a voltage having said temperature coefficient K, (b) first, second, third, and fourth constant current sources connected in parallel, having a common output terminal, and providing output currents having ratios of 1:2:4:8, respectively, (c) a fifth constant current source drivingly coupled to said first, second, third and fourth constant current sources and supplying a current I having said temperature coefficient K, said fifth constant current source comprising a self-biased field effect transistor, a first transistor having its base coupled to the gate of said field effect transistor and a second transistor having its base and collector coupled to the emitter and base, respectively, of said first transistor, (d) a resistor connected between the constant voltage source and said common output terminal, and (e) means individually connecting outputs of said counter to said first, second, third and fourth constant current sources for controlling the conductions thereof, whereby the voltage drop across said resistor is proportional to the analog equivalent of the digital value registered by said counter.