Constant illumination flashlight

Claim 1

A portable flashlight designed to provide a relatively constant illumination output over the useful service life of the battery power supply therefor, despite a relatively poor battery maintenance characteristic during discharge, comprising an incandescent lamp for providing illumination, a circuit for providing relatively constant electrical power to the incandescent lamp over substantially the entire useful life of at least one power supply battery, having a relatively poor voltage maintenance characteristic during discharge, said circuit including a drive transistor having the incandescent lamp coupled in series with the emitter and collector of the drive transistor and further coupled in series with said battery power supply, said circuit further including a control circuit including first and second control transistors coupled in a complementary manner such that only one control transistor is conducting at a time, said first control transistor being coupled to the base of said drive transistor, to drive it into a saturated conductive mode when said first control transistor is conducting, and said second control transistor being an inhibit transistor having a drive output thereof coupled to the base of said first control transistor to turn it off when said inhibit transistor is conducting, wherein said control circuit causes said drive transistor to generate a current pulse train in which the duty cycle thereof is increased, by providing current pulses having a greater pulse width to compensate for a lesser pulse amplitude, as the voltage from the power supply declines during its discharge to provide relatively constant electrical power to the incandescent lamp over substantially the entire useful life of the battery power supply, wherein said drive transistor has a relatively high beta characteristic and is driven by said control circuit in a non-dissipative manner in substantially either a fully conductive state or a fully non-conductive state to produce the pulse train, whereby power dissipation in the drive transistor during partially conductive modes is substantially eliminated, and wherein an RC timing network is coupled to the base of said inhibit transistor for controlling the frequency of the pulse train generated by the circuit.