A portable light source unit for being coupled to an instrument is provided. The unit includes a housing having a port for receiving a light guide end tip of the instrument, a battery and LED mounted within the housing, and a pull tab that extends through the housing and electrically isolates the LED from the battery. Upon removal of the pull tab, an electrical connection is completed between the battery and the LED. The unit also includes driver circuitry for the LED ensuring that a constant level of current is provided to the LED so that the LED produces light of a constant brightness. The driver circuitry also produces a current spike for breaking an inline fuse when a remaining charge of the battery is unable to provide a voltage at the constant level of current above a pre-determined threshold level thereby de-energizing the LED.
