This design has a low duty cycle to preserve battery life and underdrives the cold filament to extend bulb life, however it doesn't flash well on a near dead battery.
The circuit below has a higher duty cycle and drives the cold filament a bit harder. It seems to work well with near-dead batteries too.
Back in the 60's these lights used three germanium transistors and a thermistor for temperature compensation; I wish I still had one like that. The 70's and 80's circuitry shown above invariably used two silicon transistors, and a CdS photocell was often present to conserve battery life. Nowadays they often use "Hybrids" on a ceramic substrate and a photodiode is built right into the chip. The newest lights use ultrabright LEDs but externally they look the same.
I do not gaurantee the accuracy or safety of any information herein.
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