OK. I'm familiar with the multi vibrator. It wouldn't be practical for charging things. But I like it though! I could also half wave rectify it so it pulses. I think it would be great if I was using a 12 volt battery.Here's a sim of a crude buck converter.
A standard multivibrator comprising two L6s drives a P-MOSFET to chop the 12V supply . The chopped supply has an average voltage set by the duty cycle of the multivibrator and is smoothed by L1 and C3 to give 5V out to the load. The duty cycle is adjusted by varying R2 or R3.
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I would not recommend this for driving critical loads, since there is no negative feedback to regulate the output voltage.