Ultra small solar power bank with retro circuitry (with schematic)

I missed a resistor on the first page of the schematic while drawing a neater one. Can you work out where it goes? The answer is at the bottom of this description.

This is a small keychain solar power bank I bought from eBay. What I didn't know is that it was NOS (New Old Stock) dating back about ten years, and it shows what an amazing leap the circuitry in power banks has made. Instead of a single chip, inductor and smattering of capacitors, this unit has lots of discrete circuitry to give a similar result, but still cheats on the end of charge detection.

The unpopulated chip position does correlate to being an LTH7 charge control chip, but has no facility to turn off the LED on charge completion. The existing DW01 LED control hack would not work, since the LTH7 would terminate the charge just before the DW01 turned off the MOSFET (and disabled the LED).

It's actually quite an interesting design.


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Did you manage to work it out? There should have been a 10K resistor between the base of the LED transistor at the left hand side of the schematic, and the collector of the one that switches its base to the 0V rail. The link shown would have shunted the current intended for the LED to the zero volt rail.