'LED bulb' for 2.4 Volts:
I use the E10 base LED lamp below for caving. A very popular
headlamp is the FX2, but it is based on 2 NiCd cells, so only
I designed a tiny circuit wit 2 transistors to be fitted
into to E10 base, so its a real plug-and-play replacement!
It works fine from 1.0 V, and is optimised to deliver 25mA
per LED at 2.4V. Input current is around 130mA @ 2.4V for
For the schematic see below. All parts are SMD (0805 &
SOT23). The inductor is from coilcraft. Both transistors may
also be BC847 without sacrifying efficiency too much.
You can build it together by using the inductor as carrier.