See Many simple electronic circuit that is very important on this sites. My friends used to say the large projects, from small several circuits to combinations.
This is a 4 transistor audio amplifier circuit. Which is a 4-transistors complementary push-pull amplifier, that shows the basics of audio amplifier design. This circuit saving on battery current, which is quite low with middle volume, rising to 25 -30mA as a volume is increased. This gives us a 250 mW amplifier, enough to drive a loudspeaker to the same volume as a mobile phone or MP3 player. The input must be about 100-500 mV to drive amplifier fully. Previously, you may like LM386 amplifier circuit. But you may also like this circuit as well. The working of 4 transistor amplifier circuit …
This is simplest automatic solar led light circuit that my son try to make it for basic small solar charger. to use 2 x AA NI-MH battery total 2.4V