This is TBA820m Amplifier circuit stereo 2W+2W, supply voltage 12V, Easy to build and cheap. on DIP-8pins same LM386. but TBA820M use easy better. [...]
Here is temperature sensor diode 1N4148 silicon type and 741- op amp, so easy. The Output to voltmeter. You will learn why diode becomes temperature sensor.
This is inverter circuit, 12VDC to 220V 50Hz at 500W power.It easy to make and Low cost. using basic parts transistors 2N3055,IC-4047,