This is all simple LM741 circuits, it's so useful, LM741's general purpose single OP-AMP. UA741's high performance monolithic OP-AMP on single silicon chip.
Though very ancient, it still is many useful.
lets to see Automatic Op-amp night light circuit using IC-741. It is also very simple. As my feelings It has high sensitivity more than one,high accuracy.
This Diode tester circuit can check the diode the good or bad and also can indicate its polarity. using op-amp 741IC and LED for display. [...]