The presented circuit uses a PNP transistor, which can be the BC558. Observe the polarity of the supply. The circuit is suggested by Texas Instruments opto-electronic documentation.
We found this circuit in the National Semiconductor Linear Applications Handbook of 1994. The...
This is 4-input audio mixer using an integrated circuit 741 and two more operational amplifiers....
This shield circuit for controlling the direction of rotation of DC motors with current up to 500...