This power step for relay activation was found in the 1964 RCA transistors manual. The circuit is of the NF type and the transistors admit equivalents. R4 depends on the drive voltage.
This is one of the many versions of amplifiers with LM386. This was achieved in an old publication (1981). The...
This circuit uses unijunction transistors and drives a small speaker through an output...
This circuit can also be used as a shield if the excitation is made with 5 V and the output stage...