We found this circuit in the 1964 RCA transistors manual. The sensor may be a common LDR and the transistor BC558. The relay has an operating voltage according to the power supply.
This is a traditional complementary-output transistor amplifier sourcing 2 to 4 watts to a 4 ohm...
This is the conventional application of the UCN4202 to control a 12 V stepper motor with current...
The three-terminal positive voltage regulators, such as those in the 78 XX series (where XX is the...