Electronic contact with NPN switching transistor - has PNP control transistor and amplifier circuit with external variable element

摘要

An auxiliary n-p-n transistor (T2) base is connected through a first resistor (R1) to the control transistor (T3) collector and to the switching transistor (T4) base. Its emitter is connected to the transistor (T4) emitter, and collector through a second resistor (R2) to its own emitter, and through a third resistor (R3) to the supply voltage positive terminal. The control transistor (T3) is controlled through a further control element (W) connected either to the transistor (T3) base and to supply voltage positive terminal, or to transistor (T3) base and transistor (T2) collector.