BD139 NPN Bipolar Medium Power Transistor (BJT) 80V 1.5A TO-126 Package Surface Mount It works on the principle

It works on the principle of voltage comparison

The slide switch is for switching the circuit by sliding the mini handle of the switch to turn the circuit on or off

the Echo pin outputs a high level

Female connector with improved locking recess without "window"

BD139 NPN Bipolar Medium Power Transistor (BJT) 80V 1.5A TO-126 Package Surface Mount It works on the principleThe BD139 is a NPN complementary low voltage NPN transistor in 3 pin Its package is TO 126. This device is designed for audio amplifiers and drivers utilizing complementary or quasi complementary circuits. Specification Type Medium Power NPN transistor Maximum collector power dissipation (Pc) 12 W Maximum collector base voltage Vcb 80 V Maximum emitter base voltage 5 V Maximum collector current 1. 5 A Applications Medium Power and Liner Switching