Darlington Switch

 

A Darlington switch uses two NPN transistors connected in a way that greatly increases current gain. When the switch is pressed, a small input current flows through resistor RB into the base of the first transistor (TR1). This current is amplified and passed to the base of the second transistor (TR2). As a result, TR2 conducts a much larger collector current, allowing the load RL to draw significant current from the supply.
Because the total current gain is approximately the product of both transistor gains (β1 × β2), even a very small input current can control a large load. This makes the Darlington pair useful when weak signals need to drive high-power devices like relays, motors, or lamps.
When the switch is OFF, no base current flows, so both transistors remain OFF and the load is disconnected. When the switch is ON, both transistors saturate, effectively acting like a closed switch and allowing current through the load.
One important point is that the Darlington pair has a higher base-emitter voltage (about 1.2–1.4 V) compared to a single transistor. This should be considered in low-voltage designs. Despite this, it remains a simple and effective way to achieve high current amplification.