Use an NPN transistor as a switch

Low-side switching

In a low-side NPN switch, the load connects to the positive supply. The transistor connects the other side of the load to ground. Raising the base voltage through a resistor allows base current to flow and turns the transistor on.

The base resistor is essential: it limits base current. A transistor intended as a switch should have enough base drive to reach saturation at the required load current, without exceeding the drive pin or transistor ratings.

Try it in resistor.dev

1. Open the transistor example and run the simulation.
2. Select the transistor to inspect collector, base and emitter currents.
3. Increase the base resistor and observe the lower base drive.
4. Change the LED load resistor and see how the collector current is limited by the whole circuit.
5. Use Engineering accuracy mode to compare the educational approximation with a SPICE transistor model.

Design checks for real hardware

Confirm the transistor's maximum collector current, voltage rating, power dissipation and gain at the intended operating point. For inductive loads, add a flyback diode. For a microcontroller, check that GPIO current remains within the board's documented limit. A simulation is a design aid; it does not replace the data sheet or a bench measurement.