Tricky Circuit

NPN + LED

A small signal takes control.

Use a small base current to control a separate LED current.

Open interactive experiment
NPN · NPN transistor
n-type / p-type / n-type. N–P–N describes three semiconductor regions: n-type, p-type, n-type. Electrons are the majority carriers in n-type material; holes in p-type. Here a small base current controls the LED current.
LED · Light-emitting diode
Light-emitting diode. A semiconductor component that emits light when forward current flows. In this circuit, a resistor limits that current.
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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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Two current paths. One controller.

Simplified NPN: gain 100, VBE 0.7 V, saturation VCE 0.2 V, red LED 2 V. These are assumptions, not guaranteed ratings.

Base current flows through Rb and B–E. Collector current flows through Rc, LED and C–E. In saturation, Rc limits current. Never connect an LED without its resistor.

Difficulty

  1. Build · Switch on the light

    Power on and enable the base signal. Reach 5–20 mA in the LED.

    Need a hint?

    Try Rb = 10 kΩ and Rc = 330 Ω.

  2. Explore · Switch it off

    Keep power on but turn the base signal off. LED current must be zero.

    Need a hint?

    The base driver switches between 5 V and 0 V.

  3. Solve it · Control efficiently

    Enable the signal. Reach saturation and 8–12 mA, using at most 0.20 mA base current.

    Need a hint?

    Try Rb = 33 kΩ and Rc = 330 Ω. Real designs allow margin for gain variation.

Extra reading and circuit recipes

A little signal, a separate power path

An NPN transistor lets a small base current control a collector current. The LED gets its energy from the supply. The base signal controls that path; it does not create energy. This separation is the beginning of electronic control.

Ib ≈ (Vin − 0.7) / Rb; Ic ≤ (5 − 2 − 0.2) / Rc

With Rb = 10 kΩ, base current is about 0.43 mA. Rc = 330 Ω limits the saturated LED current to about 8.48 mA. The collector-emitter voltage is about 0.2 V in this simplified model.

Gain is not a promise

Our model uses a gain of 100 in the active region. Real current gain varies with device, current and temperature; a reliable switch needs design margin.

More input stops helping

Once the transistor saturates, the LED resistor and supply mainly determine collector current. More base current will not keep multiplying LED current by 100.

Ground is a shared reference

A separate signal source needs the correct common reference with the switching circuit. Otherwise “5 V at the input” may have no useful meaning to the transistor.

A useful trap to avoid

A motor or relay is not just a bigger LED. An inductive load can produce a high voltage when switched off; its driver needs suitable flyback protection and current ratings. That protection is not modeled in this LED experiment.

The transistor is already saturated. You reduce Rb. LED current…

  1. must double
  2. is mainly limited by Rc
  3. becomes zero
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Base current increases, but Rc and the available voltage already limit the collector current.

Signal-controlled indicator

Switch the base signal off and on. Follow both current readings.

About 0.43 mA controls about 8.48 mA. The two currents take different paths.

Less drive, same light?

Try a smaller base current while keeping Rc unchanged.

About 0.091 mA still reaches saturation in this ideal gain-100 model. This is not a guaranteed real-world switching design.

Analog Devices: transistor stages