Green LED Transistor Driver

Application

This circuit shows the Green LED as a current-limited indicator controlled by a low-side NPN transistor driver. It is similar to how a low-current microcontroller GPIO pin can drive an LED from a separate supply without the pin directly sourcing or sinking the full LED current. The DC sweep shows the driver transition from off to on as the control voltage rises.

Design Context

A transistor driver lets a logic-style control signal switch LED current while the 5 V rail supplies the LED branch. This is useful when a microcontroller pin has limited output current, uses a lower logic voltage, or should be isolated from the load current. RLED limits the LED branch current from the 5 V rail, while RB limits base current into Q1. Sweeping VCTRL from 0 V to 5 V shows the control threshold, collector voltage change, and LED branch current increase in one run.

Expected Behavior

At low VCTRL values, Q1 is off, LED current is near zero, and the collector stays high. As VCTRL rises enough to drive base current through RB, Q1 turns on and pulls the LED cathode toward ground. Current then flows from the 5 V rail through RLED and the Green LED into the transistor collector, so the LED current trace increases and the dynamic symbol should brighten.

Key Values

• LED supply: 5 V DC
• Control sweep: VCTRL from 0 V to 5 V in 0.1 V steps
• LED resistor: 330 ohm
• Base resistor: 10 kOhm

Suggested Experiments

• Compare the sweep to a 3.3 V or 5 V microcontroller output driving the base resistor.
• Change the DC sweep stop value or increment to inspect the turn-on region in more detail.
• Increase RLED to reduce LED current and apparent brightness.
• Increase RB to see how reduced base current shifts or softens the low-side switch behavior.

Featured Component

The Green LED is used here as the switched load in a practical microcontroller-style indicator driver. Its dynamic symbol is driven by simulated forward current, so the schematic visibly tracks how the indicator responds as the swept control voltage turns the driver on.