Sergi Sánchez, Federal Signal Vama SA, Vilassar de Dalt, Spain; Edited by Brad Thompson and Fran Granville -- EDN, 15/2/2007
Using a blue LED can pose problems when available power-supply voltages don't meet or exceed the LED's 3V forward-voltage drop. This Design Idea shows how to drive a blue LED from a 3V battery or another power supply.
The circuit in Figure 1 uses the On Semiconductor NCP1729 voltage inverter, IC1, to produce enough voltage to drive blue LED D1. Transistor Q1 serves as a constant-current limiter for the LED's forward current. When current through the LED and RS increases to a level that develops enough base-emitter voltage to turn on Q1, Q1's collector draws current from the voltage divider comprising R1 and R2 and forces IC1 to shut down. The voltage inverter restarts when the voltage drop across RS falls below Q1's base-emitter turn-on threshold. Pulling transistor Q2's base to ground through R2 turns on the circuit.
In this application, the LED exhibits a voltage drop of approximately 3.3V at 10 mA forward-bias current. Table 1 illustrates the LED's applied voltage, VBAT+|VOUT|, and Q1's base-emitter voltage for various battery-voltage values.