Design Idea

Drive a blue LED from a 3V battery

Two 1.5V cells and a few components can light a blue LED.

Sergi Sánchez, Federal Signal Vama SA, Vilassar de Dalt, Spain; Edited by Brad Thompson and Fran Granville -- EDN, 2/15/2007

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 R_S 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 R_S 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, V_BAT+|V_OUT|, and Q₁'s base-emitter voltage for various battery-voltage values.