Use a 3V battery to drive a blue LED

Using a blue LED can be problematic when the supply voltage used does not meet or exceeds the LED's 3V forward voltage drop. This Design Idea shows how to drive a blue LED from a 3V battery or other power source. The Figure 1 circuit uses On Semiconductor's NCP1729 voltage converter (IC1) to generate enough voltage to drive blue LED D1. Transistor Q1 acts as a constant-current limiter for the LED's forward current. When the current through the LED and RS increases to a level that provides enough base-emitter voltage to turn Q1 on, Q1's collector sinks current from the voltage divider formed by R1 and R2, forcing IC1 to turn off. When the voltage on RS drops below Q1's base-emitter turn-on threshold, the voltage converter restarts. The circuit is turned on by pulling the base of transistor Q2 to ground through R2.

In this application, the voltage drop across the LED is approximately 3.3V, and the forward bias current is 10mA. Table 1 lists the voltage VBAT + |VOUT| applied to the LED, and the base-emitter voltage of Q1 at different battery voltage values.