Electronic – Driving blue LED with 3.3V using an op-amp


I'm trying to design a circuit that drives six LEDs – two red, two green, and two blue – off a 3.3V GPIO pin. Since the voltage drop on a blue LED is in that range, I'm attempting to step the voltage up with an LM318N and then drop it with a resistor. However, all the red and green ones are working when set up like this, but the blue ones are not.

LM318N LED config

Is this a reasonable setup to accomplish what I'm going for, or is there something wrong with the way I'm approaching this?

Best Answer

LM318 is only specified to drive to within 3v of either rail, it's not a rail to rail (R2R) output opamp. As you've got it supplied from the +5v pin, its output current into a 3v load will be low at best, and might be zero.

For a drop in replacement, find an amplifier that's got a R2R output.

It might be simpler to use a switch instead, a simple transistor or FET, or a 5v rail gate with 3.3v capable input like an HCT74xx part. LM339 makes a nice low current FET driver as well, output pulls close to ground, and the inputs are very flexible.