Please Educate Me On parallel LED Laser Basics

Other Hardware LEDs and Multiplexing
1

Greetings,

Thanks to responses, I corrected my LED polarity and realized that I am dtrying to drive more current than my microprocessor will allow. Please see the notes on my final drawing. Thanks to all who contributed.

I am trying to energize two 3.3vdc 5mw Laser LEDs. I am including two schematics in the link below. In the first schematic, I simply took the output from my single PC817 because the current limit was greater than twice the current rating of the LEDs (in my limited scope of knowledge). This resulted in a significant dimming of both LEDs.

I then decided to send my digital signal into two different PC817s (see the second schematic). This also results in both LEDs being significantly dimmer than when I remove one of the PC817s from my breadboard. I do not understand this. With each PC817 having a discrete output (one Laser), why should it matter how many PC817s are being energized by my digital output pin? It seems to me that the second schematic is the same as having separate digital outputs to each PC817 that are controlled together.

You may notice in the JPGs that I also tried using a LTV-817H which is an opto-isolator with a higher current rating, to no avail.

If anyone can shine some light on this matter and bring me up to speed (or, at least, shoew me how to do this), I would really be grateful. Thanks, ahead of time, to anyone who can offer some insight.

1_PC817
1_PC817801×1019 104 KB

2_PC817s
2_PC817s1920×1338 128 KB

PC817_C
PC817_C1920×1338 129 KB

2

The laser diodes are connected backwards in the posted schematics, and there are no current limiting resistors in series with the optoisolator inputs, as is required.

Make sure that the optoisolator output transistor can handle the required laser diode current, and that the optoisolator input LED drive current is high enough, or the output transistor will not be fully saturated.

3

My guess is that if you measure the 3.3V it's not "holding up" when you draw more current.

4

Care to share that rating?
If we know the ratings, maybe we can help

6

That was the issue. Too little current to drive the optoisolators. Solution is a simple software change and duplicating the output signal on another digital output to make things right. Thanks!

7

For a 3.3V digital signal the correct resistor value would be (3.3V-1.2V)/20mA = 105Ω
You need to use the voltage across the resistor not the diode to determine the resistor value
60Ω would result in a current of (3.3V-1.2V)/60 = 35ma, way to much.

8

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