Troubleshooting Laser Diode Circuit with Arduino and PN2222a Transistor

[Stonestreecty]

TL;DR Summary

I have a circuit where the base transistor (PN2222a) leg is plugged into the pin 9 of the Arduino, but my laser diode doesn’t light up. After some adjustments, the circuit doesn’t work...

Hi, all
I have a circuit where the base transistor (PN2222a) leg is plugged into the pin 9 of the Arduino. When I power everything up and send the signal from the Arduino to the transistor base leg, the laser diode doesn’t light up, but my design strictly follows Complete Tech Guide to Laser Diodes. In addition, the Arduino is powered through a cable that is plugged into the computer and I am using a LED rocker switch if that helps.

Is there anyone has ideas? Thank you in advance.

 

[Borek]

What is the diode model/make/specification? Turn-on voltage, current?

Do I understand correctly that you use external 9V for power, not the standard 5V supplied through the Arduino?

I am not sure I understand the schematic - what is connected in parallel with the diode and to A0? Looks like photoresistor, but they have two legs?

 

[berkeman]

Summary:: I have a circuit where the base transistor (PN2222a) leg is plugged into the pin 9 of the Arduino, but my laser diode doesn’t light up. After some adjustments, the circuit doesn’t work...

but my design strictly follows Complete Tech Guide to Laser Diodes

Please post a link to that; your schematic makes no sense to me.

EDIT/ADD -- Please also post a link to the laser diode datasheet. Is it a visible laser? And are you wearing the appropriate laser safety glasses while trying to get this to work?

 

[DaveE]

1) your battery is backwards.
2) it doesn't matter because it's in series with a capacitor, which will quickly charge up to the battery potential. The net voltage from the two will be zero.
3) I don't believe this schematic is from "Complete Tech Guide to Laser Diodes" (whatever that is). It doesn't really make sense. Are you sure you've drawn it correctly?

 

[DaveE]

Try something like this:

 

[berkeman]

Try something like this:

And don't forget to put your goggles down over your eyes before you flip the switch (not like this guy, whoever he is)...

https://superradnow.files.wordpress.com/2011/11/real-genius_480_poster1.jpg

 

[DaveE]

This circuit will turn the laser on and off at the PWM frequency. You probably won't get a dim laser output at any instant in time. What you'll get is a laser pulse that is either on or off with a variable fraction of the time it's on. This can "look" dim to many processes that effectively average the light they "see" over a time period longer than the PWM period (like a human eye, for example). Assuming your detector is also fast compared to the PWM signal (most are), your uP won't measure a dim light, it will either measure all or nothing.

If you want a continuous, but dimmable, light source, you will need to add some sort of low pass filter between the PWM signal and the diode. Ask if you want to see how you might do that.

 

[berkeman]

Try something like this:
View attachment 287495

If the diode circuit is powered with 9V and the uP with 5V, what voltages will A0 see coming off of that sensor? It may need clamping to 5V, depending on what-all is going on.

 

[Stonestreecty]

Thank you for you all replies.
It is a 5V laser diode and the power the battery is 9V wouldn’t that fry my diode if I didn’t include the resistor?
I included the capacitor to prevent spikes from damaging the laser diode. It is 220uF.
how does not having the resistor between the base and the arduino affect the whole circuit?

 

[Baluncore]

@Stonestreecty
It would be good if you could give us a link to the "Complete Tech Guide to Laser Diodes".
Then we would know what circuit was recommended.

 

[DaveE]

Thank you for you all replies.
It is a 5V laser diode and the power the battery is 9V wouldn’t that fry my diode if I didn’t include the resistor?
I included the capacitor to prevent spikes from damaging the laser diode. It is 220uF.
how does not having the resistor between the base and the arduino affect the whole circuit?

Sorry, I just can't make sense out of your schematic, so I'll pass on your questions. You'll be better off asking questions about my schematic, I think. Laser diodes aren't 5V unless there is extra stuff built in, like a series resistor. So, if you want good answers, you'll need to tell us what parts you are using. Real part numbers and datasheets.

In general, most semiconductors need something like a series resistor to limit the current that flows, as do diodes. Although there are so many variations that I'm not sure this advice helps at all.

Adding a capacitor to filter "spikes" isn't a bad idea, but you have to put it in the correct place. That is usually a pretty complex subject.

Please draw a schematic of what you intend to build, I doubt that it's the drawing you previously included.