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Purpose of visible green LED on a green laser pointer driver

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josecanuc:
I was given a cheap green laser pointer as part of a conference's swag kit. These seem to be quite ubiquitous for "cheap Chinese green laser pointer".

I tried to work out, as best I could, the schematic of the driver circuit, as shown.

I haven't yet reverse engineered how it works, but I gather that there is a phototransistor that's detecting the output power of the laser diode and using that to modulate the power output to the 5mW specified on the box. (And in fact, if I give it input voltages from 2.1V to 4.0V, the power drawn is exactly 5mW in every case.)

What struck me initially was that there's a standard visible-light green LED on the circuit, which would normally be encased in the metal case/tube of the laser pointer. What is the purpose of this LED? Is it to provide a cheap voltage reference per the forward voltage of the LED?

josecanuc:
Just double-checking the schematic and I see a couple of mistakes, which make me re-think the polarity of the laser diode. But the main question is about the purpose of the non-laser LED.

ledtester:
It might be related to monitoring the laser diode. This video has a tear-down of what's in a laser diode assembly:



Some assemblies use a photo-diode to detect the amount of light actually being emitted by the laser.

Dabbot:
This schematic doesn't add up. Q2 is going to short everything after the phototransistor via its base-emitter.

mikerj:

--- Quote from: Dabbot on July 21, 2019, 05:36:14 am ---This schematic doesn't add up. Q2 is going to short everything after the phototransistor via its base-emitter.

--- End quote ---

There are significant errors in the schematic, the entire circuit including the laser diode can only draw current through the feedback phototransistor which is definitely not correct.

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