Electronics > Beginners
Purpose of visible green LED on a green laser pointer driver
mariush:
Maybe they reused the circuit board in multiple products. Maybe a particular OEM product had a tiny view port that showed the led (or some light ring around the laser pointer) while powered on.
KL27x:
^+2. I feel like it's most likely not used as a voltage reference, because there's no node across it that doesn't also include a series resistor?
chris_leyson:
Thanks josecanuc for posting corrected schematic, that makes a lot more sense.
LED D1 doesn't serve any purpose apart from being a battery indicator. and now for the verbose
Q2 drives the laser diode and is biased on by R3. When the laser diode reaches it's threshold current and starts to emit light then the photodiode produces photocurrent that flows out of the anode and into Q3 base. Q3 collector then shunts Q2 base drive and provides negative feedback. The laser diode operating current and output power is controlled by the photodiode current because the photodiode detects the amount of light emitted from the laser diode rear facet.
In your schematic R4 and R5 set the required photo current to set the laser power. Back of envelope calculation goes like this, say you need 0.7V bias voltage to get Q3 to conduct, then you need about 60uA photodiode current flowing through R4 R5. The data sheet says 150uA typical photodiode current at 10mW output power, so at 60uA the laser output would be about 4mW if you trust the data sheet.
Is it eye safe ?
EDIT: R1 C1 provide a snubber or a transient suppressor RC network cross the laser diode.
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