I've got a small red laser pointer from a chinese ultrasonic distance meter, which has a small regulator circuit (6mm x 7mm) with SMD components.
I guess that is a constant current to drive the laser, but I was unable to understand the circuit.
I've used a microscope and a continuity tester to check all connections several times, as I was thinking I got some wire incorrectly attached. But I was not able to find any error.
The transistors are labeled L6 in a SOT-23 package, and I assume they are KST1623-L6 or 2SC1623-L6, both similar NPN transistors. I was not able to measure the capacitor without desoldering it, and I'm not confident in being able to take it apart and put it back being so small (~ .5mm x 1.4mm).
I powered the circuit using my Arduino regulated output at 3.3V during about 10 seconds, and the laser lights up, without any overheating observed.
I don't understand how Q2 works, as the base is attached to GND with 39 Ohms resistor, so as far as I know, ideally, it should be in cutoff mode. I'm curious how this circuit can work and why they decided to use it.