Started 7th October 2020
There is a common type of laser, that goes by something like "12mm Red Laser Head Diode Tube 650nm 5mW Dot or Cross Line Module" on ebay. They come in versions that display a line, a cross or a dot. Interestingly they contain laser diodes with a built in photo-diode and electronics which use the photo-diode to control the power output. The model number is SYD1230.
The first set of photos show a version which uses an integrated circuit for power regulation. Click to view full size.
The second set of photos shows a version using discrete components.
Removing the lens, different lenses and the laser diode.
Searching for clues on the discrete driver, there are:
KEYCHAIN LASER DRIVER CIRCUIT by Le Magicien
Sam's Laser FAQ, Toshiba Laser Diode Power Supply (TO-LD1)
The typical three pin semiconductor laser package consists of a laser-diode and photo-diode with a common connection which goes to the positive supply voltage. The two separate connections are the laser cathode and photo-diode anode.
In normal operation the laser-diode is forward biased and the photo-diode is reverse biased (pin 2 is positive relative to pins 1 and 3), as the light output increases the current flowing through the photo-diode increases.
In general for a discrete driver there would be a transistor driving the laser, and another transistor connected between the driver base and the photo-diode in such a way that more photo-diode current reduces the driver base current and hence the current in the laser-diode.
The actual schematic is shown below (click to expand):