Started 6th October 2020
It's cheap to buy 10 laser diodes from ebay, I was not that impressed and they stayed in their bag. One reason, there's no specification, how do I know what current to run them on, or how to drive them. Another aspect is safety, traditional bigger is better specmanship is on display "Military grade laser cat toy", but also "safe low power". It is said that many of the laser pointers are more powerful than described, either because that makes them more attractive or less likely to be stopped at the border.
Eventually I took a closer look and found some interesting features.
There is a current limiting resistor built in (the white rectangle in the photo on the left), it is 90 Ω. With a forward voltage drop of 1.9 V running from 5 V that gives a current of 33 mA which is in the right area.
The front will unscrew, this allows some focusing, eventually a spring and lens will pop out. Naively I expect a laser to produce a non-diverging beam, here it is the result of a lens.
The back consists of a PCB with the laser diode fabricated on it, pushed into a slot in the circular metal shell.
The result of this is that there's a hole at the back that some laser light escapes from.
It is possible to use a sensor here to monitor the light output of the laser.