Why does the brightness of the ky-008 laser head sensor module differ greatly between being connected directly to the 3.3V power rail of a bread board and to a GPIO pin?
The laser emits a very bright beam and a "star field" around it when connected directly to power or two AA batteries in series, but a small, tight beam when powered by a "HIGH" pin. From what I read the GPIO pins should operate at 3.3V.
It is likely that the current the Raspberry Pi's GPIO pin provides is too low for the laser diode to operate properly.
I did not find a data sheet for the laser diode, but this page claims 30 mA at 5V.
The Raspberry Pi's GPIO pins however have a limited capability to support high current, see here: Maximum current on each GPIO pin for Raspberry Pi 3 Model B
The party line has always been that you shouldn't draw more than 16 mA from an individual GPIO or more than 50 mA from the GPIO as a whole.
It might thus be necessary to use an additional transistor to control the laser diode from the GPIO pin, see here: How to control a 3V laser from GPIO
