The circuit works by measuring the voltage across the 5.6 Ω resistor, since the voltage across the resistor will be proportional to the current flowing through it. The voltage is kept small to lower power dissipation and prevent necessitation of a larger voltage source. Because the voltage is so small it is amplified by a non-inverting amplifier. Adjusting the gain of the amplifier allows the magnitude of the sensed current to be adjusted, allowing a wide range of regulation currents. The measured current is compared to the stable reference voltage provided by the TL431, and an error is sent to the BD139. The op-amp will attempt to keep the measured current signal equal to the reference signal, and thereby keep the current constant by constantly adjusting the base current to the BD139. The 1N5815, 1µF capacitor and 22nF capacitor are for protecting the diode from ESD, reverse voltage, etc. I've made a spreadsheet calculator to ease modifying the circuit for different regulation ranges. Download.


I tested my laser between 60 and 90 mA, though it's said to tolerate 110mA. Burning stuff can be done with DVD lasers, so I don't plan on pushing my violet diode. I had to look up the pin out at "Laser Pointer Forums", since I failed to determine it empirically. This is what the pin out is:



Here are some images of the beam before collimation, ie from the bare laser diode. When purchasing bare laser diodes remember that they require additional optics to make a nice laser pointer beam. Commonly something such as an "Axiz module" or the optics from an old laser pointer are scavenged for this purpose.