Build a Laser 3D Printer - Stereolithography at Home

Step 8: Laser Driver Board

There is most likely a better way to do this but I am not an Electrical Engineer, the parts I used were on hand. This setup has worked fine for me for hours of printing so far. 

The drawings below are a little different than what I have wired but should work. You could also put this all on one board, this was the biggest board I had on hand.

The Laser driver board takes 9V from a wall wart power supply to drive the relay and the LD33V 3.3V Voltage Regulator that powers the laser. The TIP120 transistor switches the relay which switches the ground of the laser to turn it on or off.

Comments

[eranrom.me]

To my understanding the laser ground switched by the relay needs to be 'switched' to the 9V ground. That is - when ever the TIP 120 transistor gives a signal, the relay coil is 'activated' and the laser ground needs to be connected to the 9V ground.
Now, in the circuit picture, the 9V ground is connected to the lower right leg of the relay (when looking from above), which according to the relay spec is indeed the leg that is 'switched' when the coil is 'activated'. In the Fritzing diagram, however, the 9V ground is connected to the lower left leg of the relay. Is that a mistake?

Thanks very much!
Eran