Laser cutter, part IV

Now that I have the EiBot board running the steppers, I'd like to have it turn the laser on and off too. Normally, the Egg-Bot this board was designed for has a servo to raise and lower the pen. This is controlled with a pulse width modulation (PWM) output. But I needed an on/off signal to feed to the laser. Looking at the documentation, I noticed references to a solenoid output, which sounded like just what I needed. Looking at the earlier versions, I could see the circuitry to drive the solenoid, but that circuitry isn't present on the board I have. Looking at the schematics, the I/O pin that drives it is now used for another purpose. I downloaded the firmware source code and tried to dope out what the current code did, but I wasn't able to figure it out. So I sent a note to the designer and asked him about it. He sent back a prompt and helpful reply, stating that the RB4 output should have the solenoid signal. I soldered a header on the appropriate pins and hooked up an LED (and current limiting resistor). At first, I had the LED hooked from +5V to RB4, but that voltage isn't supplied by the board when it's running from just USB, so no joy. So I hooked the LED between RB4 and ground, and sure enough, it followed the pen up/pen down state.

( Sweet! )