If I understand how the fire button works correctly, you can use an NPN transistor (2N3904 is fine). With base at or below emitter potential (less than voltage drop of base-emitter junction), unbiased, no current flow. Base forward biased (3.3V is plenty), current flow. Fire terminals would be +12 side to collector as shown and the other side replaces where ground is shown to the emitter resistor. CPU output pin to base resistor (your CPU output pin is simulated by the 0 and 3.3V sources in the screen grabs). Hopefully the simulation makes sense to you. Forward bias current through base of transistor (base emitter junction) and it conducts. Zero bias current, it doesn't conduct. Classic emitter follower. You may have to experiment with what the DNA wants to see at the other side of the fire button. Another approach would be classic open collector / common emitter configuration. See the third simulation screen grab. Some CPUs have at least a few open collector outputs. If yours does, you don't even need a transistor, just a current limiting resistor. [/QUOTE] This worked perfectly! I got caught up over thinking it after reading that other thread where he settled on using a relay. Thanks again Mad Scientist [/QUOTE] Awesome, great news! Can you post the schematic of what you settled on? I'm curious as to whether the emitter follower configuration allows enough current to flow to fire the DNA 200. Or did you go with common emitter? Something else entirely?[/QUOTE] I ended up with a common collector configuration that seems to work well. I can trigger a single fire by pulling HIGH for > 10ms.