[SteveWill]

I stand corrected. As I said I am not familiar with G540. With pull down output putting a meter on the output of the G540 may not show any movement. If it was me I would get a 1000 ohm resistor put 12VDC on one end of the resistor and tie the other end of the resistor to the output of the G540 Now when you put a meter on the output it will read 12VDC when the output goes low( less then a volt) the output is turned on.

Once you find the right control to make the output happen when you want it then tie g540 output to the relay where you had the ground wire and remove the ground wire.. The 12VDC wire will be tied to the other lead of the relay. DO NOT TOUCH TO THE OUTPUT OF THE G540 with a wire that is connected directly to a voltage, it has to have a resistor or coil between the power and the output of the G540.

This part is still correct:
OK lets see if we can get the relay to work.
Connect the 2 wires you have taped to get the unit to fire to the output of the relay ( this would be the contacts/switch side).
Now the coil/control side of the relay connect one pole/contact to ground and take a wire with 12VDC and touch the other contact on the coil, does it fire?
Now that the relay works, we need to find a way to control the relay.

[greenjeens]

I stand corrected. As I said I am not familiar with G540. With pull down output putting a meter on the output of the G540 may not show any movement. If it was me I would get a 1000 ohm resistor put 12VDC on one end of the resistor and tie the other end of the resistor to the output of the G540 Now when you put a meter on the output it will read 12VDC when the output goes low( less then a volt) the output is turned on.

Once you find the right control to make the output happen when you want it then tie g540 output to the relay where you had the ground wire and remove the ground wire.. The 12VDC wire will be tied to the other lead of the relay. DO NOT TOUCH TO THE OUTPUT OF THE G540 with a wire that is connected directly to a voltage, it has to have a resistor or coil between the power and the output of the G540.

This part is still correct:
OK lets see if we can get the relay to work.
Connect the 2 wires you have taped to get the unit to fire to the output of the relay ( this would be the contacts/switch side).
Now the coil/control side of the relay connect one pole/contact to ground and take a wire with 12VDC and touch the other contact on the coil, does it fire?
Now that the relay works, we need to find a way to control the relay.

Im in the process of checking to make sure the output turns on, with the resistor. This is gunna sound rediculous and my lack of electrical knowledge will show. I have the positive probe of my voltmeter on the resistor, with there a 12 volt power wire attached, on one end, and the other end into the output of the g540. Where do i put the negative side of the probe? Also i bought a cheap 12 volt power adapter with a positive and neg wire, what do i do with the neg side of the power adapter, is this where the neg side of my volt meter goes?

[holobox]

Here's my recommendation using opto-isolator, mechanical relay, etc. If you mind your polarities you'll only need the lower opto channel. (actually the 390ohm resistor in the BOB is good for the single channel, if you use both channels cut that value in half!) What you see here is active (torch on) with a high logic level on the ~spindle control pin from the BOB, that assumes a switch closure does the trick! Probably good idea to procure a relay with normally open and normally closed contacts. You really don't want to be powering that relay when you aren't cutting. About anything will work with the snubber diode shown at the arrow. (1N914, !N4004, etc.)

If you can't find a +12Vdc supply in your plasma cutter, some resistor values will need to change. You want to make sure that no electrical signal paths (supplies, gnds, etc.) are shared between your BOB and cutter! Put the entire circuit inside of the cutter, two wires only out to the BOB!