How can I provide a 12v no-load output with a 5v source?
I'll add some specifics. Before I do, a short discussion of my thinking.


I do not know what you are talking about when you discuss an amplifier and a camera. It would have helped clarify my own understanding as to whether or not anything I say here would be helpful, if I knew more about what you are discussing. But I don't. So I'm going to completely ignore the camera and just assume we are talking about an amplifier that has "special features" I don't apprehend well.
You already have tested the idea of supplying \$+12\:\textrm{V}\$ to an "input." From this, I adduce that you (a) used the existing supply in the amplifier to achieve this; (b) just used a single wire jumper; and (c) didn't need a second reference point since an assumed shared ground reference already existed. I am not sure if this input has 1 pin or 2 pins (a + and -?) But for our discussion purposes, I'll just assume 1 contact point is required and that there is an assumed shared reference, regardless.
These things tell me you do have a way of accessing the amplifier's \$+12\:\textrm{V}\$, single input point you are trying to control, and also the amplifier's ground point.
I'm also going to assume that you can connect the Arduino ground to the amplifier ground without causing any trouble in either system. (This may be true, but opens the door to some unlikely but possible problems.)
You've said (but apparently didn't measure) that very little current is probably required by your amplifier's input-under-consideration.
You've asked for the simplest way.


The following is based on a pretty simple idea. Just tie the input to your amplifier's \$+12\:\textrm{V}\$ supply rail through a resistor, to start. This will cause it to be always active, if you don't do something else. Then, add a BJT switch to pull that input down to ground (or close to it.) If your I/O pin is HI, then the BJT switch is ON and the control input to the amplifier should be close to ground. If the I/O pin is LO, then the BJT switch is OFF and the control input to the amplifier should be close to \$+12\:\textrm{V}\$.

Examine the left side first:







simulate this circuit &ndash; Schematic created using CircuitLab

On the left, \$Q_1\$ and \$R_2\$ help to isolate your Arduino input from the \$+12\:\textrm{V}\$ supply and it's probably all you really need. But if you are risk-averse then you could also consider the right side, where the added BAV99 diode pair clips any unusual voltages and limits unexpected currents from them into your I/O pin. Cheap protection, but perhaps over-kill too.



You could also just go with an opto, if you wanted. This would keep the two systems galvanically isolated (good thing) and doesn't cost much (about half a dollar) and uses a part that is supplied from multiple manufacturers. Let me know if that's what you'd prefer to use. If so, I'll add a circuit for it.