the actual circuit is a bit more complicated and has a press button switch that grounds the circuit to turn it off (including pulling an IC reset pin low). I need to add an Arduino controlled thing in there to which can ground the circuit in the same way as the switch.
If I understand what you've got, you can connect a single arduino pin, leave it as an input (with no pullup enabled). Connect ground, too.
To "activate" the circuit, change the pin to an output with a value of 0, delay a bit, then change it back to an input.
-j