The "opto in the circuit" doesn't control the voltage whatsoever.
The Vcc to the op-amp does. The op-amp is connected as a feed back comparitor, meaning as long as any voltage above 0V is applied to the + pin, the output will be driven high which as we can see is 3V.
I took 10 sec to modify that circuit in paint for what is the equivalent effect with fewer parts, I even added a LED indicator just for kicks so you know its on, using the un-used op-amp.
I didn't add it in but 40mA seems a bit high (the sourcing current of the op-amp) for a plant, if you wanted to be safe with your plant I'd add a resistor in series to the plant of 1K ohm and then lower it as required if you get no movement.
Edit: There is one thing wrong with my hasty circuit, you must use a voltage divider to bring the 5V D2 pin to a voltage range of 1-2V before connecting to the + pin on the op-amp.
