If you look carefully on the calc = button on the pcb you'll see 2 contacts, usually in the shape of 2 E's, one interlocked with the other, what you want to do is follow the tracks that go to each 'E', scrape off some of the green or whatever colour laquer on the board and solder 2 thin wires to it, if you can find an old coil and unwind some of the copper wire this is great for the job as you can fit it in the calc easy.
Connect the calc to your circuit using a headphone connector then you can just unplug it.
You also need to be carefull of background light with this circuit, whatever interrupts the light source needs to make a noticeable shadow on the photocell.