This (original source is here) with some "explanation".
This is a pretty dubious circuit in general and I really doubt an ordinary non-differental photo interruptor will work satisfactorily as a smoke detector at non-lethal smoke densities. If you crank P2 down all the way it will burn out T2. It depends on leakage to keep T4 off. There are other issues.
Modern smoke detectors usually use a sensitive ionization chamber with a tiny bit of Americium-241 radioactive material. Ones that operate photoelectrically usually use a scattering principle. The method suggested here (beam detection) is prone to all sorts of problems.
If you want to use it as a beam break detector, you could try connecting another MOSFET gate to the one driving the buzzer and using that to switch a relay (diode across the coil). But I'd suggest staying away.. this is strictly amateur hour stuff.