The basic idea is taken from Atmel's application "AVR182 ZeroCrossDetector". The circuit diagram can be subdivided into the zero voltage detector (with transformer less power supply), the phase cut control (an ATtiny25 microcontroller with 5V voltage supply) and the power section.
Since the Pentax P80 movie projector has a metal housing, it's safer to separate the controller from the mains voltage, to attach the potentiometer R9 to the housing. The aim of the circuit design is to make the phase-cut control potential-free. Thus, the optocouplers OK1 SFH6156 (signal transition: zero voltage switch / phase cut control) and OK2 MOC3023 (signal transition: phase control / motor control) were used.
In the part of the zero voltage detector, the transistor T1 reduces the threshold voltage and thus increases the sensitivity of the zero voltage detector for a switching threshold close to the zero point of the mains voltage.
The trigger signal is forwarded potential-free to the microcontroller via the optocoupler OK1 SFH6156.
The phase intersection control runs in a microcontroller ATtiny25,, written with avrStudio/WinAVR.
The motor starts approximately three milliseconds after the mains voltage has been zeroed. To bridge this non-active range, the potentiometer R9 was adjusted and a 68 kΩ resistor connected in series. 
This phenomenon is based on the inertia of the motor and has also been taken into the software. The triac is only active about three milliseconds after the mains voltage has passed through the zero point. This means that the motor is voltage-free in this area.
When an inductive load is triggered by a phase cut, noises can occur. The circuit diagram has three measures to prevent noises, R11/C10, L1/C1 and varistor R13.
The MOC3023 optocoupler is designed internally without a zero voltage detector and is thus suitable for phase cuting. Various types (BTA12, BT139, BT136) can be used for the Triac T2.
The circuit diagram is taken from the application description of the optocoupler MOC3023 (TI) and has been adapted to the requirements. The attached image of the oscilloscope shows the full power consumption of the motor. 
