Circuit for converting motor tacho-sensor sine wave output to 5V TTL for MCU
If you can share a GND with the sensor then a good way to convert that signal to a TTL digital signal would be to use a simple common emitter NPN transistor configuration. Feed the signal through a resistor of say 10K ohms into the base of the transistor. Place a diode with its cathode also to the base of the transistor. The transistor emitter and the diode anode both connect to GND. The purpose of the diode is to protect the transistor when the input signal goes negative. The collector of the transistor goes to a resistor of say 1K ohms that comes from the MCU +5V rail. The collector side of the 1K resistor also goes to the MCU input.

You will have to carefully evaluate the sensor connections however. If they are in any way not isolated from the mains AC voltage then it would not be recommended to connect your MCU to the sensor without some isolation. An opto-coupler would be a suitable way to provide isolation. Recommending a circuit connection for the opto-coupler could only come when more is known regarding the sensor connections.