Optical sensor

The one i like as it installed on my Nissan Skyline. As we didn't have one at Unitec i have to bring my own to test.

Inside this sensor is a infra red diode and photo electric cell with interrupter plate with a lot of open windows for the sensor to know where top dead center and number 2,3 and 4 cylinders.
Here is oscilloscope reading which shows that a negative trigger switch. On a picture we can see supply voltage of 5 volts main line and when interrupter ring rotates and blocks the LED we can see voltage goes to ground to 0 volts. Its isn't a perfect pattern, but its visible that sensor works and give reading. And it basically work as a Kettering ignition system by braking the power supply to coil to induce high voltage for spark plug to fire. This is the most correct speed sensor compare to hall-effect and magnetic pick up ones, as its got a scale of 360 degrees. This is the most advanced and also most expensive to build so progress decide to take advantage on cheep magnetic pick up sensors which can be found in late model cars ABS, TRS and ignition systems this days. No one wanna pay extra for quality and reliability.