VRS sensors (sometimes called crank sensors) are basically a coil of wire around a ferrite core. They are located close to a toothed wheel which is usually mounted on the crankshaft or the teeth are part of the flywheel itself. Here’s an example of one:
As the teeth pass the sensor, they create a change in magnetic flux, and this change induces a current in the coil. The current is proportional to the rate of change of magnetic flux. This means that as the engine speeds up, not only do the pulses from the sensor get closer together, but they also get larger in amplitude. At high speeds this can mean that the voltage across the coil can become quite large. In the test set up I used, at 6000 RPM, the peak to peak voltage from the sensor was around 26 Volts. Hence, the signal from the sensor can not be directly fed into most microcontrollers which may only be able to handle up to 3.3 or 5 Volts. Therefore, a conditioning circuit is required to interface between the sensor and the microcontroller. One possible circuit to do this uses an op amp as a comparator and some zener diodes to clamp the VRS sensor output and protect the op amp inputs. A potential divider is used to set the threshold to 2.5 V.