This circuit can be used to amplify the signal of a magnetic/inductive or photo barrier type RPM sensor. the supply voltage of 5 to 12V DC should be applied to JP2 pin 1 (+) and 3 (-). Pin 2 is the digital output.
If you use a photo barrier, connect the LED (sender) to pin 1 (anode) and pin 3 (cathode) of JP1 and the photosensitive transistor (receiver) to pin 4 (C, positive) and pin 2 (E, negative). Be careful to select a higher value for R9 if you use a supply >6V otherwise the LED current gets to high.
The coil of an inductive or piezo sensor can be connected between pin 3 and 4 (polarity doesn't matter).

R6 and C3 is a filter to suppress noise from the supply line. Together with R8 it supplies DC bias to the receiver. C1 is used to block that DC signal off the input of IC1, a voltage comperator. R1 and R2 bias the input of the comperator to 50% of the supply voltage independent of the bias at the sensor which may vary (sunlight, temperature drift). C4 filters the bias from supply noise, again. R3 and R4 set a small hysteresis to prevent the comperator from oscillating. R5 forms a high-pass together with C1.

I routed a printed circuit layout in SMD style which can be easily etched at home because it's single layer:

The layout can be downloaded in postscript format. You can print this layout directly to transparent paper or overhead film for transfer to photosensitive PCB material. If you don't have a postscript printer, use the import function of Corel Draw or some similar program.