12 V Tach Signal to 5 V Converter

This circuit is a level shifter which converts a 12 V tach signal from a car to 5 V TTL level. Since the input signal is at 12 V peak-to-ground, and the phase lock loop (PLL) chip expects 5 V TTL levels, I decided to use a metal-oxide-semiconductor field-effect transistor (MOSFET) for the conversion. I decided to use a MOSFET because it has high input impedance and gate capacitance that will ensure it does not interfere with the tachometer circuitry. The gate threshold of the transistor is 3 V and can accept an input voltage of up to 20 V DC. In the voltage repeater configuration, the output can never exceed the 5 V rail voltage. This is one of my elegant ways of sensing square wave signals without interfering with any external circuitry.

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