Digital tachometer

摘要

A digital tachometer for an internal combustion engine includes a circuit for receiving a pick-up pulse having a frequency related to the rotation speed of the engine and outputting a wave shaped pick-up pulse. This circuit filters the pick-up pulses and regulates the filtered voltage to provide a d.c. output for powering the tachometer. A crystal controlled oscillator and divider provides update pulses at intervals of 1 to 10 seconds and provides sampling pulses at frequencies of 1 to 5 kilohertz. A display counter and driver counts speed pulses indicative of speed between update pulses and latches the count until the next update pulse. A numerical display displays the count currently latched in the display counter. A multiplier/divider recieves the pick-up pulses and divides them by the number of pulses per revolution to provide pulses indicative of the engine rotational speed. It also multiplies the pulses in a sampling phase locked loop by a factor corresponding to the period between update pulses to provide speed pulses to the display counter. The sampling phase locked loop is strobed by the sampling pulses produced by the crystal controlled oscillator. The multiplier/divider provides a d.c. output reference voltage indicative of speed in addition to the speed pulses.