Fig. 1: RPM meter circuit Diagram
The circuit has three stages—pulse detection, frequency-to-voltage conversion and display of voltage by the LED bar graph. To sense the pulses from the ignition coil, 18SWG PVC-covered household wire is wrapped around LT side of the ignition coil. The number of turns should be around 50. Induction pulses from the sensor coil must be reduced, rectified and filtered. Resistor R1 reduces their voltage level, diode D1 rectifies them and capacitor C1 filters them.
These filtered pulses are fed at pin 1 of frequency-to-voltage converter IC1. Voltage output corresponding to the frequency of pulses is available at pin 4 of IC1. The level of this output voltage can be controlled by preset VR1. LM2917N-8 (IC1) is available in 8-pin and 14-pin versions; we have used 8-pin version here. The output of IC1 is fed at pins 5 of IC2 and IC3 through a filter formed by resistor R8 and capacitor C5. This filter removes any variations in the output and provides stable reading. IC2 and IC3 are connected to each other in cascade mode. Pin 9 of both these ICs are held at high voltage to select bar mode.