Construction and testing
An actual-size, single-side PCB for the RPM meter is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on PCB, enclose it in a suitable box. Use two-pin connectors on the PCB for connecting the sensor coil and the battery terminals.
To check the circuit for proper functioning, verify input supply at TP1 with respect to TP0. This supply is reduced to 9 V, which can be checked at TP2. Check the sensed pulses from the ignition coil at TP3 using an oscilloscope. The voltage at TP4 will vary as per frequency of pulses.
To calibrate the system, as we did, use a function generator. Give 20Hz signal at TP3 and, using preset VR1, set its amplitude such that only LED1 glows. When you increase the frequency, you will see more LEDs glowing. Your system is now ready to use. Use preset VR2 to define the frequency range for each LED bar display.