Normally we often encountered the frequency meter can be used in speed sensor, tachometer, measurement or signal recurring. This frequency to voltage converter (FVC) can be used to change the voltage into a digital or analog tachometer. Circuit that consists of three blocks. The first block is squarer, the input signal to a square wave. This block is protected from high input voltage up to 400V, but remember this only works if the value of capacitor C1 is for 400V. Input Impedance around 560k, so it is safe to connect the ignition pick-up coil in parallel with the CDI (capacitor discharge ignition) circuits without a problem. Supply is protected from voltage spike by Zener diode D3. Here is a schematic picture:
t = 0.2 x R5 x C4 x ln (VDD-VSS)
R and C are in ohm and Farad, VDD-VSS is in the pin 16 voltage minus the voltage on pin 8, and t in seconds. Minimal value of R9, the monostable multi-vibrator output pulse width will be 0.2 * 4700 * 22e-9 * ln (12) = 5.139e-5, and this will change the frequency of 19.460 kHz to 12 Volt output. This gives a conversion factor 1.622kHz/Volt. If you set the maximum value to R9 (100k) then the pulse width of monostable multi-vibrator output akan 1.145 MS. This setting will give the maximum voltage output of 12V at 874Hz, or about 72.8Hz per Volt conversion. If you use the tachometer to the application, wide rage this adjustment will accommodate almost any type of engine.
A last block of the first order low pass filter about U2, about 0.1 seconds of time set by R6 and C5 constant. With the slow time, you can not read all the frequencies below 10Hz or close, but OK for a variety of applications. Although the diagram does not show the scheme decoupling capacitor for bypassing the supply line for u3 noise, it’s good to add a 100nF cap u3 as close as possible to the power pin (pin 8 and 16), since the monostable multi-vibrator is sensitive to such noise.
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