The simple analog solution shown in figure 1 allows monitoring the frequency of the power grid using a low cost analog instrument. The circuit can be calibrated to have in the central band frequencies of 50 or 60 Hz and the power is made directly from the power grid.
The zener diode is 400 mW and the capacitor C1 must be polyester with at least 100 V working voltage. Scale adjustment is done in P1 consisting of a wire potentiometer with at least 3 W dissipation.
In figure 2 we have a suggestion of printed circuit board to implement this frequency.
MATERIAL LIST
D1 - 6.8 V x 400 mW - zener diode
D2, D3 - 1N4148 - general purpose diodes
R1 - 100 k ohm x 2 W - resistor
C1 - 220 nF - polyester capacitor
P1 - 5 k ohm wire potentiometer x 3 W
M1 - 100μA full scale microammeter
Several:
Printed circuit board, mounting box, wires, solder, etc.
