This simple circuit makes use of neon lamps to indicate the voltage of a power grid. The lamps are common neon and the circuit indicates whether the voltage is 110 V or 220 V.
We found this circuit in a 1977 factory documentation. The operational amplifiers can be 741 and...
This circuit with regenerative action was found in the June 1986 issue of Radio Electronics...
Equivalent FETs can be used according to the frequency of the crystal which can be up to a few...