The circuit is using four LEDs, which will show the low, medium, high, and full signal strength indications. The operating voltage of the circuit is 12 volt DC but you can also operate the circuit with 9 volts. For operating with 9 volts simply remove D5 and D6 diodes.
Note: It is important to use all LEDs from a same manufacturer, same color and type because there is some difference between brightness and other specifications of each manufacturer's LEDs due to which they will not show equal light on equal milliampere and the circuit will not show accurate results.
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Here is a very useful project / schematic of a sensitive field strength meter circuit.
A field strength meter (FSM) is a very useful circuit to detect and measure the RF signals and their strength. The circuit shown here can be used as a wide band field strength meter and can