Here are the projects of very sensitive FM field strength meters or we can also say them RF detectors. Both the circuits can be used to measure the field or signal strength of FM transmitters. The circuits are quite sensitive and can detect a very low power transmitter from 8 to 10 meters away and
provide a visual and sound indication by activating an LED and a peizo buzzer.

The field strength meter or RF detector circuit shown in figure 1 is a simple circuit using not more then 8 parts and a battery. The circuit is built around a CA3140 IC, which is an operational amplifier.

There is not much difference between the two circuits but only the 2N3906 PNP transistor used at the output of the figure 2 circuit to further increase the sensitivity. L1 is an air cored coil and can be made easily by winding 6 turns of #24 enameled wire on a 5mm form. Remember to remove the insulation from the two ends of the enameled wire to make solder connection. For antenna use an 85 centimeters telescopic antenna.




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Sensitive FM Field Strength Meter Or RF Detector Circuit
Figure 1.
Figure 2.