Circuit Detects and Locates Hidden RF Bugs
Figure 1. This circuit sounds a buzzer alarm when it detects an RF signal in the range 100MHz to 3000MHz, above approximately -35dBm.
At low input-power levels such as -35dBm at 3000MHz, the MAX2015 generates a DC output of approximately 0.5V. This voltage increases in proportion to the input RF power level, up to approximately 2.0V. The reference voltage at the inverting terminal of the MAX9075 is preset to 0.6V by resistors R1 and R2. (To avoid false alarms, you can fine-tune the reference voltage by adjusting resistor R2.) On detecting an RF signal, the MAX2015 triggers comparator MAX9075 to generate an output voltage of 3.0V, which in turn activates a loud buzzer.
A modification to this circuit not only detects RF bugs; it also locates their hidden positions (Figure 2). Simply replace the MAX9075 comparator with the MAX4480 amplifier, configured for a gain of 1.5. The MAX4480 then amplifies the MAX2015 output voltage (0.5V to 2.0V) to the range (0.75V to 3.0V) for driving the buzzer. You can adjust resistor R4 to obtain the desired gain and output-tone level. The audio level generated by this buzzer is proportional to the input voltage, so as the detector approaches a hidden bug, the MAX4480's output voltage increases, intensifying the alarm and making it easy to pinpoint the bug's location.
Figure 2. Substituting an op amp for the comparator in Figure 1 enables the audible alarm in this circuit to increase in volume as you approach the RF source.