The F.E.D.

Detects pesky agents and other prowlers.

Whether you're deep in the woods, cultivating a "crop" or hiding in your basement, planning a tea party, the Fed is coming for you! The F.E.D. (Frequency Emissions Detector) will detect the signals from agents' fancy-pants digital radios, computers, and phones for a distance of several yards. Place a few of these simple Fed detectors around the area and it will be hard for Special Agent Gibbs to sneak up on you. Even he carries a cell phone, albeit one he swiped from one of his subordinates. The output of the detector may be used to operate a counter for the passive-aggressive type who would rather not be present when the SEALS arrive, and would rather look at the count at a later, safer time. Or, the output can operate a wireless doorbell for an instant "heads-up" or, perhaps, "heads-down" as the case may be. The prototype includes two outputs to drive a counter and doorbell. The doorbell output is a universal solid-state switch that could be used for all sorts of other purposes, including the simple porch light deterrent or the more sophisticated sprinkler system surprise.

The device will, of course, detect anyone's cell phone, so the detector really responds to just about anybody with more than two pennies to rub together. Most phones occasionally transmit short bursts to the nearest cell tower, even when the phone isn't in use, so a typical 21st century loiterer will eventually set off the detector. On the other hand, a deer can stare at it for an hour with no effect. Let's face it, even most burglars emit digital signals these days. So, in a sense, the F.E.D. is a human detector. With that in mind, consider using the output to activate a digital camera or video recorder to capture a record of the prowlers. A recorded barking dog or, one of my favorites, a recording of police radio traffic will scare off most crooks. Add some hidden flashing red and blue lights to the police chatter and the culprits will actually run away.

The F.E.D. may also be used to detect yourself. If you carry your phone around with you (of course you do), the occasional signal it emits may be used to enhance the effectiveness of your backyard motion detecting lights. Those tend to turn off at inconvenient times, especially on hot evenings and adding the F.E.D. will help to fill in those gaps. That cell signal can be used to control lights and other appliances in the home in synchrony with your movements, too.

Construction Details


see fed.pdf


The simple amplified detector responds to a characteristic of digital communications, the abrupt amplitude shifts that sound like a 'buzz' or white noise in an ordinary AM receiver. As shown in the schematic above, a simple broadband antenna responds from the UHF band through microwave frequency and a biased Schottky diode AM detector drives a high gain audio amplifier. The output of the amplifier is rectified and applied to a comparator with hysteresis. When the rectified voltage exceeds the voltage on the positive input, the comparator output drops low, briefly lighting the LED in the LH1547 and turning off the VN0300. After about 20 seconds, the comparator resets and the CUB3R counts the transition. The long time-constant limits the 'counts' to one per 20 seconds so that the variable digital signal doesn't produce excessive counts. The counter therefore displays the number of 20-second windows that contained RF activity. If a cell phone is in use within range for 3 minutes, the counter will register about 9 counts. The doorbell signal occurs upon first detection of the signal and the switch closes for about a second.


The circuit of the prototype is built on a piece of tinned circuit board with little islands of board soldered into strategic positions. The op-amp is a surface-mount type and a little prototyping board makes it easier to connect other components to the tiny leads. The only critical part of the circuitry is the diode detector section. Notice how the diode and 100 pF capacitor are connected in series between the ground plane and the antenna. The 1 megohm and 0.1 uF capacitor aren't as critical and the rest of the circuitry is quite forgiving. Any sort of construction technique will work for most of the circuit. The antenna and ground plane boards are simply glued to a thin piece of fiberglass board using spray adhesive.


The various pieces are installed in a roomy plastic case (not metal). The white box is an old Radio Shack wireless doorbell sender. These have two terminals on the back for connecting an additional button. A quick voltmeter check will determine which terminal is most positive and this terminal should be connected to pin 4 of the LH1547. A slide switch was added in series with one of the terminals to allow for easy disabling of that function. Not shown is a 4.7k resistor in series with a 1 uF capacitor between pin 1 of the op-amp amd an earphone jack, with the ground terminal of the jack connected to the ground plane. This jack allows for the connection of an external amplifier or PC soundcard for listening to the action. It isn't a good idea to add an audio amplifier to the box; there's already a lot of gain in there and additional gain will result in instability, especially if everything is running off the same battery.

Comments about the Components:

Other experiments:

Connecting an external speaker amplifier to the output of the first op-amp (pin 1) through a 4.7k resistor in series with a 1 uF capacitor will allow the signals to be heard. If the amplifier has sufficient gain, a low-level hiss should be heard with no signal present. A cell phone anywhere near the thing will give quite a loud buzz when it starts transmitting. Active wireless devices will also produce a racket. It's interesting to listen to a typical Wi-Fi device, the amount of communication required to receive a web page is surprisingly little - just a little 'bzzt' for a typical static page. Many types of cell phones can be heard for quite a distance and the signal comes and goes dramatically as the user moves around.

Other antennas could be connected to count or detect other RF signals. For example a tuned circuit and whip antenna could look for nearby AM modulated signals like aircraft communications (see Passive Aircraft Receiver for a front-end design), CB radio, lightning crackles, and any number of digital transmitters. A 'proper' antenna for a particular band should give excellent performance. If power isn't a problem, increase the diode current by lowering the two diode resistors from 2.7 meg and 1 meg to 270 k and 100 k. The bias current will be about 15 uA. 

The RF detector could be replaced with other sensors like a seismic pickup made from a speaker, a microphone, a photocell or phototransistor, or other transducer that produces occasional weak audio signal bursts that could be counted.

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