Vibrating Pocket Bug Detector

Wide Spectrum RF Detection
Sensitivity: 20mW @ 6 FEET
6 V to 12 V alkaline operation

SCHEMATIC: RF-V DETECTOR

PICTORIAL DIAGRAM:

There's no tune-up involved, so everything is essentially plug 'n play. I've attempted to be explicit in the fabrication so that the novice builders will be successful. If you run into any snags, drop me a line and I will get you through it. Those of you who have built or used the comparable high priced vibrating detector will truly appreciate the value of this plan. My sincere wish is for you to enjoy constructing this project, and I want you to realize that the elegance here is in the simplicity of the design. I also offer other easy to construct high quality designs.

THEORY OF OPERATION

Two components stand out that allow the RF-Vibe circuit to be concise and effective. They are the universal operational amplifier, LM741, labeled as IC1 and the small inexpensive dc motor which we shall fabricate into thevibrator. The 741 op amp is known as the 'workhorse of industry' because it can be configured into many different functions by simply rearranging a few external components and the manner in which they are connected to the device pins. For our purposes, the op amp is setup in a comparator-schmitt trigger configuration which forms the r-f detector part of the circuit. The 741 incorporates balance(null) circuitry that ensures equal conduction of the inherent differential input amplifiers. It allows the use of a balance potentiometer to bias the chip so that you get 'zero out' for a 'zero in' condition. In our case, we utilize the balance pot, R6, as a "sensitivity adjustment" to lower the threshold to the trip point, so that we are able to detect a relatively small signal induced into the antenna wire. In fact, the RF-V has responded to as little as 20mW at six feet away from the antenna lead. By adjusting the sensitivity, you can use the RF-V as a sniffer to pinpoint hidden transmitters.

Adding some resonant components would make the detector frequency selective. We could install preamp circuitry which would permit us to hear the r-f generated by an angry gnat rubbing its hind legs together. People have suggested many desirable add-ons, but increasing complexity only adds to the cost and size; both of which negate the intent of this design. I could provide you with a design that is 1/2 the size of the RF-V case, limits detection to the VHF & UHF bands, and needs no external antenna lead. If you're interested, send me a cashiers check in the amount of $1200 and be willing to sign a proprietary agreement. You see folks, in a phrase, "it's all economics." Believe me when I tell you that there are not that many engineers in this country who are willing to create economical designs for public consumption. They are able to glean the maximum yield from simplistic circuits and comprise a design which rivals the performance of many high priced commercial units. This is done for the shear satisfaction of building the better mousetrap. For me, there are not enough buyers to support even a part-time endeavor in this work. My reward is in the knowing that I have provided a quality plan to someone whom can replicate the device and who realizes its worth. Maybe it will spark an interest, and the builder will continue the tradition. Well, enough of this noise. Thank you for your forbearance. Now, please step back while I jump off of this 'soap box'.

Detailed Description: IC1 is fundamentally used as a comparator. With the addition of R2, positive feedback is applied to non-inverting input pin 3. R1 drops the feedback level and then applies it to the inverting input pin 2. This sets up a span range between inputs and converts the circuit function into a schmitt trigger. R6 null potentiometer is initially adjusted to unbalance the input bias currents such that IC1 trips on, placing output pin 6 low. This in turn will cause Q1 to conduct, thereby energizing LED1 and the vibe motor. Now R6 is 'backed-off' until IC1 just resets (with the LED and motor off). At this setting of R6, the input amplifiers are unequally conducting with the non-inverting input (pin 3) being slightly more positive than the inverting input (pin 2). This is the reset state, with output pin 6 being high, thus keeping Q1 turned off. As you gingerly move R6 closer and closer to the trip-point level, the imbalance between the input bias currents is lessened. Essentially this increases the 'sensitivity', since now less input signal is required to cause pin 2 to become slightly more positive than pin 3, which then sends output pin 6 low, again turning on Q1. Realistically R6 acts as an offset pot which narrows the margin to trip-on IC1.

In summary; when an input r-f signal is induced into the antenna lead wire, the level is limited by D1/D2 to keep from saturating the inputs. C1 has low impedance to the r-f signal and couples it to the op amp inputs. It is applied directly to pin 2, but delayed to pin 3 by the time constant effect of R1/C2. On the positive alternations, the r-f level aids the voltage at pin 2 such that IC1 trips-on. As long as the r-f is present with sufficient amplitude, the op amp remains in the set state. When the r-f level drops or disappears, then the established balance currents return the op amp to the reset condition.

R3, R4, and R5 are current limiters. D3 ensures that the vibe motor doesn't conduct any armature current until Q1 turns on enough to forward bias D3. This precludes the battery from being drained by the op amp responding to low-level noise or R6 settings that may cause the op amp output to allow Q1 to weakly turn on. When this happens, the low impedance path through the dc motor would prematurely kill the battery. We wouldn't realize this occurring because the current flow wouldn't be enough to illuminate the LED or cause motor rotation. So you see, some things aren't intuitively obvious to the casual observer.

Of course you can shorten the antenna lead. It doesn't affect circuit design parameters in any manner. It's just that you can "trap more bugs with a larger net". The longer the antenna wire, the more apt you are to detect low power transmitters. 'Mr. Businessman' usually puts the RF-vibe in his vest pocket with the antenna lead running down inside the pants leg. Sometimes paranoia sets in when a faulty lighting ballast or a r-f belching computer system triggers the detector. Most often, you can readjust the sensitivity in successive increments and walk right to the source. It's quite a feeling of empowerment when you discover that the signal is emanating from a live 'body'. One firm reported that the little RF-vibe saved them from making a costly mistake during high power negotiations.

In some RF-V units, the dc vibe motor commutator produces enough r-f chatter to keep the op amp in the set state once it's triggered. If this occurs, R6 is then adjusted to re-set everything. The inherent hysteresis and pseudo-sensitivity are affected by varying the values of R1, R2, and C2. You can experiment and 'prune' to meet your specific needs, but the circuit depicted in this plan will give you an overall reliable operation.

COMPONENT LAYOUT: click to enlarge

ASSEMBLY

The pictorial diagram shows a typical layout for the parts listed in this plan. No interconnecting leads are shown as to keep an unobscured view. Some fabrication adjustment of the parts will be necessary in order to fit them into the case, but it doesn't require any difficult craftsmanship. When we finish, the parts will fit nicely in the enclosure with plenty of room for the interconnnecting wires. You can use the SMD pc board along with the other components to be installed in the R.S. 'pager style' case with 9v battery. But that's like packing your clothes in a steamer trunk when only a small suitcase is needed. I have even soldered the through-hole components together without a pc board and was able to install them in the smaller enclosure.

The RF-V pc board (SMD) is actually about 40% larger than it needs to be, but it gives the novice builder ampleroom to easily install the 9 components and solder in the interconnecting leads. The component layout sheet will help ensure accurate placement of all the SMDs and indicate where to attach the wire leads. Notice that R5 and D3 are not placed on the pc board. These are standard size components and are to be wired in series with one of the motor leads. They are placed beside the motor in the case bottom, or they can be glued to the side of the motor. The vibe motors that I tested draw anywhere from 80ma to 120ma at an applied 1.5 volts. So we need at least a 1/4 watt rating for R5. Rather than parallel three SMD resistors on the pc board, we use a standard 1/4 watt thru-hole component. It also permits us easy access to change the value of R5 to satisfy different motor choices or higher battery voltages. For example, if you opt to build the circuit in a Radio Shack 'pager style' case and use a 9v battery, then make R5 a 47 ohm resistor. D3 is a standard 1/2 watt 1N4148 diode. D1 & D2 can also be standard 4148's, but in the SMD model a series-connected dual diode in a SOT-23 package is used.

R6 POTENTIOMETER The Digi-Key pot comes the with SPST switch built onto it. The pot body will need to be modified slightly to clear the case. First cut the switch contact terminals to 1/8" long. Cut the pot terminals 1/16" above the edge of the fiber board. Now snip and file the fiber board until its width falls within the diameter of the pot body or is almost flush with the contact mounting crimps. Cut off and file down the metal locating lug until both sides of that surface is flush with the pot body. When you place the potentiometer in the case, it will be on its side with the contact terminals extending over the pc board. Using the pictorial drawing as a guide, temporarily locate the pot inside the case bottom and mark the edge of the case on each side of the shaft. Screw the case together and drill a 1/8" hole in the center of the seam (where the top and bottom come together) between the marks you made to center the pot shaft. When you're ready for final assembly, you can put a dab of adhesive under the pot side that touches the case bottom and screw the case together until the glue sets up. When you do this, pull the pot shaft tight to snug the pot against the end of the case. Later, you can add more glue to the case bottom around the pot to make sure it stays solid. [ I like to use FIX-ALL cohesive sealant made by MULTI-MIST(TM) because it sets up faster than most silicones, is a little more rigid, and yet remains flexible, and can still be picked away if you need to make changes.] All of the interconnecting leads are made up from #30 gauge solid wire (wirewrap, kynar,etc.). You can use 30 AWG stranded wire, but it usually has a tough teflon insulation that makes it difficult to form and stay put in tight places.

Three wires are 'drill' twisted together to make a short connecting cable for the R6 potentiometer. If you look at the pot from the front view with the contact terminals pointing downward, the terminals are numbered 1, 2, 3 from left to right. Connect each terminal to its corresponding pad on the pc board. When properly oriented, a clockwise rotation of the pot wiper shaft will produce an increase in 'sensitivity' (turn on the vibe motor and LED). If not, then swap the two outer leads to correct the polarity. Twist two lengths of wire together to make a switch cable. Connect one end of the cable to the switch contact terminals. At the other end of the cable, one of the leads is connected to the positive(+) battery terminal. The remaining lead is soldered to the (+)power pad on the pc board. The negative(-) battery terminal is wired to the (-)power pad on the pc board.

BATTERY HOLDER: The '1/2AA' holder listed on the parts sheet is an excellent holder for the PX28AB 6.0 V alkaline or PX28L lithium Duracell batteries. Albeit a glove-fit for the battery, some honing is necessary in order to fit the holder into the enclosure. We will have to reduce the height of the batt holder by at least 5/32" and round the top shoulders a bit for a custom fit. Start by filing down the top edges 1/16" or until the top of the contacts are fully exposed. (In order to have more control, I secure a wide flat multicut file on the bench top, and then I move the work on top of the file. When almost finished, I switch to a smooth file to polish it up.) Next we'll have to remove 3/32" more from the flat of the bottom of the holder. But, before beginning to file on the bottom, we must move the contact terminal lead out of the way. Do this by cutting a notch on each side of the wire lead, extending down to the contact surface. After removing the plastic notched material, slowly and carefully bend the lead back away from the bottom until you have enough clearance to file on the holder bottom without nicking the contact lead. You don't want to crimp it or completely bend it over; at least not yet. Now you can move the holder back and forth on top of the file until you just about reach the contact surfaces. (You might be able to move the contacts up out of the holder just a bit, so that you gain some leaveway while you file.) When you near the contact surfaces, file away the metal on the outsides of the terminal lead wire, thereby lengthening the lead so that more of it can be bent back away. Now you know why I said to carefully and slowly bend the lead out of the way. You will know that enough of the bottom has been filed away when the molded holes in the bottom of the battery holder begin to enlarge or match the size of the upper wall of the hole. When finished filing down the bottom, you can then form the contact leads up against the holder ends. The last filing to do is to take down some of the width until the holder fits into the case between the side wall and center screw post. You can use an exacto or razor knife to trim away some of the flange on the center post. Don't get carried away here, because you need the strength of the center post to remain intact. It's the only thing that holds the case together.

If all this custom tailoring is more than you want to deal with, then you can opt for the 'N' size batt holder. It's a bit of a tight fit for the 1/2AA battery diameter, but if you center the battery in the holder, it works fine. However, you'll have to insert a small screw in the positive terminal contact of the holder in order to extend its length so as to make contact with the battery when it's centered in the holder. Other than this little modification, you won't have to do anything else. You can even use the 'N' size holder as is with a 'N' size 12 volt alkaline battery. The problem with using this battery is that it has no wealth of capacity. It's a 12v, 34ma-hr battery. Five minutes of vibe motor runtime will suck the life out of the battery. Although it would still have enough voltage to power the detector circuitry, there isn't enough reliable instantaneous current available to consistently startup the dc motor. And without that, you definitely won't have good vibes. But to be more optimistic, a fresh batt would probably give you days of reliable standby operation. And after several minutes of 'proof positive detection', you can simply insert another fresh battery to ensure a continued vigilance.

VIBE MOTOR Here we have two viable choices, either of which satisfy our operating parameters, but vary greatly in cost. The first choice is economical and is the one indicated on the parts list. It is a small rectangular dc motor that operates on 1.5 to 4.5 vdc and costs about 33 cents. All we have to do to turn it into a vibrator is add an eccentric weight to the armature shaft. It is the motor depicted in the pictorial drawing. The second choice is to use a vibrator motor out of an available vibrating-type pager. Almost all of the pager services use a Motorola Bravo Pager. They just stick their own logo on the front of the case, but invariably when you flip it over you'll read 'Motorola' on the back. Inside is a cylindrical vibrating dc motor that is clamped to the circuit board. It is about 7/8" long and 0.3" in diameter. It's manufactured in Japan by a company called NAMIKI. Motorola contracts them in lots of ten thousand, and then sells them to their authorized service centers. The Motorola Radio service outlets then sell them to 'Joe Public' for $38 a pop. The 'profit misers' won't sell you used ones and they won't come down on the price. Only a few of the local pager service outfits will bother to sell you one at that price. Even they have to pay an outrageous cost for a seemingly $2 motor. If you locate a source for $5 or less, please let me know so I can pass it on.

Here is one example that might be an option for you. Although the little vibrators are efficient and pretty darn reliable, sometimes as they wear they become sloppy and begin to vibrate extremely well; too well. Since, in the pagers, they are clamped to the circuit board, the excessive vibrations cause the pc board to act as a diaphragm. Then the pager will buzz with more sound instead of silently shaking. The pager customers who rent the devices will turn them in for servicing. The pager technicians simply change out the vibe motor. The service department always seems to have an ample supply of scuffed cases and used vibrators in the shop. Do what I did and just ask the service manager if you can purchase some of the replaced motors for trial use in some experimental electronic projects. Chances are that he will do you a one-time favor by giving you a few vibe motors and maybe even kick in some cases. If you don't live near a town that has a firm which services pagers, or if the service manager is a jerk, then you revert back to fabricating your own vibrator. The little dc motor from All Electronics may not be as efficient as the Namiki motor, but it is sized just right for our purposes, and with a good battery it performs equally well. And the price is right!

Now, let's make it dance. The pictorial that I've drawn is a very close representation of what your dc motor will look like with the off-set weight mounted on the shaft. You'll need a chunk of 1/4" round metal stock. I used brass, but of course steel, iron, lead, and even aluminum will work. Probably dense wood or plastic will be okay [OK ..... derivative of 'Ol Korrect]. Use a hack saw and cut the round stock to a length of 3/16". Don't worry if the cut is not straight, just try to drill the hole parallel to the length. Measure 3/32" in from the edge towards the center to mark the spot for drilling a 1/16" hole through the length of the eccentric weight. The shaft is slightly less than 1/16" diameter so we will have to shim the hole for a snug fit. Untwist the strands to a 1" piece of 18 to 22 gauge stranded wire. Take one of the strands and thread it through the hole in the off-set weight. Now when the weight is pressed onto the shaft, the wire will deform enough to make for a very tight fit. For insurance, put a drop of super glue in the hole (a tiny drop) before you press the weight onto the shaft. Move the weight onto the shaft until the end just noses out of the hole. Whatever you do, don't let any of the superglue run down the shaft into the motor casing! If it even gets near the end bearing, then you might as well toss that motor and start over. Take a 6 volt dc source in series with a 47 ohm resistor and apply to the vibe motor. How does it perform? Lower the voltage to 5 volts and note the effects. If possible, build up several of these vibe motors so that you can choose one with the best starting response at low voltage.

When installing the vibe motor into the case, as like the pagers, we don't want the vibrations to be translated into too much sound. By mounting the motor onto a piece of foam rubber and then securing that to the case, we dampen the transmitted sound while still receiving plenty of vibration. Cut a piece of spongy foam rubber in the shape like a picture frame, which matches the outline of the vibe motor casing. Make certain that it doesn't touch any of the moving part of the motor. Use as little adhesive as possible to secure it to the motor and to the bottom of the RF-V enclosure.

LED The Led is installed in the bottom portion of the case just above the height of the pc board. A 1/16" hole is drilled through the case wall and the LED lens is centered behind it. The connecting leads are soldered onto the LED terminal leads right next to the body of the LED, and then the surplus is cutoff. Dab on some adhesive around the LED body and you're done.

ANTENNA If you can, use 18 to 22 gauge rubber insulated test lead. It's very flexible and easy to deal with as an antenna lead wire. Solder it onto the pc board pad and route it straight back between the battery holder and vibe motor. Drill and pass the wire through a 1/16" hole in the lower center of the end of the bottom half of the case.

CONSTRUCTION HINTS:

This information had originally appeared in the SM-2 planspec. Because so many newcomers said that it had really helped them to succeed in their attempts to acquire new skills, I opted to present it again in the RF-V series. It's just some general tips that may help some to make the transition from through-hole assembly into the smaller, more densely populated constructions using SMT components. Once you get the hang of it, it's very satisfying to be able to re-do the layouts into smaller and smaller configurations.

If you're an old hand at SMD work, the following suggestions will ring very true. If any of this is new to you, don't fret! SMD work does require attention to detail, but it is not all that arduous, and the rewards are big. So accept this as a learning tool.

There are probably many other tidbits that the old-timers would consider to be crucial advice, but eventually you have to test the water on your own. The best thought that I can leave you with is this: When you start to become tired, stop working! Don't go on to make an irrevocable mistake. Remember, "Tomorrow is another day!"