__ __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.
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