THE SMITH-KETTLEWELL TECHNICAL FILE

A Quarterly Publication in Braille Talking Book and IBM Diskette Editions from The Rehabilitation Engineering Center The Smith-Kettlewell Eye Research Institute

Bill Gerrey, Editor

Supported, in part, by The Smith-Kettlewell Eye Research Institute and National Institute on Disability and Rehabilitation Research

Braille Edition Produced by Clovernook Printing House Cincinnati, Ohio

Talking Book and IBM Diskette Editions Produced by The Smith-Kettlewell Eye Research Institute San Francisco, California

1998 SUBSCRIPTIONS

Braille Edition . . . . . . . . $18 per year

Talking Book Edition . . . . . $14 per year

IBM 5-1/4" Diskette Edition . . $16 per year

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The Smith-Kettlewell Eye Research Institute
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Please address all correspondence to: Bill Gerrey, Editor at the above address or call: (415) 561-1677

VOLUME 15, NO. 1 SPRING 1998

TABLE OF CONTENTS

"PROBOND" POLYURETHANE GLUE

PAGER VIBRATORS AS TACTILE OUTPUT DEVICES

THE "VACUVIBE" -- AN ATTACHMENT FOR "TANK" VACUUM CLEANERS -- A VIBRATING NEGATIVE AIR-FLOW ALARM

THE SMITH-KETTLEWELL VIBRATORY VOLUME LEVEL METER FOR OPEN-MIKE RECORDINGS

"PROBOND" POLYURETHANE GLUE

Always in search of cements, I came across this in my local, rather small, hardware store. It is made by Elmer's Products Corp.; an 8-ounce squeeze bottle cost $9. The proprietor said "It's all the rage." Well, here's what I think about it.

My application was bonding plastics -- cementing items to a plastic vacuum-cleaner tube (see the "Vibrovac" of this issue). Using a half- round file and other crude methods, I made a sort of "plate" which could interface between the flat bottom of a project box, and t 1-1/4-inch- diameter (rigid) part of the vacuum-cleaner hose.

Naturally, even though the surfaces to be joined were large in area, their "fit" was not perfect; there were gaps in which a cement that could act as a filler would be the glue of choice. The product described here has the interesting property that it bubbles up and expands while curing. It can fit cracks as well as bond things together. (Perhaps some indication of intended usage is that it comes in a half-pint squeeze bottle with a big spout as applicator.)

As it comes out from the squeeze bottle, it is a very runny sort of stuff, fairly thin and not at all viscous. Much ado is made of avoiding skin contact with it. I am a great advocate for applying glue with fingers, but you needn't bother feeling this stuff, as its lubricated texture doesn't even give you a good indication where it is puddling. My only indication while applying it is to note how much I squeeze the bottle.

With it still fresh on my fingers, it felt like a synthetic lubricant; it has no substance, but just feels slippery. They say to wash with soap and warm water; as I did that, my fingers felt a little sticky, and that's about it.

Back at the project, however, rubbery/bubbly beads were oozing out of the joint which were solid enough to examine by touch in a few hours. By quitting time at work I felt confident enough to trim these beads off the joints with a razor blade. Being still "rubbery" (flexible) at that time I thought, "Oh, it's like hot glue in its texture and behavior." Nope, by the next morning, it was a hard plastic.

I like it. Counting on its expansive property, I have used it to glue one of those little cylindrical motors (see "Pager Vibrators" in this issue) into a piece of plastic "angle" stock, and used it to secure loose grill work to a wooden case. My impression is that large surfaces and crevices are its specialties.

What follows are quotes from its label. Note the mention of "dampness" being required for curing. Also, note the mention of clamping the work pieces for a few hours; while it is slippery and slimy, there is ample lubrication to promote drifting out of position.

"'PROBOND': Polyurethane glue -- The ultimate universal adhesive."

"Water proof; bonds virtually everything."

"Not for continuous submersion or below water-line use."

"Contains: Diisocyanates "

"Do not use at temperatures below 40dg F. or above 90dg F."

"Bonds: metal, leather, wood, ceramic, brick, stone, glass, and most plastics, to themselves and to each other."

"Sands easily. No volatile solvents. Water proof. Does not dull saws or tools. Stainable/paintable. 100% solids."

"Surface Prep: Surfaces must fit snugly and be free of dust, oil, wax, paint, old glue, etc."

"Important: Curing requires moisture. For porous and semiporous surfaces (wood, pottery, etc.), wipe both surfaces lightly with a damp cloth about one minute before gluing. For nonporous surfaces (metals, rigid plastics, glass, etc.), dampen one side and apply glue to the other before bonding."

Application: "Small Areas -- Apply and "swirl" pieces to spread a thin, even coat over the entire surface."

"Larger Areas -- Apply a thin, even coat with a brush or spatula. Use rubber gloves to avoid skin staining/irritation. Seal cap tightly after each use." "Clamping: Within 15 minutes, clamp glued pieces together for 1 to 4 hours. Curing time will vary depending on temperature, humidity, and porosity. Glue will expand as it cures to fill gaps and further penetrate porous surfaces."

"Curing: Allow 24 hours drying time before using or coating."

"Cleanup: Wet glue -- Use acetone or mineral spirits. Cured Glue -- Sand or scrape: Wash hands thoroughly with soap and warm water immediately after handling."

"Staining: Sand cured glue before staining."

"Warning: Contains Diisocyanates. Exposure may result in acute or delayed damage to the lungs. Eye irritant. May irritate or cause allergic reaction with skin. Inhalation of aerosol may result in respiratory allergies, including asthma. Symptoms may include coughing, difficulty in breathing, and a feeling of tightness in the chest. Effects may be delayed. Persons with asthma or other chronic respiratory condition should not use this product."

"PRECAUTIONS: Work in well-ventilated area. Do not spray or heat product. Avoid swallowing, inhalation, or contact with skin. Keep away from eyes and skin. Wear impermeable gloves. Contact may stain skin. Wash hands immediately after use."

"Keep out of reach of children."

"First Aid: If skin rash or breathing difficulties occur, see physician immediately. In case of eye contact, rinse immediately with water, then flush with running water for 15 minutes. If skin contact occurs, wash thoroughly with soap and warm water. If ingested, drink large quantities of water and see physician."

"For emergency medical information, call 888-435-6377."

PAGER VIBRATORS AS TACTILE OUTPUT DEVICES

Abstract:

Vibratory output devices have long been of interest to the blind and the deaf-blind. With the advent of so-called "silent pagers" (paging units which vibrate in the users pocket instead of beeping out loud), vibrators of various sizes and shapes have become available as components. Typically, these are DC motors which swing an eccentric weight (mass whose center of gravity is not concentric with the motor shaft). Three models tested by the author have all been designed to run off 1.5 volts, and the two described here draw under 100mA.

Smith-Kettlewell has long-since been interested in "vibrotactile" devices. We were the organization who, from 1966 to 1978, designed equipment to present TV camera information onto the back and abdomen via panels of vibrators, and we even used bipolar electric current pulses to simulate "vibration".

While motors described in this article cannot stimulate pin-point areas of the skin for high-resolution displays, they are good "go/no-go" indicators for alert/alarm devices (such as the "Vibrovac" and the "Vibratory Volume Level Meter" in this issue). Other Smith-Kettlewell applications have included prototype medication reminders and a vibratory adjunct to the "Talking Signs" (R).

The only reason you can't make a vibrator yourself -- using a toy motor and a weight fitted to its shaft -- is that the "resolution" of the commutator leads to dead spots in the rotation where the motor is too weak to start reliably. They make these vibrator motors with such high resolution that you cannot feel pole pieces of the armature as they are attracted by a permanent magnet when you slowly rotate the shaft. These motors never fail to start.

Obviously, vibration is only produced when shaft rotation is fast enough to swing the mass with significant momentum; these are not instantaneous indicators. Moreover, there is a delay after they are de-energized while their heavy weights coast to a stop. The torque of a DC motor is maximum at a standstill; thus, startup is fairly noticeable. However, coasting to a stop takes a few revolutions. Thus, they are not the ideal devices to present fast pulsation information.

The best news is their ready availability and cost -- under $12 for both models presented here.

The first model tried here was comparatively large, 3cm long and 1cm in diameter. We paid $17 each for two that a small manufacturer was willing to part with for our research. They bore no brand name, and I have found no ready source for those particular ones (although I suspect equivalents could be had from our Motorola source).

Subsequently, two less-expensive, smaller units were identified which can readily be ordered as "replacement parts" from Motorola's "Communications, Parts and Accessories division" in Schaumburg, Illinois. (The Motorola service technicians couldn't provide a catalog; they just looked through a parts listing in repair literature to find these by hit and miss. The technicians chanced upon one vibrator component in the $35 class, so I expect many configurations could be had with more research.)

I am deliriously happy with the two described here -- models of very different shape. One strongly resembles the aforementioned nameless one; in fact, I suspect that several cylindrical vibrators might be made by the same manufacturer. The other is a "pancake-motor" -- round and flat like four stacked dimes. I neither see, nor feel, any advantage in the larger one for which we paid more money; they all vibrate strongly (they shimmy so as to put Sister Kate to shame, you might say).

The "Bravo Pager" Vibrator, Motorola Part No. 59-5046H03:

On this cylindrical version, the motor diameter is 7mm (0.275inches); the radius of the weight is slightly less than that of the motor's body so that it can be strapped down without fear of the weight's motion being obstructed. The body of the motor is about 6.6mm (0.653 inches); the over-all length, including the weight, is just over 22cm (0.875 inches).

One-inch-long flexible leads (perhaps 28-gauge) emerge from the rear of the motor -- one red and one blue. The motor seems to run fine in either direction, though, so polarity shouldn't matter.

Running from a fresh 1.5-volt cell, I measured the current drain to be 95mA. This could vary somewhat, depending on the rigidity of the mounting arrangement. If allowed to, the motor will toss about, running slower as it "throws its mass around," so to speak. Clamped firmly, its rotation speed is about 150 revolutions per second (150Hz vibration). Its ultimate vibration rate will be lower in projects of hand-held size and mass.

It is unlikely that "gluing" the round body of the motor to a flat surface with hot glue, or some filler-type material, will last. Rigid mounting of these units can be done in two ways:

I have successfully held them in screw-down cable clamps -- the kind that wrap around and that can be secured with a number 8 bolt and nut. Given the motor's odd size, you may wish to wrap it in tape until it properly fits a 3/8-inch clamp. This method does not, however, take advantage of the component's small size.

Another way I tried was to glue the body of the motor into a piece of angle stock, thus providing a flat surface on the assembly for cementing to the circuit board. The angle stock,, available from hobby and architectural supply stores, can be gotten in the selection of glue-together plastics called "Plastruct". The right-angle material I bought is one-fourth-inch on a side; the pieces are a couple of feet long, but inexpensive.

The glue I used, "Probond", is described in this issue; it has the advantage of expanding as it cures so as to make a "cradle" for the motor as it lies in the corner of the right-angle. I clamped it in place with a rubber band. To keep the glue from creeping over to the weight and/or the bearing, I cut a slot in a small square of pasteboard and forced this around the shaft between the weight and the motor. (Wisely, I periodically stopped by to turn the cardboard, and the weight, while the glue was still in its aggressively expanding stage.)

The result of this second mounting method (a bit risky, I'll admit) is that the assembly is only an inch long and about 3/8-inch wide -- little more than an electrolytic capacitor.

Optec "pancake-Style" Vibrator, Motorola Part No. 59-02890W11:

These are the diameter of a dime (amazing looking!) and, including a rubber pad on the back, stand about four dimes high. They bear the manufacturer's name, Optec, and in addition to the "2890W11" portion of the Motorola number, the designation L723 is printed on them as well.

These are obviously of different construction. "Pancake motors", as I understand them, use printed-circuit technology to make the armature on a disc. What's inside these shall remain a mystery until I have sufficient surplus to tear one apart. From the outside, they appear as a round PC board (with two solder points for connection), this board fitted into a shallow steel cup, formed by stamping. This assembly is only 3/16 of an inch thick, and just under 3/4 inch in diameter. (Being stamped and crimped, none of the measurements can be precise.)

The flat bottom of the steel cup has a thin (0.050 inch) disc of foam rubber cemented to it, this pad being about 1/2 inch in diameter. Lastly, an L-shaped tab, part of the stamping of the cup, is folded down against the side and provides a projection almost even with the bottom of the cup.

I have no notion as to how the rubber backing and the "tab" are used in mounting; perhaps the tab is to restrain the motor from rotating in a socket.

Polarity is marked by a plus sign at the solder point farthest from the tab. As with the former, my experiments show no difference in the way they run when the polarity is reversed.

Depending on how you anchor this model, it draws between 50 and 70 milliamps at 1.5 volts. If you glue its pad down with a good rubber cement (such as Goodyear "Pliobond), its vibration rate is about 80Hz (80 revolutions per second). Held in a vise, this goes up to 150 rps, similar to the cylindrical one.

Rigid mounting would be tricky because of irregularities in its shape. I don't know what they used to secure the rubber pad, but it is very well glued to the steel casing. The vibrations are very intense, so your glue job has to be equally as strong.

Drive Circuitry:

A disadvantage of these is that they are intended to be powered from 1.5 volts. (I suppose the pagers are supplied by low-voltages.) Five volts would be my preference. Therefore, you have to dump a lot of power somewhere -- in a voltage regulator, a transistor or a resistor.

The scheme I came up with shares the power dissipation between a half-watt resistor and a 2N2222. Also, the input impedance is high, very high under 2 volts of drive, and equal to whichever base resistor you choose at higher driving voltages. Moreover, the motor speed can be varied for low drive voltages; before the current limiting resistor (in the collector) drops the collector voltage down to perhaps 2.1 volts, the transistor is an emitter follower (hence the high impedance at the base).

The output of an op-amp, perhaps a comparator or an amplifier with finite gain, goes through a base resistor (I use 10K in my circuits) to the base of a 2N2222. The emitter goes through the motor to ground. The collector goes through a half-watt resistor (from 15 to 47 ohms, depending on how hard you want to drive your motor) to VCC (5 to 15 volts, perhaps).

Naturally, the components get hot so give them some breathing room.

The projects in this issue are not intended to run the motors for long periods of time; thus, I picked 15 ohms to limit the current and kick the heck out of them. For long duty cycles, subtract 3V from VCC and calculate E over R to be 100mA (R equals the quantity VCC minus 3, divided by 0.1 amp).

THE "VACUVIBE" -- AN ATTACHMENT FOR "TANK" VACUUM CLEANERS -- A VIBRATING NEGATIVE AIR-FLOW ALARM

Abstract

Conceived as an aid to the deaf-blind homemaker, this device, mounted on the "wand" (hose) of a "tank style" vacuum cleaner, vibrates when the cleaner is inadvertently obstructed by debris that will not pass through the orifice. Subsequent use by this author, a blind homemaker, has convinced him (that's me) that partial obstruction of air flow very often goes unnoticed; the vibratory feedback can aid anyone to keep the appliance operating efficiently.

Introduction

Back in the early 1970's your editor did a lot of work adapting "vacuum tweezers" with audible feedback. These devices use a vacuum pump attached to a hypodermic needle; operating like tiny vacuum cleaners, they are used to pick up tiny bits for microscopic assembly (Smith-Kettlewell, in collaboration with Hewlett Packard, was demonstrating "tactile vision substitution" for assembling point-contact diodes under a microscope -- yet another story.)

I needed to know when my vacuum tweezers caught hold of a tiny silicon wafer; which I would then jettison into the glass capsule by releasing the vacuum on the needle. For that feedback, I arranged for a VCO to sound as soon as airflow was restricted. The greater the restriction, the higher the pitch. If I mistakenly caught two or more wafers, the needle would most likely be only partially occluded (giving me a low-pitch), and if only one wafer covered the opening, I would hear the desired high pitch. (The wafers were as small as grains of pepper.)

Years later, I wondered if deaf-blind homemakers might gain benefit from knowing, through vibratory feedback, when debris has clogged the vacuum cleaner. Mostly, I notice when this happens by listening to the motor speed up when the wind load is removed from the blower; inadvertently snagging a paper napkin, which renders the cleaner impotent, causes the motor to change pitch.

I designed an attachment which monitors the negative air pressure behind the "nozzle". When debris obstructs the intake of air, the hose in the user's hand vibrates to indicate that the vacuum cleaner has been rendered ineffective. Two products make this device possible: the availability of relatively low-cost vacuum sensors ($25), and ready available vibrators used in "silent pocket pagers" ($12).

Physical Description

This prototype consists of a battery-operated device, mounted to the so-called "wand" of a "tank style" vacuum cleaner. (Battery power, while not so desirable, makes for easy installation on the vacuum cleaner.) A small plastic tube (5/32-inch i.d.) connects the sensor of the device to an opening in the "wand".

The vibrator, a small motor spinning an eccentric weight, vibrates the whole assembly when air flow is restricted. The "vacuum" behind the restriction is measured by the sensor, and when the sensor's output voltage exceeds a preset threshold, a variable drive voltage operates the vibrator motor. The speed of the motor's flying weight gives the user a sense of the extent of air flow restriction.

My unit is built into a project cabinet measuring 4-1/2 by 2-1/2 by 1-1/4 inches (Radio Shack No. 270-221). As I often do, I chose to mount the 9-volt battery on the outside at one end. At the opposite end, a hole accommodates the nipples of the sensor (Sensym SX05DN, plus/minus 5 p.s.i.). Next to the sensor is an on/off switch. Finally, I deemed it advisable to drill an access hole so that the "threshold" adjustment (the 2K pot in the circuit) could be reached from the outside.

Some sort of fitting must be created for a short length of plastic tubing to connect between the sensor and the vacuum hose. The Eureka I modified has an adjustable collar which exposes a port to diminish the suction. I never open that port on vacuum cleaners. Thus, I mounted a "tip jack" in-side-out on the collar, glued the collar in place so that the jack is permanently over the port, and used the bushing of the jack as a fitting for the tube.

Any scheme for mounting the project box to the rigid part of the vacuum hose will work. The more rigid the mounting, the more vibration will be transferred to the hose. I chose to secure the bottom of the cabinet to the hose. So that it could be easily removed for repair or refinement, I mounted all the electronics on the lid; everything but the on/off switch comes off with the lid.

{There are good reasons to consider quite a different configuration. See the "Discussion" at the end of this article.}

The perforated board onto which everything is mounted, including the motor and the sensor, measures 2 by 4 inches. The corners of the board are filed away to accommodate the corner posts of the box.

The sensor is almost diamond-shaped; the most distant corners have clearance holes for No. 4 bolts to secure it. The other two corners are "squared off"; the flattened surface of one has four connecting pins emerging from it, while the opposite side has the 11/64th inch fittings. It has both positive-pressure and negative-pressure nipples coming out this one side. The one nearest the printed face ("Port 1") is for positive pressures; I cut this one off to avoid confusion and so that a single drilled hole in the end of the box could suffice for connection to "Port 2".

The pins are at standard one-tenth-inch spacing. Therefore, if you bend them downward at right-angles -- making this bend about 3/16ths of an inch away from the body of the sensor -- a socket on the board can accommodate them.

I used pins 5, 6, 7 and 8 of an 8-pin DIP socket to accept the bent pins of the sensor. Moreover, by wiring across both columns of socket pins -- tying pins 1 and 8 together, pins 2 and 7 being connected, etc. -- I created test points for convenience. Using 26-gauge wire, I could test the 5 volts, the sensor outputs, and ground was available on pin 1.

The sensor is mounted at one end of the board, the op-amp package and the 5-volt regulator are behind it, the trimmer adjustments are next, and there is plenty of room for the motor and its driver transistor at the other end of the board. If the "pancake" style of vibrator is used, the back side of the Motorola motor can be glued to the board with Goodyear "Pliobond" (or other good rubber cement).

The only things mounted to the main part of the cabinet are the on/off switch and the battery holder (this battery holder being the simplest clamp-type, mounted to the outside). Holes in the cabinet are made for access to the sensor fitting and the all-important "threshold adjustment".

Technical Description

A differential "gauge-pressure" sensor was chosen over an "absolute-pressure" one. Vacuum is applied to one port while the other one is left open.

Testing vacuum cleaners of three different manufacturers revealed that pressures of minus 2.5 p.s.i were typical; the smallest unit (using a "6-amp motor") pulled about 2 p.s.i. Sensym has gauge-pressure units of plus/minus 1 and plus/minus 5 p.s.i. Thus, the SX05DN (5 p.s.i) unit was chosen.

Data on the use of these Sensym transducers was published in SKTF (Vol. 9,No. 3, Summer 1988). Sensym describes them as using "Piezo-resistive ceramic strain gauge elements"; they are Wheatstone bridges with differential outputs. Assuming a 5V supply to the bridge, the rated output is 15mV per p.s.i.g.; 0.0375V for our 2.5 p.s.i requirement.

Using the high-impedance differential amplifier suggested (comprised of two op-amps), the gain is calculated by the expression:

Differential gain equals 2 times the quantity of 1 plus the ratio of the 10K resistors divided by the feedback resistor (a low-value resistor between the inverting inputs). Thus, choosing a 160-ohm feedback resistor, the calculated gain is 127.

Proper interpretation of the sensor data predicts that 2.5 p.s.i will yield a differential signal of plus 37.5mV; this times 127 gives an amplified single output of 4.76 volts.

These sensors have an unpredictable offset error. Again, accounting for scaling (given a 5V supply for the Wheatstone bridge), this offset error can be anywhere from 0 to 33.3mV. The 10K trim pot feeding pin 5 of the LM324 is to be adjusted for zero volts output from pin 14 of the 324 with no vacuum applied.

Some proportionality, over a small range, is provided for by choosing a finite gain for the "comparator" of the op-amp of pins 1, 2 and 3 of the LM324. The gain of approximately 3 (100K feedback and 33K input resistors)can be changed to suit your liking. If a lower gain is chosen, you can expect slow motor speeds for some partial vacuums, and these vibrations maybe hard to feel.

Drive to the motor is via an emitter follower (using a 2N2222). However, this drive must be current limited so as not to burn something out for highest partial vacuums. The 15-ohm resistor in the collector accomplishes this; note that a base resistor (10K) is required as the driving op-amp would otherwise try to pull the base positive with respect to the collector.

At quiescence, the battery drain is 10mA. Naturally, this jumps to as much as 150mA when the motor is energized.

Circuit

A 9-volt battery is used. Its positive goes through an on-off switch to the plus 9V line. The negative of the battery goes to the cathode of a 1N4001 diode; the anode of this diode goes to ground. The plus 9V line is bypassed to ground by 470uF (negative of the cap at ground).

A 7805 5-volt regulator has its "Common" terminal grounded. Its "Input" terminal goes to the plus 9V line. The output of the 7805 goes to a plus 5V line. Both Input and Output of the regulator are bypassed to its common terminal (both caps close to the regulator), these caps being 0.1uF discs.

Pin 11 of an LM324 quad op-amp goes to the junction of the negative of the battery and the 1N4001 cathode. Pin 4 of the 324 goes to the plus 9V line.

Pin 1 of a Sensym SX05DN is grounded. Pin 3 goes to the 5V line. The outputs of this sensor are pins 2 and 4 (positive and negative, respectively). Pin 4 goes to pin 10 of the LM324; pin 2 of the sensor goes to pin 12 of the 324. (Pins 10 and 12 of the 324 are non-inverting inputs of "A1" and "A2" op-amps, respectively.)

A1 and A2 comprise a differential amplifier. Between pins 8 and 9 of A1, from output to inverting input, is a 10K 1% resistor. The inverting input of A1 goes through a 10K 1% resistor to pin 7 of the same chip (this being an offset circuit to be described later).

Between pins 14 and 13, from output to inverting input of A2, is the parallel combination of a 10K 1% resistor and a 0.1uF disc capacitor. Pin 13, the inverting input, goes through 10K 1% to pin 8, the output of A1.

The gain-determining feedback resistor is 160 ohms 5%; this goes between inverting inputs, between pins 9 and 13. The output of this differential amp is pin 14.

The offset circuit, using pins 7, 6 and 5 of the LM324, has a 10K 5% feedback resistor between pins 7 AND 6; pin 6, the inverting input, goes through another 10K 5% resistor to ground.

Pin 5, the non-inverting input, goes to the arm of a 10K multi-turn pot. The bottom of this pot goes through 10K (5%) to ground; the top of the pot goes through 10K (5%) to the 5V line. The top of the pot is bypassed to ground by 10uF (negative of this cap at ground).

The output of the gauging circuit, pin 14 of the 324, goes to the non-inverting input, pin 3, of the fourth op-amp in the package. Between pins 1 and 2 is a 100K feedback resistor. The inverting input, pin 2, goes through 33K to the arm of a 2K multi-turn trim pot; the bottom of this pot is grounded, while its top end goes to the 5V line.

The output of this fourth op-amp, a sort of "soft comparator", goes through 10K to the base of a 2N2222 transistor. The collector goes through a 15-ohm 1/2-watt resistor to the plus 9V line. The emitter goes through the vibrator motor to ground.

"Port 1" is left open, this is the nipple closest to the printed side of the sensor. "Port 2", the nipple farthest from the printed face and opposite pin 1, goes through a small length of 5/32-inch i.d. tubing to a corresponding fitting on the vacuum cleaner's hose.

Calibration

First, the offset error must be accounted for by adjusting the 10K trimmer to get 0 volts at the output of A2, pin 14 in this layout. This adjustment should be at the point where turning the pot in the upward direction just begins to cause the output voltage to climb (with no vacuum applied).

Of course, setting of the 2K threshold adjustment will depend on the "strength" of the vacuum cleaner. Beyond this, the preference of the user may be a factor; for example, you might want to adjust it so that a gentle vibration will occur when firm contact with a high-pile carpet is made.

As mentioned above, the range over which there is a dynamic effect -- vibrator speed versus vacuum -- can be selected with the choice of input resistor off pin 2 of the LM324. The higher this value (shown here as 33K), the larger the range over which the speed will be variable. However, the vibration of slow motor speed is often unnoticeable, so restricting the dynamic range makes the indications seem more definite. A "hard threshold" can be gotten by removing the 100K feedback resistor between pins 1 and 2.

With the gain shown (127, determined by the 160-ohm resistor), the output at pin 14 is 1.9V per p.s.i. As a starting point, I set the 2K pot so as to present 0.95V to the comparator op-amp (measured at pin 3 of the LM324). Theoretically, then, my vibrator starts turning at 1/2 p.s.i. I threw some cocktail napkins on the floor, and vibration reliably told me when I snagged one. (My cleaner has a so-called 10-amp motor", the "strongest" I could buy.)

Pin Assignments

Sensym SX05DN Differential Pressure Sensor (With the printed side up and the pins facing you, the pins are, from left to right, 1, 2, 3, 4):

78L05 (TO92 package; With the flat side away from you and the leads pointing upward, they are, from left to right):

Parts List

Resistors:

Trim Pots:

Capacitors:

Semiconductors:

Miscellaneous:

Discussion

The self-contained assembly of this prototype has three disadvantages: (1.) Being battery-operated, failure to turn the "Vibrovac" off will render it inoperable and cost you money. (2.) The assembly is bulky and may get in the way for some jobs. (3.) Most importantly, restrictions caused by a full bag or by something further down the hose are not detected by this system.

The best possible place to sample the vacuum is in the bag compartment. A full bag, or any other restriction, would be detected by the gauge.

If the vibrator alone were affixed to the hose, the assembly in the user's hands would be small indeed.

Electricity is available down at the main body of the vacuum cleaner. A 9-volt DC supply would have to be fashioned to power the circuit, but there might be room in the bag compartment for this and the circuitry.

Suppose the appliance were large enough to accommodate everything in that compartment -- sensor included. AC mains power would not be hard to get. Two wires would have to be run up the hose to the vibrator; some sort of plug and socket might be desirable so that the hose could be completely disconnected. Finally, with the pressure transducer in the evacuated chamber, Port 2 would be left open; a tube connecting Port 1 to the outside would be required.

This is more assembly than I cared to do. As an "adapter kit" for modifying vacuum cleaners, the first assembly would be a much more likely product.

Passive vacuum switches can be gotten with which one might turn the circuit on when the appliance is running. This would relieve you from having to remember to turn off the battery power when cleaning is done.

Address List

Player Piano Co. is mentioned here because of their wide variety of tubing, "Y connectors" and other fittings. So-called "tracker-bar tubing" is 5/32-inch i.d.; "expression tubing" is 3/16-inch i.d. (which also fits the tapered nipples of the Sensym unit.

Player Piano Company: 704 E. Douglas, Wichita, KS 67202; Phone: (316) 263-3241.

Sensym: 1255 Reamwood Ave., Sunnyvale, CA 94089; Phone: (408) 744-1500.

Motorola "Parts and Accessories": (800) 422-4210, Menu Choice 2.

THE SMITH-KETTLEWELL VIBRATORY VOLUME LEVEL METER FOR OPEN-MIKE RECORDINGS

Abstract

Standard "VU metering" equipment for blind recordists has auditory output; typically, an audio oscillator signals the user when a preset level has been exceeded. The only way to use such equipment in the proximity of live microphones is by way of earphones. Moreover, these earphones must be of an isolating type; otherwise, the sounds made by the VU meter would be audible in the room. It can be argued that a sighted recording engineer has the advantage of being free of those extra sounds and, perhaps, free of special headgear. A vibratory output is an obvious alternative; it can be relatively quiet, and it addresses a sensory modality different from hearing.

Introduction and Background

In the 1970's, this author designed a vibrotactile VU indicator which used a Smith-Kettlewell-designed solenoid with a vibrating plunger. This was an okay system, but those "tactors" never became commercially available. Besides that, the user had to put a finger on the face of the instrument in order to feel the tip of the plunger.

What makes this version practical is that the vibrators in so-called "silent pagers" are now commonplace. These vibrators are tiny motors which swing an eccentric weight. Luckily for this application, they are fairly fast -- not instantaneous, but fast enough to indicate major transgressions of over-recording.

The unit described here is single-channel, it can only monitor one microphone at a time. How to expand this is left up to you. You could make two or three units which could be dropped into different pockets. Or, one vibratory unit could contain a mixer; it would vibrate whenever any channel exceeded the limit. As for me: once I have adjusted the artistic balance of channels, I look for the one most likely to exceed 0VU and connect my single-channel unit to monitor that.

Description

My prototype is built into a small project box that easily fits into a pocket, or that I can hold onto when I wish to feel every little "kick" of the instrument. "Smaller and sleeker" is not necessarily better; you do want it to be thick enough to remain in contact with you while it is in your pocket.

I made the mistake of choosing a trade-show sample cabinet which has a battery compartment with a sliding door. I now must carefully stuff the compartment with bits of foam to keep the battery from rattling and making noise. If I build another, I shall use a standard 1- by 2-1/8- by 3-1/4-inch project box, arranging to clamp the 9V battery to the outside. To avoid audible vibrations, build this instrument rigidly. Bolt the board in place and avoid chance contact of the various items.

The circuit is fairly simple; it shares similarities with "the Smith-Kettlewell Auditory Volume Level Meter" (SKTF, Vol. 2, No. 1, Winter 1981). It can easily fit on a 1.8- by 2.5-inch piece of perforated board. One half of a dual op-amp amplifies the audio for half-wave rectification. The second op-amp is a comparator that "triggers" the vibrator when the rectified audio exceeds a stable voltage.

Initially, I tried building in some dynamic information into the instrument -- giving the second op-amp a finite gain so that the motor would run faster as the level exceeded the threshold by various degrees. The motor does not respond fast enough to make this subtlety practical.

So given a hard threshold, I tried speeding up the onset of vibration by paralleling the 15-ohm current-limiting resistor with a large-value electrolytic (1000uf), but I noticed no difference.

All in all, though, when holding the vibratory unit in my hand while listening to an auditory meter simultaneously, a good correlation between indicators is evident and convincing.

Circuit

The negative side of the 9V battery is grounded; its positive goes through an on-off switch to the plus 9V line. The 9V line is bypassed to ground by 470uF (negative of this cap at ground).

An LM317 is set up to supply a "standard" voltage, about 5V. The 317's input goes to the 9V line. Its output goes through 240 ohms to its "Adjust" pin, with this pin going through 680 ohms in parallel with 10uF to ground (negative of this cap at ground). The output pin, 5V, goes through 470K, then through 910K to ground. The junction of these resistors is bypassed to ground by the parallel combination of 4.7uF and 0.047uF (negative of the electrolytic at ground).

Pin 4 of an LM358 dual op-amp is grounded. Pin 8 goes to the plus 9V line. Pin 5, the non-inverting input of the first stage, goes to the voltage divider -- the junction of the aforementioned 47K, 910K, and the bypass caps.

The "cold" side of the audio signal is grounded. The "hot" audio lead goes to the top of a 100K trim pot (calibration). The bottom of this pot is grounded. Its arm goes through 0.47uF, then through 47K to pin 6 of the LM358. Between pins 7 and 6 is a 910K feedback resistor.

Pin 7 of the 358, the output of the amplification stage, goes to the anode of a 1N914 diode. The cathode goes through 10K, then through the parallel combination of 910K and 1uF to ground (negative of this cap at ground).

The top of this parallel combination, its junction with the 10K -- goes to pin 3 of the LM358, the non-inverting input of the comparator.

The comparator's inverting input, pin 2, goes to 5V (the output of the LM317). Pin 1, its output, goes through 10K to the base of a 2N2222. The emitter of this transistor goes through the vibrator motor to ground. The collector goes through a 15-ohm 1/2-watt resistor to the plus 9V line.

Note: This unit can accept more than one channel of audio, since the input op-amp can be made into a mixer. Several input potentiometers (100K) can be fed by separate input jacks; each then requires its own 0.47uF coupling cap and its own 47K resistor going to pin 6 of the LM358.

Parts List

Resistors (1/4-watt 5%, unless otherwise stated):

Potentiometers:

Capacitors:

Semiconductors:

Miscellaneous:

Pin Assignments

LM317LZ (TO92 package; with the flat side away from you and the leads pointing up, they are, from left to right):