pedmond@A.GP.CS.CMU.EDU (Patrick Edmond) (04/10/88)
This may be a terribly basic question; if so, please forgive me - I'm pretty much a novice at this! I want to build a circuit that senses the pressure someone's foot is exerting on a surface, without using springs, rheostats etc, because it has to be small & simple. Is there such a thing as a small (e.g. < 0.5" tall) transducer or some such component whose resistance/capacitance/whatever varies with the pressure exerted on it? (The weight would range, I guess, from 0 up to ~ 200 pounds). Basically what the circuit is to do is this: there should be a potentiometer or some way of adjusting a reference 'weight', and if the weight on the transducer exceeds this reference, an LED should light up, or a buzzer go off, or whatever. I can manage this part of the circuit, but any information/pointers regarding the existence of transducers (oh! and cheap ones as well, if possible!) or else alternative ways to implement this, would be much appreciated. Thanks in advance Patrick Edmond pedmond@a.gp.cs.cmu.edu
blarson@skat.usc.edu (Bob Larson) (04/10/88)
In article <1368@PT.CS.CMU.EDU> pedmond@A.GP.CS.CMU.EDU (Patrick Edmond) writes: >I want to build a circuit that senses the pressure someone's foot is exerting >on a surface, without using springs, rheostats etc, because it has to be small >& simple. Is there such a thing as a small (e.g. < 0.5" tall) transducer or >some such component whose resistance/capacitance/whatever varies with the >pressure exerted on it? Commercailly there are things called "pressure transducers". I remember seeing data sheets in one of my old National Semiconducter catalogs. They don't seem to be readily available on the hobby market, (so you get to pay distributer price) and may not be able to handle the pressure you want. A cheap, easy to make pressure sensitive resistor can be made with conductive foam. (The black foam used in shipping static sensitive ICs.) Just hook contacts on opposite corners. Have fun! Bob Larson Arpa: Blarson@Ecla.Usc.Edu blarson@skat.usc.edu Uucp: {sdcrdcf,cit-vax}!oberon!skat!blarson Prime mailing list: info-prime-request%fns1@ecla.usc.edu oberon!fns1!info-prime-request
jbn@glacier.STANFORD.EDU (John B. Nagle) (04/11/88)
Pressure-sensing mats are widely available commercially. See any issue
of Sensors magazine for ads. Such mats are widely used to control automatic
doors, and are very reliable and not outrageously expensive. Fail-safe mats
are available for machinery-guarding applications. This is the standard
approach. Tapeswitch is one vendor.
If you want to detect motion, rather than presence, as in an alarm
system, you might look into Kynar film, a DuPont product which generates
electricity via piezoelectricity when squeezed. If you're thinking of
a security device that is hidden under carpet, this could be an effective
approach. It has the significant advantage that static weight will not
cause output, and therefore heavy carpets, even with furniture on them,
won't cause false triggering.larry@kitty.UUCP (Larry Lippman) (04/12/88)
In article <8266@oberon.USC.EDU>, blarson@skat.usc.edu (Bob Larson) writes: >> I want to build a circuit that senses pressure someone's foot is exerting on >> a surface, without using springs, rheostats etc, because it has to be small >> & simple. Is there such a thing as a small (e.g. < 0.5" tall) transducer or >> some such component whose resistance/capacitance/whatever varies with the >> pressure exerted on it? > > Commercailly there are things called "pressure transducers". I > remember seeing data sheets in one of my old National Semiconducter > catalogs. They don't seem to be readily available on the hobby > market, (so you get to pay distributer price) and may not be able to > handle the pressure you want. Pressure transducers are used for measuring pressure of a fluid (i.e., gas or liquid). Unless one used a hydraulic cylinder or hydraulic bladder of some sort whose pressure was transmitted to the pressure transducer, a pressure transducer would be of no value. Assuming that a liquid was used, some rather simple math determines how the surface area (or piston area) of the bladder (or cylinder) determines the range and sensitivity of the device when connected to a suitable pressure transducer. A more useful transducer for this application is called a load cell. It transforms mechanical pressure to an electrical signal, which may be in the form of a 1,2 or 4-arm resistive strain gage bridge, LVDT, etc. Piezoelectric load cells exist, but they measure only dynamic pressure changes - not static pressure. For those with experience, a strain gage element may be directly installed upon the measuring surface to measure deflection; knowing the elastic modulus of the measuring surface plus some relatively simple math determines pressure in engineering units. Strain gage elements themselves are inexpensive, but they are extremely fragile and require suitable protection for direct use. A load cell is most commonly a strain gage whose element is protected within an enclosure. > A cheap, easy to make pressure sensitive resistor can be made with > conductive foam. (The black foam used in shipping static sensitive > ICs.) Just hook contacts on opposite corners. This is true; however, the output of such a device is highly non-linear and will change over time as the foam undergoes permanent deformation through repeated use! <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|utzoo|uunet}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"
mark@adec23.UUCP (Mark Salyzyn) (04/12/88)
In article <1368@PT.CS.CMU.EDU>, pedmond@A.GP.CS.CMU.EDU (Patrick Edmond) writes: > Is there such a thing as a small (e.g. < 0.5" tall) transducer or > some such component whose resistance/capacitance/whatever varies with the > pressure exerted on it? (The weight would range, I guess, from 0 up to ~ 200 > pounds). Try: CMI-DEARBORN, INC. 5353 Wilcox Montague, Michigan 49437 (616) 894-9051 They have several pressure transducers. They manufacture Weigh-in-Motion equipment for weighing vehicles on the highway. They have two pressure transducers that may fill your needs; capacitive and DYNAX(r) I think. I've seen the DYNAX sensor, it can act as a pressure switch. They say that it can also be set into a more linear transducer range. It is supposed to be cheap, but I've never actually purchased it.
ssr@cos.com (Dave Kucharczyk) (04/13/88)
In article <1368@PT.CS.CMU.EDU> pedmond@A.GP.CS.CMU.EDU (Patrick Edmond) writes:
<Writes about wanting sources of pressure transducers.>
try these:
data instruments
617-264-9550 (Acton,MA)
entran
201-227-1002 (somewhere,NJ)
tedea
818-349-6116 (Northridge,CA)
expect to pay about 100-400 per unit depending on capacity and accuraccy.
dave
PS i in no way endorse any of the above companies, as a matter of fact i never
even heard of 'em, i just made them up as i went along, yea, that's the ticket..sic@ritcsh.UUCP (Eric A. Neulight) (04/16/88)
In article <1368@PT.CS.CMU.EDU> pedmond@A.GP.CS.CMU.EDU (Patrick Edmond) writes: >I want to build a circuit that senses the pressure someone's foot is exerting >on a surface, without using springs, rheostats etc, because it has to be small >& simple. Is there such a thing as a small (e.g. < 0.5" tall) transducer or >some such component whose resistance/capacitance/whatever varies with the >pressure exerted on it? (The weight would range, I guess, from 0 up to ~ 200 >pounds). About two years ago I helped my little brother assemble a Heathkit digital bathroom scale. It sounds like what you want is a simple scale. The transducer they used was a relatively long, thin (5"x1/4"x1/4"??) metalic strain gauge with a few leads coming off the tail. I suppose it was piezo-electric or capacitive (redundant?). You would most likely need to situate a hefty platform centered on the end of the gauge just like the scale. How big an area does the sensor you have in mind have to cover? If you call Heathkit I'm sure you could order the part. Check out their catalog. I wish I had the old schematics to talk more intelligently. Alas, I am 300 miles away. ============================================================================== CLAIMER: Well -- I wrote it! Eric Alan Neulight Electrical Engineering "INSANITY is just a state of mine." Computer Science House Rochester Institute of Technology BITNET: EAN4762@RITVAX UUCP: ...!rutgers!rochester!ritcv!ritcsh!sic ==============================================================================
jad@dayton.UUCP (John A. Deters) (04/20/88)
A cheap* transducer can be made from the anti-static foam that many CMOS chips come packed in. If you are looking purely for detection and not measurement, you might consider it. It decreases in resistance with compression. -- -john deters Dayton Hudson Department Store Company uucp: rutgers!dayton!jad MIS 1060/700 on the Mall/Mpls, MN 55402 So many pedestrians ... WARNING: Do not leave these opinions ... so little time ... lying around -- they attract flies.