RIDICULOUSLY SENSITIVE CHARGE DETECTOR

Transcription

1 RIDICULOUSLY SENSITIVE CHARGE DETECTOR (C)1987 William J. Beaty (The earth-grund is nt required.) (The 1-Meg resistr is nt required.) This simple circuit can detect the invisible fields f vltage which surrund all electrified bjects. It acts as an electrnic "electrscpe." Regular fil-leaf electrscpes deal with electrstatic ptentials in the range f many hundreds r thusands f vlts. This device can detect ne vlt. Its sensitivity is ridiculusly high. Since "static electricity" in ur envirnment is actually a matter f high vltage, this device can sense thse high-vltage bjects at a great distance. On a lwhumidity day and with a 1/2 meter antenna wire, its little light will respnd strngly when smene cmbs their hair at a distance f five meters r mre. If a metal bject is lifted up n a nn-cnductive supprt and tuched t the sensr wire, the sensr can detect whether that bject supprts an electrstatic ptential f as little as ne vlt! Nte: I use the term "electrificatin" rather than "charging", in rder t avid cnfusin between charge (the stuff n the negative electrns and psitive prtns) and net-charge (the imbalance between psitive and negative particles that appears n everyday bjects.) Realwrld bjects becme "electrified" when their pre-existing + and - charges are nt equal. PARTS LIST: 1 - Standard 9-vlt battery

2 1 - MPF-102 N-channel Field Effect Transistr (FET) Radi Shack # Red Light Emitting Dide (LED) Radi Shack # MISC: Battery cnnectr (# ) Alligatr Clip Leads (# ) slder, if desired 1-meg resistr (ptinal) plastic, fur, fil, cmb, tape dispenser, plastic cup NB. BF 245 mnteres på maven altså med flad side pad da Gate g Drain er byttet m i TO 92 huset. (Tiny versin bult atp a 9v battery cnnectr) Shrtcuts: 1.CONSTRUCTION HINTS 2. SENSE E-FIELDS 3. SENSE POSITIVE ELECTRIFICATION 4. CHARGE IS CONSERVED 5. PEELING CAUSES ELECTRIFICATION 6. JUMPING ELECTRONS, "VOICE CONTROL" 7. VARIABLE GAIN 8. FIELD DISTORTIONS 9. VANDEGRAAFF SENSING 10. HOMEMADE CAPACITORS 11. DIPOLE ANTENNA 12. THE SKY VOLTAGE 13. UNTESTED SUGGESTIONS 14. HOW IT WORKS 15. FET-PANEL MUSEUM EXHIBIT 16. OTHER LINKS CONSTRUCTION HINTS

3 Warning: dn't cnnect the battery until yu are SURE yu've hked everything up exactly right. It's pssible t burn ut the FET r the LED if they are cnnected incrrectly. Dn't let the transistr's wires bump tgether even briefly, r it will flash the LED and burn it ut. NOTE: Dn't ever cnnect any LED directly t a 9-vlt battery, it will burn ut the LED. A bare LED needs a 1000-hm resistr wired in series t limit the maximum current frm the 9-vlt battery. Warning: Avid tuching the Gate wire f the FET. Small sparks jumping frm yur finger t the Gate wire can damage the transistr internally. The 1-meg resistr helps prtect the FET frm being harmed by accidental sparks t its Gate lead. The circuit will wrk fine withut this resistr. Just dn't intentinally "zap" the Gate wire. T test the circuit, charge up a pen r a cmb n yur hair, then wave it clse t the little "antenna" wire. The LED shuld g dark. When yu remve the electrified pen r cmb, the LED shuld light up again. IF IT DOESN'T WORK, the humidity might be t high. Or, yur LED might be wired backwards, r the transistr is cnnected wrng, r maybe yur transistr is burned ut. Make sure that the transistr is cnnected similar t the little drawing abve. ALs, if the plarity f the LED is reversed, the LED will nt light up. Try changing the cnnectins t yur LED t reverse their rder, then cnnect the battery and test the circuit again. If yu suspect that humidity is very high, test this by rubbing a balln r a plastic bject upn yur arm. If the balln des nt attract yur arm hairs, humidity is t high. SENSE E-FIELDS Cnnect the circuit t its battery, and the LED will turn n. Cmb yur hair, then hld the cmb near the Field Effect Transistr (FET) gate wire. The LED will g dark. This indicates that the cmb has an excess f negative charge, and the FET respnds t the electrstatic field surrunding the cmb. It acts as a switch and turns ff. Remve the cmb and the LED brightens again. Wiggle the cmb, and find at hw great a distance the circuit still detects it. It's amazing hw far an e-field extends arund an electrified bject. (But then, e-fields shuld extend t infinity, n?) On a very lw-humidity winter day the circuit will respnd at a much greater distance. This happens because, when humidity is lw, the cmbing f yur hair then generates a much strnger separatin f charge upn the cmb's surface. Nte that a metal cmb will nt wrk, since any separated charge immediately weakens by spreading t yur hand and acrss yur whle bdy. A plastic r hard rubber cmb wrks well because rubber is an insulatr and the imbalanced charge can't leak ff the cmb. Try simply TOUCHING a plastic pen briefly t hair. The FET will detect even this tiny negative net-charge n the pen. The sensr will usually nt indicate the equal psitive that appears

4 n yur hair, since hair is made cnductive by humidity, and the psitive net-charge leaks t yur head. The plarity f the surface charge n the cmb r plastic pen is negative. The rule fr this FET is, negative charge turns the switch (and the LED) ff. SENSE POSITIVE ELECTRIFICATION This FET sensr is nt an ideal educatinal device because it respnds differently t psitive than t negative Ptential Difference at it's "Gate" wire. Create sme psitive net-charge by affixing a small tuft f hair r wl t the end f a plastic bject (pen r ruler), then rub the hair upn anther plastic bject. (If we electrify sme hair, we can avid leakage lsses by nt tuching it with fingers r ther grunded bject.) Bring the psitively-electrified hair near the FET. Nte that the LED becmes brighter, but when the hair is remved, the LED ges dark and stays that way. Bring the hair clse by again, and the LED lights up again. Rules fr this FET: negative bjects turn the LED ff, it lights again when remved. psitive bjects make the LED bright, then dark when remved. Turn the LED back n by simultaneusly tuching fingers t the "Gate" wire and t sme ther part f the circuit. Or, tuch a plastic pen t sme hair, then wave it near the sensr, and the LED will light up. Remember this trick when ding ther demnstratins. (Nte: prfessinal electrmeters d nt suffer frm this "reset" effect, but prfessinal electrmeters cst several hundred dllars at the very least!) CHARGE IS CONSERVED Munt a tuft f hair n a plastic rd, verify that it is cmpletely discharged and des nt affect the FET. Take a secnd plastic rd (r plastic pen!) and verify that it is als cmpletely neutral. (Fndle the whle pen with damp hands if nt.) Nw hld the plastic handle and tuch the hair t the tip f the pen, separate them, then hld them up t the sensr ne at a time. Yu'll discver that the end f the plastic rd is nw negative and turns the LED mmentarily ff. The hair is psitive and turns the LED n, then ff. Cntact between the hair and the plastic caused sme assymetrical sharing f the equal psitive and negative "electricity" within them. When they separated, sme negative charges stayed with the plastic, leaving it with mre negatives than psitive (net negative charge.) At the same time, the hair was left with fewer negatives than psitives, fr a net psitive charge. Atms were trn apart, "inized", and pairs f electrns and prtns were yanked apart and separated t vast distances. Nte: "static electricity" is nt caused by frictin, it is caused by cntact between dissimilar materials, fllwed by separatin. We culd say that it's caused by "peeling". PEELING CAUSES ELECTRIFICATION The "peeling" effect can be demnstrated with a rll f plastic adhesive tape. Peel a few inches f tape ff the rll and hld it near the circuit. The LED will shw that the tape is strngly electrified. Nw use the sensr t test the tape dispenser. Yu will discver that the rll f tape has an ppsite plarity cmpared t the strip f tape. This illustrates that "static" electrificatin des nt require frictin, it nly requires intimate large-area cntact between dissimilar materials. Matter is made f psitive and negative charge, and the peeling f tape can separate the charges that were already

5 there in the matter. Because the plastic backing f the tape is a different material than the adhesive, when they tuch tgether there is assymetric bnding and electrn-sharing. This leads t separatin f ppsite charge when we peel tape frm its rll. Als, try taking tw strips f tape, stick them back t frnt (fld little tabs s yu can separate them again,) pat them dwn with mist hands t discharge them, then peel them apart. Hld each near the sensr. One strip indicates strngly psitive, the ther is equally negative. The strips will attract each ther. Try ther demnstratins frm Sticky Electrstatics, using the Charge Detectr t shw plarity f varius parts f the tape. JUMPING ELECTRONS, "VOICE CONTROL" If yu build a tiny cmpact versin f the FET circuit (slder it t a trn-pen battery cnnectr), yu can try the fllwing trick. Hld the circuit in yur hand, make sure the LED is lit, stand n a rug, then jump up and dwn. The LED will flash n and ff. Walk arund, and the same thing happens. As yur she sles make cntact with the rug and then peel away frm it, yur entire bdy becmes electrified. This makes the sensr respnd. If yu place yur shes back nt the electrified ftprints, yu cancel ut the net charge and the sensr indicates anther plarity change. Scuff yur shes, stmp up and dwn, jump arund, and the sensr will flash wildly. Demnstrate t nlkers that the sensr des nt respnd when yu shake it up and dwn, but it des respnd when yu jump. On a dry day, yu can cntrl the sensr with the tiniest mtin: scuff ne she, then lift the te t turn the sensr n and ff. Say "n", "ff" while mving yur te, and yu have a "vice cntrl" magic trick. Let sme pr fl examine the sensr, yell at it, etc. It will nly respnd t yur vice! (grin!) VARIABLE GAIN Obtain a small capacitr with a value belw 100 picfarads. Cnnect it between the FET gate lead and ne f the ther leads. This reduces the sensitivity f the device. In situatins where the sensr is TOO sensitive, this can make a big difference. Capacitrs larger than 100pF can be used, they REALLY wipe ut the sensitivity in inverse prprtin t the capacitance value. Obtain a clip-lead, and cnnect it t the Gate lead f the FET. Let it hang lse withut tuching anything. Yu'll find that this has vastly increased the sensitivity f yur FET circuit. On a dry day it will respnd t hair-cmbing frm 20ft away. If a TV screen is present, the sensr will act weird (especially when peple walk between the screen and the sensr.) The clip lead acts as an antenna, and the lnger it is, the mre sensitive the FET circuit becmes. FIELD DISTORTIONS Electrify a plastic bject, place it n an insulating supprt, place the FET sensr near it, then make sure the LED is turned n. If yu nw wave yur hand near the bject r the sensr, the LED will respnd. Yur hand causes the e-field arund the bject t distrt and change. Even thugh yur hand is nt electrified, the FET respnds. Yu've created a srt f "DC Radar" system which sends

6 ut a signal and then respnds when nearby bjects "reflect" it. Sme types f industrial sensrs ("prximity" sensrs) use this effect. Sme burglar alarms d as well. VANDEGRAAFF SENSING See at what distance yur FET electrmeter can sense the e-field frm an perating tabletp VandeGraaff electrstatic generatr. Suddenly discharge the generatr by using a grunded sphere electrde, and watch the distant FET respnd. Yu are actually sending ut radi waves with nearly zer frequency when yu d this. The FET des nt actually respnd instantly, there is a speed-flight delay (abut ne nansecnd per ft f distance.) It takes a shrt while fr the wave f vanishing e-field t reach the sensr. Radi waves are simply prpagating changes in electric fields, s yur VDG machine and FET sensr cnstitute a simple radi transmitter and receiver. HOMEMADE CAPACITORS The FET circuit is s sensitive that it will detect the energy stred n a tiny hmemade capacitr. Build a simple capacitr ut f aluminum fil, styrfam (frm a cffee cup), and wires. Stre energy in the capacitr by briefly cnnecting it t a 9V battery. Nw tuch ne capacitr wire t the negative battery terminal f the FET circuit, and tuch the ther capacitr wire t the Gate terminal (avid tuching the wires with fingers, this will discharge the capacitr.) The LED will indicate the stred energy. Use the 9V battery t reverse the plarity f the capacitr, then test it again with the FET and nte that the plarity is indeed backwards. Nte: dn't use paper fr yur capacitr dielectric, paper becmes slightly cnductive when humidity gets high, and yur stred energy will mysteriusly vanish because the paper ffers a leakage path s the separated charges can recmbine. Anther nte: this experiment demnstrates that "static electricity" and battery circuits are the same. The FET detects the ptential difference created by the 9V battery, just as it detects the much larger ptentials in the space arund electrified bjects. It is nt t far wrng t say that "static electricity" is simply "vltage." Everyday circuits are driven by the "static electricity" prduced by their lw vltage pwer supplies. DIPOLE ANTENNA After yu use this FET device fr awhile, yu'll get the idea that it has just a single antenna terminal. Hwever, like all vltmeters, it actually has tw. The rest f the circuit acts as the ther terminal. T demnstrate this, build a miniature versin f the detectr circuit nt the tp f a 9V battery. If yu hld the battery as usual, the Gate des act as the antenna, and negative bjects make the LED g dark. Nw carefully grasp the Gate wire between fingers and lift the whle device int the air. Avid tuching the battery. If yu nw hld a negatively electrified bject near the battery, the LED will get brighter instead f dimmer. Plarity f peratin has been reversed. If yu lay the whle unit dwn upn an insulating surface and apprach it with electrified bjects, yu'll find that the FET gate wire respnds with ne plarity, while the battery and the rest f the circuit respnds with the ther. Try cnnecting the gate wire t earth grund, then suspend the rest f circuit with an insulating handle. If yu hld up bjects having varius plarities, yu'll find that plarity f peratin is ppsite that f the gate wire.

7 'SCUSE ME, WHILE I SENSE THE SKY All ver the earth, thunderstrms are transprting negative charge dwnwards and psitive charge upwards. As a result, the earth is electrified negatively everywhere, while the sky is psitive. (Actually, it's the cnductive insphere which is psitive.) The FET sensr can detect this. Take it utdrs, away frm trees r buildings. Hld it high in the air, then lwer it t the grund while watching the LED. (Maybe get a tall adult t d this.) The LED will get darker when the device is lwered, and get brighter when it is raised up. The earth is negative! Maybe hang a cliplead antenna n the sensr wire t imprve sensitivity. (This plarity reverses when there is a thunderstrm directly verhead, but I wuldn't suggest standing ut in the pen when there is a chance that lightning may strike!) UNTESTED SUGGESTIONS Here are a cuple f things t try ut. I haven't tested them, I dn't knw hw well they wrk. Yu be first! Electrify a large plastic bject while n ne sees, then have a grup f peple with FET charge detectrs try t find which bject in the rm has the imbalanced charge. Have everyne build FET electrmeters. Line them all up in a rw, electrify a plastic bject, then sweep the bject back and frth. Yu'll be able t "see" the electrstatic field that surrunds the bject. Hld yur hand near the rw f detectrs while standing n a rug. Jump up and dwn and see what happens. Use a piece f clth t create a small electrified spt n a plastic bk cver. Use the FET device t find the spt. Draw an electrified shape using the clth as a paintbrush, then see if yu can use the sensr t figure ut what the shape is. Build many FETs and LEDs in a rw n a wden stick. Cnnect them all t ne battery. Place a negatively electrified bject n a table in a dimly lit rm, then sweep the FETstick rapidly past the bject. G back and frth really fast, and yu shuld see a rw f red lines caused by the mving LEDs. In the middle f the red lines will be a black spltch caused by the electrstatic field surrunding the negative bject! Repeat this test, but this time use a bit f clth t write the letter "A" n a plastic bk cver in invisible, negative net-charge. Can yu see the "A" when yu sweep the stick back and frth? Munt yur rw f LEDs n sme srt f mtrized prpeller, and yu'll have an autmated "charge detectr disk." HOW IT WORKS

8 A cmplete descriptin f this device requires delving int the physics f slid state electrnics. Instead, here is a quick descriptin based n the fluid analgy fr electric charge. Metals act as cnductrs NOT because charge can pass thrugh them. Instead, they are cnductrs because they cntain charge which can mve. Think f a metal wire as being like a hse that's aways full f water. And remember, vacuum is an insulatr, even thugh it presents n barrier t charges. The "sea f charge" in a metal is nt cmpressible, and t remve even a tiny bit f it wuld take a huge amunt f energy. In metals, each atm cntributes ne electrn t an "electrn sea", where the electrns dn't stick t single atms but instead rbit all thrughut the material. If we culd remve all the mvable electrns frm a metal, that metal wuld becme an insulatr. Unfrtunately, remval f electrns frm even the thinnest metal wire requires gazillins f Newtns f electrstatic frce, and develps gazillins f vlts f ptential difference. ("Gazillins" means sme huge number with way t many zers!). Metals are cnductive, and we can't easily change that. This is where silicn cmes in. While a metal's electrn-stuff within acts like a dense fluid, the mbile charges in silicn act like a cmpressible gas. In silicn, nly very few atms cntribute an electrn t the "sea." In fact, the silicn desn't really cntribute electrns at all, and ultra-pure silicn is an insulatr. Instead, nly the impurities in the silicn cntribute mvable electrns. If we nly put a gazillinth f a percent f impurities int the silicn mix, then the resulting material's mvable electrn-stuff becmes much mre cmpressible than the "electrn sea" within a metal. This reduces the vltage and frce (by a gazillin times!) that is required t cnvert the material frm a cnductr t an insulatr. The electrn-sea f a metal is nt very cmpressible. The electrn-gas within silicn is very cmpressible. S what? Well, if we can push the "electrn sea" ut f a cnductr, we can change it int an insulatr. It wuld be like turning ff a switch, but almst n wrk is required t d it. Just apply an electrical "push" in the frm f electrstatic repulsin, and large currents can be switched n and ff. The Field Effect Transistr is basically a tiny wafer f silicn with its edges cnnected t the Surce and Drain leads, and the Gate lead cnnected t a metal plate layed upn the wafer. When the gate lead is electrified negative, it repels the electrn-gas ut f the silicn and cnverts it int an insulatr. It acts like a switch that is turned ff by pure

9 vltage. If we picture the silicn as being like a rubber hse full f water, then the gate applies a sideways frce which pinches the hse clsed. Placing a negative net charge n the gate wire causes the "switch" t turn ff and the LED t g dark. Merely hlding a negatively electrified bject near the Gate lead will apply a frce t the electrns in that little lead wire, which pushes them int the metal plate, which repels away the electrns in the silicn, which pinches the cnductive path clsed. Interesting part: it really takes n energy t turn ff the FET. It des take electrstatic frce, but frce is nt energy! And s, even a very distant bject with a feeble net-charge can affect the FET and cntrl the much larger energy directed t the LED. The FET is nt really turned ff by negative net-charge. That is an verly simplified descriptin. It is really turned ff by a DIFFERENCE in the net-charge f the silicn and f the metal plate. Yu can either electrify the metal plate negatively, r electrify the silicn (and the battery, LED, and circuit wires) psitively. Bth will turn the FET ff by pushing (r pulling) the electrns ut f the silicn. Think f the rubber hse again: either yu can squeeze it shut with fingers, r yu can lwer the pressure f the whle water circuit, and the hse will be cllapsed by "suctin" (by air pressure, actually.) What are FETs gd fr? Well, mst mdern cmputers are cnstructed almst entirely frm FETs. The megabytes f memry are frmed frm little grids f millins f micrscpic FETs, each with a net-charge stred n its gate lead signifying a zer r a ne. The prcessr chips are built f lgic switches with Gate vltage as their input, and n/ff switching as their utput. Other things: super-fets can be built which actually cntain many thusands f small FETs hked in parallel. These VFETs r HEXFETS are ften used as the main transistrs f large stere amplifiers. A tiny vibrating vltage n their gate lead can rute many amperes f sund-frequency charge flw thrugh the ludspeakers, and a handful f FET wafers the size f yur fingernail cntrl the audi pwer fr a whle rck cncert.