Repair Keyoard

7/17/2019 Repair Keyoard

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This instructable will guide you through diagnosing and possibly repairingof a USB keyboard.

In today's throw away society, we tend to run out and buy a cheap $1keyboard to replace our pre!ious high "uality keyboard. This is fine as ate#porary solution, but e!entually we're going to want that "uality feel andfunction back.

ost of the ti#e, "uality keyboards die because of abuse. %ot necessarilyintentional, but abuse nonetheless. & few drops of any be!erage with acid

it in will surely cause an e!entual failure. This would include al#ost anyuice (#ost are )!ita#in fortified) which included citric acid* or soda.

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Step 1: Supplies
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e're going to need dri!ers for our specific keyboard. ost need only a

s#all -hillips screwdri!er. So#e, such as this one, #ay also need a tiny&llen or Tor dri!er. I ha!e a kit that has about e!ery tiny dri!er one woulde!er need... It's handy to ha!e.

e'll also need an /h# eter or ulti#eter and a co#puter. I'# using aac here, but any 0o#puter will do. e'll also need an /nScreen2eyboard for testing.

3astly, we'll need about 45 inches of 67 guage insulated wire, co##onlycalled wirewrap wire and a conducti!e pen (a!ailable fro# +adio Shack*.

Step 2: Prepare the Computer.


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The first thing we need to do to is diagnose the keyboard. 8ind out what'wrong with it. /therwise, we'd ha!e no hint as to what to fi.

/n a ac, we need to enable and bring up the 2eyboard 9iewer. /pentheInternationalpreferences pane located in System Preferences(under

the &pple #enu*. -lace a check#ark net to Keyboard Viewer. 0lose thepreferences pane. %ow, we can select the 2eyboard 9iewer fro# the 8lag#enu.

/n a indows #achine, go to the Start #enu, point to All Programs,point toAccessories, point to Accessibilityand finally select On !Screen Keyboard . %ote: & #essage bo with a link to #oreinfor#ation about the /n;=?@Screen 2eyboard #ay appear. To closethe bo, select OK.

Step 3: Test the Keyboard


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-lug the keyboard into an a!ailable USB port. It's okay to lea!e anotherkeyboard attached. The co#puter can see both si#ultaneously. To pre!ententering erroneous data into an i#portant docu#ent, we #ay want to open

a new tet file in TetAdit, inord or our fa!orite tet editor...

Starting fro# any corner, start pressing one key at a ti#e and note that thesa#e key displays grey on the screen keyboard. -roceed testing e!ery keyand note those that don't respond. /n this keyboard, keys '#inus', 'leftbracket', 'se#icolon, and Space failed.


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%ote how the bad keys follow a pattern. This is indicati!e of a failure in thekeyboard #atri. ust one trace would suffice to cause this failure.

Step 4: Disassemble Keyboard


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Cisasse#ble the outer case. This consists of re#o!ing three &llen screws

fro# the underside of the keyboard. The keyboard asse#bly then lifts outof the base.

%ote that two ribbons are co#ing fro# the keyboard asse#bly andconnected into the keyboard encoder circuit. +e#e#ber that keyboard#atri fro# the pre!ious stepD These ribbons are the E and F aes of that#atri.

Cepending on the #anufacturer of our keyboard, these ribbon cables #aybe latched in or held with a pressure connector. In this case a pressure

connector was used. To re#o!e these, ust pull on the tabs on both sideson the cable. -ull straight back to a!oid da#aging the cable. /n thelatched type connectors, we would need to open the latch on both sides ofthe connector. The ribbon should then slip easily out of the openconnector.


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+e#o!e all screws holding the encoder board into place. e need toaccess the )solder side) of the board.

Step 5: Construct Test umper


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%ow we need a u#per wire to test the keyboard encoder.

Si#ply cut a 45) length of 67 guage wire and strip about 1G6Hnd to 1G14thof an inch fro# each end. Using your finger and thu#bnail, cur!e one endof the eposed wire to for# a tiny hook. This will help us hold that end inplace while we probe with the other end.

Step !: Test the "ncoder #oard.


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Using the u#per wire fro# the pre!ious step, we will test the 2eyboard

Ancoder. If this test is successful, our chances of successfully repairing thekeyboard ust went through the roof. & failure here would nor#ally #eanthe keyboard's a doorstop.

Aa#ine the two connectors. /ne will be s#aller than the other. Since thefailing keys are positioned !ertically fro# each other, the failing line willlikely be one fro# the larger connector. If the failed keys had been acrossthe keyboard, we'd look for the bad line on the s#aller connector. e willuse the hook on the suspect connector and use the other to test it.

Using one hand, hook the hooked end of the u#per onto pin one (it shouldbe #arked* and hold it reasonably tight. Too tight, we'll straigten the wireand ha!e to fi it again. Too loose and it falls off the pin.

%ow carefully drag the other wire across each side of the other connectorspins, watching for flashes in the Screen 2eyboard. -ay particular attentionto the failed keys.


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If you see any flashes at all, that line on the hooked connector is good. If itturns on the 0aps 3ock key, touch that pin again to turn it off. This way wea!oid shorting out the 0ap 3ock 3AC.

-roceed to #o!e the hook to pin nu#ber H and retest. 0ontinue with eachpin of the suspect connector. If they all pass but none of your bad keysappear to be pressed, test again then re!erse your wire and test the otherconnector the sa#e way.

If any of the lines produce no reaction at all when u#ped %ote that line. It#ay not be a proble#, but a ground line for the asse#bly. But then is couldbe our entire proble# too.

hen you do see one or #ore of our proble# keys appear during this test,#ark down the nu#ber of the hooked pin that it appeared on. +echeck thatline to !erify that all of the proble# keys appear when u#pered withdifferent lines on the other connector.

In this case, the proble# keys all appeared when testing line 15 (of 1J* onthe large connector. This is goodK It #eans a lot #ore work, but theproble# is in the #atri itself and probably fiable.

If the bad keys did not appear while testing either connector than the

Ancoder chip is bad. e could carefully ea#ine that all the traces on theAncoder Board are intact and repair any break we find. Then test again.e're not going to go into detail of that rare proble# because it isn't thesituation here.

Step $: %n to the Keyboard &ssembly


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/kay... In the pre!ious step, we deter#ined that pin 15 of the large

connector is the culprit. That trace is da#aged so#ewhere within the2eyboard &sse#bly. e now need to disasse#ble the keyboard asse#bly.Take care that there are a lot of little pieces in this asse#bly. Con't looseany of the#K Use a parts bo, dish or other organiLed container to holdparts.

There are 66 screws on the #etal plate of this keyboard asse#bly. This is"uite typical of keyboards. &ll of the# ha!e to be re#o!ed. There are threeadditional raised screws that should be left alone. These )etra) screws actto ground the plate with the encode board when the keyboard is

asse#bled.

hen you re#o!e the screws #ake sure that the keyboard is laying on astable surface. Con't use your lapK

/nce all of the anchoring screws are re#o!ed, carefully lift off the plate toepose the fleible printed circuit boards. These are the actual #atri of


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the keyboard. 3ook carefully for any discoloration that would possiblyindicate the proble# area.

hen we lift off the #atri take great care. There is a tiny fleible cap ate!ery key position. These #ust be collected and stored safely while wework.

e can set aside the keys after we'!e put away the little caps. e onlyneed to work with the #atri sheets.

Step ': (ollo) and Test the #ad Trace.


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Fes. e already spotted where we think the proble# is, but we could be

wrongK %ote that each ribbon connector lead to opposite #atri sheets.There's an unprinted sheet between the# with holes at each key position.Turn the sheet so the suspect connector is on top.

%ote that the three sheets are attached together. C/ %/T SA-&+&TATMAK e can pull the# apart where they aren't #elted together, but if webreak the bond(s* they will ne!er align properly again.

8ollow the trace fro# the ribbon connector that we noted fro# the Ancodertest. It should e!entually lead to one of the offending keys. &ctually it will

lead to all of the#, but we're satisfied when we co#e to the first one withthe proble#.

In this case, that line also led to the &sterisk (N*, -lus, inus and Anterkeys on the %u#eric 2eypad before leading to the offending Space key.Since we had no proble# with the afore#entioned keys, we can assu#ethe proble# is in the trace so#ewhere between the Anter and Space


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keys.

The pre!iously noted discoloration happens to be right on this traceK So weknow ust where to test.

Using our /h# eter, #easure the resistance between two pre!iousknown good key points. In this case, it reads about oh#s. %ow #easurefro# the last known good key point to the first bad key point In our case,this reads about 52 oh#s. FepK The trace is badK

Use a cotton swab dipped in clean water and lightly clean the bad area.3et it dry co#pletely.

+e#easure the trace fro# before the bad spot to after the bad spot. Mere

we read about O4 2 oh#s. This needs repairKStep *: +epair the Trace


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Using the 0onducti!e Ink Cispenser, carefully dab the ink fro# a good spot

of the trace before the proble# area, o!er the proble# area and into agood spot after the proble# area.

Con't )draw) it on like an ink pen or felt #arkerK The li"uid co#ponent inthis pen will dissol!e the original tracing #aterial and break it if you scratchacross it. ust dab lightly to co!er the area. &lso be careful not to get toonear another trace.

Use a weight to hold the other sheets away fro# this area for at least 17#inutes to let it dry.

&fter 17 #inutes, test across the proble# area. Con't #easure on thepatched area, but the original traces before and after the patch. e now#easure about H oh#s. I'd call that a successK

%ow for the hard partK :*


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Step 1,: +eassemble the Keyboard &ssembly


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+eturn the keyboard to the solid surface and carefully place the fleible

caps at e!ery key position. %ote that so#e keys on the edges are pushedup by the weight of the asse#bly. ake sure that the little nub on the capis in the key's hole. /n the others the caps si#ply need to fit the recess.ake sure that e!ery key has a capK If we're #issing any, get on our kneesand find the#K They're easy to bounce away.

Take etre#e care that there are no cat, dog, hu#an or whate!er hairsanywhere around the caps or onGin the #atri sheetsK If there are, cleanout the hairs with that wet cotton swab.

%ow we need to place the #atri sheets eactly in the right position. %otethat there are a nu#ber of )keys) that appear as raised plastic on the 2ey

&sse#bly. These will pass through #atching holes in the #atri sheets.&lso note the position of the 0aps 3ock 3AC. There will be dark contactson the #atri sheets that will line up with the 3AC perfectly.


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9ery carefully place the #atri sheets in the proper position. Con't slide itaround or we'll likely #o!e our caps fro# where they're supposed to be.

Step 11: +eattach the #ac- Plate


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0arefully place the backplate o!er the atri Sheets in the correct position.

%ote how the anchor holes line up.

This plate was attached with )selftapping) screws. 8or this reason, we#ust align the threads before feeding the screws in. e do this by turningthe screw counterclockwise until we feel the click of the threads lining up.Then carefully turn clockwise to screw in the screw. If the screw starts totip, pull back and realign the screw. &ny crossthreading (cutting newthreads* could destroy the keyboard.

%ote how the holes in the plate are elongated. when we're done, they

should all be roughly centered.

Start with a corner hole and screw in the screw #ost of the way. e wantall the screws slightly loose to do our final align#ent. -roceed to theopposite corner and insert the screw (#ost of the way*. %ow do the othertwo corners. 0ontinue adding screws at least two holes away fro# thepre!ious screw until they'!e all been placed.


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&s you're adding your screws, note where the plate see#s to ha!e thelargest gaps fro# the screw hole. &dd your net screw there. This waywe're balancing the plate as we add screws.

Step 12: Test the +epaired Keyboard&ssembly


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0arefully reattach the 2eyboard &sse#bly to the 2eyboard Ancoder board.

%either has to be screwed down... e're ust testing.

0heck e!ery key and #ake sure it works. If any new keys don't work or twokeys press each other, you'!e got a hair in thereK +e#o!e the screwssurrounding the proble# area to the side of the keyboard. Mold it slightlyopen on the edge and blow hard into the space. Con't spitK e really don'twant to start o!erK :* %ow, put the screws back in.

/nce the keyboard tests okay, disconnect the 2eyboard &sse#bly andtighten the screws using the sa#e e!ery other screw techni"ue. This

process aligns the tor"ue across the plate, #uch like tightening a car'swheel.

Step 13: +eassemble the Keyboard


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%ow reinstall the keyboard encoder, #aking sure the tension relief on the

USB cable is correct.

+eattach the 2eyboard &sse#bly cables and finish putting the keyboardback together.

/ne last test, of e!ery key and we're ConeK

-at yourself on the back for a ob well done.

HOW TO HACK A KEYOARD

Mow to hack a USB keyboard or any keyboard for that #atter. Send inputs into the co#puter

without a pesky #icrocontroller.

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Step 1: %pen up the -eyboard
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/pen up the case. Fou can use a screwdri!er or an any ridiculous looking

#ultipurpose tool. So#eti#es e!en after all the screws are re#o!ed thekeyboard still won't open up. In this situation, don't worry, ust forcefully prythe case apart with your screwdri!er. It doesn't #atter if you break thecase. Fou don't really need it.

Step 2: Trace the letters bac- to the pins


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The inside of the case should ha!e two plastic sheets (one on top of the

other*. /ne plastic sheet will ha!e printed conducti!e tracings that go toone set of pins on the circuit board and the other sheet will ha!e tracingsthat go to another set of pins on the circuit board.

hen you press down on a key, the tracings on both sheets will toucheach other. This co#pletes the circuit and tells the board to send a letter,nu#ber or co##and back to the co#puter.

hat you need to do is to label each conducti!e circle on both sheets withthe letter key that corresponds to it. So, for instance, if you were to press

down )&) on the keyboard, label the plastic circle on both sheets of plasticthat would be pushed together if the )&) key would strike (see picture oflabeled sheets*.

/nce you ha!e labeled both plastic sheets with all of the correspondingkeys (that you plan to use in your proect*, the net thing you need to do isto trace the letters back to the circuit board with a Sharpie (see notes on


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pictures*.

/ne way to si#plify the #atter is to consider one sheet )SICA &) and theother )SICA B) .

Below is a chart that I #ade for the particular keyboard I was working on.-lease re#e#ber that all keyboards are different. It will help youi##ensely if you #ake a si#ilar chart for the keyboard you are working on.

-I% 3&F/UT:

P P P P P P P P EEEEEE P P E P P P P P P P P E1H6Q 4O5 1H 11 17 J5O4 Q6H 1

SICA & SICA B

-I% SICA &

&71 7, J, 5, O, Q, 6, H, 1

&7H 4, , &76 %, B, D(slash*&7Q (period*, (co##a*, , 9, 0, E, R, (enter*&7 M, , (space*, '&74 3, 2, , 8, C, S, &, &7O F, T&75 -, /, I, U, +, A, ,

-I% SICA B

&7H 1, , &, R&76 H, , S, E&7Q 6, A, C, 0&7 Q, , +, T, 8, , 9, B&74 4, O, F, U, M, , %,


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&7O 5, I, 2, (co##a*&75 J, /, 3, (period*&7J -, 7, D, , ', &11 (enter*&1H (space*

Step 3: & note on pin layouts and shitre/isters


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Basically a keyboard is a specialiLed shift register. It takes in a lot of inputsand sends out one output to the co#puter (that being a an &S0II code orcontrol co##and*.

hen a switch is closed, the shift register processes which two pins areconnected and interprets it as one particular output.


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So, if you had ten pins going to each plastic sheet, then you would ha!eonehundred possible co#binations. This is because e!ery single pin onone sheet can be co#ined with e!ery single pin on the other side. Thiswould produce ten rows of ten possible co#binations. In other words, youha!e ust produced a )17 E 17) Hdi#ensional array.

8or instance, if you connect )-in Q) on SICA & and )-in 4) on SICA B youwill produce the letter )) on the co#puter (see picture*.

If you connect )-in 5) on SICA & and )-in O) on SICA B you will producethe letter )I) on the co#puter (see picture*.

It's really "uite si#ple.

Step 4: &ttachin/ )ires


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So, now you know which pins produce which letters. %ow you need to

connect your own #ad creations to the boards.

But waitK

Before you can connect things to the board, you need to connect wires tothe boardK

Sigh...

Fou're going to need:

wire (ideally in a nu#ber of colors*"uick setting epoy (fro# your local +adioshack or Mo#e Cepot*a raLor bladea soldering iron

So, here is what you do. 0ount the nu#ber of pins you need to attach


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wires to. /nce this is done, prepare all of the wires you're going to need.They need to be stripped on both ends about a "uarter of an inch andshould be about fi!e to si inches in length. Try to use #any differentcolors of wire so you can tell the# apart later. /n one end, bend thestripped part of the wire so that it can hold itself to the board (see picture*.

rap the wire around the board so that the stripped part is touching oneand only one of the pins that it needs to be connected to. ake sure noneof the wires are touching. hen all of the wires are in place (touching thepins and not touching each other*, you're going to need to glue.

-repare so#e epoy and co!er the back side of the board in epoy so thatthe wires are glued in place on the side opposite fro# the conducti!e pins.3ea!e it for how e!er #any hours the epoy says it needs to sit for for

#ai#u# strength.Siteen to twenty hours later or so, you're going to need to solder. So,solder the wire to the pin. If the solder won't stick to the pin, scratch the pinwith a raLor blade a couple of ti#es and try again. If the pin is coated withso#ething, scrape off the coating with a raLor blade and then solder to it.

If you ha!e a lot of #oney, ust glue the wire to the pin with conducti!eepoy !ery, !ery, carefully.

hen e!erything is dry and in place, test the keyboard. If it works, thenyou're #ore less done.

-ut it in a nice case or put it in a radioshack case. It doesn't #atter.

Step 5: %ther -eyboards and considerations


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So#e keyboards ha!e sockets. If it does, it sa!es you a lot of work. hat

you need to do is get your own socket or set of header pins and solderwires to each connection. /nce you ha!e a socket or header pins withwires attached, plug it into the socket on the board. Test it to see if it worksby touching a wire on each side together. If it works, glue the header pinsor socket into the socket on the board and you're done. It's so si#ple toconnect, in fact, that you #ay not e!en want to bother tracing the plasticsheets and ust try to figure out the key co#binations by trial and error.

&lso, there are ac USB keyboards that allow you to connect other USBde!ices to the co#puter through the#. I wish I could tell you so#ething

#ore enlightening about the USB connections on those boards, but I can't.aybe you can do so#ething profound with the#. /therwise, you can ustattach wires to it using the #ethod shown in Step Q.

other considerations:

The USB cable has a tendency to rip off the shift register board. Fou #ay


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want to glue it in place.USB de!ices are ! 177#aSo#e boards ha!e 3ACS attached (see picture*. Fou #ay be able to senddata back fro# the co#puter to light the# up. If you can control the 3ACS,then you can attach low !oltage relays to the# and ha!e outputs as well asinputs. I ha!en't tried to figure it out yet, but if you want to gi!e it a go, agood place to start #ay be here:

http:GGwww.beyondlogic.orgGkeyboardGkeybrd.ht#

Step !: %ne step beyond0
http://www.beyondlogic.org/keyboard/keybrd.htmhttp://www.beyondlogic.org/keyboard/keybrd.htm


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/nce you ha!e a hacked keyboard you can use it for a nu#ber of

functions and attach a nu#ber of different types of switches.

Fou can build your own typewriter keyboard. 0heck the picture and !ideo.It #ay not autoload and it #ay take a long ti#e to load when it does (it'saround H7 B*, but here is the !ideo of the typewriter:

http:GGa.parsons.eduGrandyG!ideoGtypewriter.#o!

Fou can use a photocell as a switch (as seen in the picture and !ideo*.

Fou can hook it up to a capacitance sensor and use ust about anything totrigger an e!ent in a 8lash #o!ie.

Fou can hook it up to so#e floor switches and de!elop your own CC+ga#e.

Fou can do #ore things than I could e!er drea# up.
http://a.parsons.edu/~randy/video/typewriter.movhttp://a.parsons.edu/~randy/video/typewriter.mov


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Build an ohmmeter

A very simple circuit to measure low resistance values from 0.001 up to 1.999 Ohm.

With a "Direct Resistance Readout in Ohms". You must use two separate atteries.

One for the D!! and one to supply power to the !#1$%. & recommend the

!#1$%' which is the 100 mA' (0)9* version of the normal !#1$. +ut you can also

use the !#1$' in the (0)**0 pac,a-e' if you want. (he trimpot must e set

precisely to deliver 100.0 mA out to -et truly accurate resistance measurements. o

you need a very accurate !illi)Amp !eter to ad/ust this orrectly. And li,e Any (est

23uipment' (his aliration should e Re)hec,ed once a year or so.4


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55 !y aliration !eter is a 6 17* Di-it D!!' "o 100.00 mA settin- apaility".

(he !eter eads !8( onnected D&R2(Y Across the Resistor or Wire to e

(ested. DO O( O2( &( to the lip On eads' that connect to the Resistance

under test' As this will -ive :A8(Y Readin-s' as a Result of ontact Resistance.

You ould use a D!! instead of this ;anal !eter' +ut you will ;roaly ose One

Decimal point of ensitivity.

(he :ront Decal was created in a small ad ;ro-ram and Reverse ;rinted on

"(ransparency :ilm".

(han prayed with "#!


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AW> +are opper wire that was aout 1? inches lon-' "ut e@actly 1* inches lon-

etween my !eter (est eads". And it measured 0.01? Ohms. oo,in- at my opper

wire hart' it tells me= ** AW> has 1?.6? Ohms per 1000 feet. o that is 0.01?6?

Ohms per :oot. o & A! A;;Y with this resultB

& could also create a !illi7!icro)Ohm Resistance !eter' +ut the real ;rolem would ethe Accurate aliration of the current source.


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Low-Ohm Meter

Back to the Britih Amateur Electronic Clu!"

If you need a rough check on the value of a resistors the ohms ranges of your

analogue multimeter will provide it, so long as the resistance value is between

10 ohms and one or two megohms, but the non-linear scales reduce the

accuracy of the higher readings. A digital multimeter will provide greater

accuracy - perhaps 1% but neither meter can be used for values below 1 ohm. It

may be argued that one does not very often need to measure such values, but

the emitter resistors of output transistors in power amplifiers fall into this

category, and so do many coil and transformer resistances. It is also useful to be

able to check the resistance between switch contacts, especially if you suspect

that a switch is faulty.

here are a number of ways of measuring low resistance, and for many years I

used a shunt ohmmeter with mid-scale readings of 10 and 1 ohms on its two
http://baec.tripod.com/newsletter_index.htmhttp://baec.tripod.com/newsletter_index.htm


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ranges. I described this in a workshop article in the !anuary 1"#$ ewsletter

&o. '().

*n the ohms ranges of a normal multimeter the unknown resistance is

connected in series with a battery and the meter and the scale reads backwards.

A variable resistor is included in the circuit so that the meter can be ad+usted toread full scale with the test terminals shorted &ig 1). In the shunt ohmmeter a

battery and variable resistor are connected across a milliammeter and the

resistor is ad+u ted so that the meter reads full scale with the test terminals

unconnected &ig.(). Any resistance across the test terminals will bypass some

current so the meter reading will fall. o commercial meter that I have

encountered uses this circuit. In my case the shunt ohmmeter uses the two

lower ranges of a three range milliammeter &ig ).

In April 1"#1 a design for a low ohmmeter by ay /arston was published in

lectronics oday International. I built a modified version of this meter and

found it very satisfactory. It will measure resistance from 100 ohms down to a

few milliohms, and has a linear scale. A short while ago I was asked about

measuring low resistance by a 2.A..3. member and recommended this circuit

to him. I thought it might interest other members so I obtained permission from

the editor of I to publish details in the ewsletter - and here they are.

he meter contains two independent and independently powered circuits4 a

multi-range constant current generator and a 5.3. millivoltmeter with a full

scale sensitivity of 10m6. he generator is used to apply a known fi7ed current

to the resistor being measured, and the millivoltmeter measures the voltage

developed across the resistor, eliminating the effects of the test leads.


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he complete circuit is shown in ig $. A 86 regulated supply feeds the

constant current generator and the output test current is determined by resistors

1 to $. *n each range the value of 7 is very low in relation to the currentlimiting resistor, and the full scale test voltage &l*m6) is very small compared

with the 86 regulated supply. As a result, the test current is virtually

independent of the effects of lead resistance9s etc.: on the most sensitive range

&l**m6) one ohm of lead resistance will introduce a ma7imum full scale error

of (%. *n the other ranges the effect of lead resistance is negligible. In

practice, the reading errors are primarily determined by the accuracy of the

resistors 1 to $.

he 5.3. millivoltmeter is based on a 3A1$0 operational amplifier, which can

respond to inputs down to ;ero volts. o allow the output to go slightly negativefor ;ero setting, a -


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he =32 track layout is shown in ig.8, and ig.< shows the componentpositions I used a 1> multiturn pot for 6( but the commoner si;e is ?,

which would re@uire slight repositioning of the pads for this component. 1 to

$ can conveniently be mounted on 1.

If batteries are used to power the meter, 21 must be capable of providing a

current of l00mA on the lowest range, and a ==" is recommended. he op-amp

current is only around 8mA so here a == would suffice. o power my version

of this meter I use a power supply which provides 86 and "6 regulated

supplies: in this case all the components to the left of the dotted line in igs. $,

8 and < can be omitted. he 86 supply should be connected at the pointsmarked 7.

he meter / should have a full scale reading of 16: the 1v range of a

multimeter is perfectly satisfactory. If the meter has a sensitivity of 10BC6 or

better, " should be l0B as shown: for lower sensitivities " should have twice

the ohms-per-volt value of the meter.


92/99

he various fle7ible leads from the ohmmeter should be terminated

appropriately. hose for connection to 21 and 2( should have battery clips &or

suitable plugs if a mains power supply is used). he / leads can have plugs to

fit the meter sockets. =robably the simplest way to terminate the 6 and I leads

is to use crocodile clips, though these are liable to come off at a vital moment.

Dhen construction is finished, connect batteries or power supply to the unit and

connect the / leads to a 16 5.3 meter or multimeter. hort the 6 leads

together, and ad+ust 6( for a ;ero reading on the meter. hen turn 1 to the

100 ohm range and connect the I leads across a 100 ohm resistor &1% if

possible). 3onnect the 6 leads across the resistor on the resistor side of the I

leads, with like polarities, and ad+ust 61 to get a full scale reading on the

meter. he unit is now calibrated and ready for use on all ranges.

ake a orkbench ulti#eter 0ircuit ith the I0 OQ1-osted by hit#anTesting and troubleshooting electronic proect circuits re"uires a #ulti#eter, sowhy not #ake the circuit of a ho#e#ade #ulti#eter itself as your net electronicproect. Interesting ho#e#ade circuits like an /h##eter, !olt#eter, a##eter arediscussed here using the I0 OQ1 and ust a few other passi!e co#ponents.

&lthough #ulti#eters are a!ailable plentifully in the #arket today, but buildingyour own ho#e#ade #ulti#eter can be real fun. oreo!er the attributes in!ol!edcan beco#e thoroughly useful for the future electronic circuit building and testingprocedures.

"irc#it Ill#strations
http://images.brighthub.com/4d/9/4d987dceb6e7d76d4353b3157a401f5a595afb2f_large.jpg


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& si#ple configuration for #easuring C0 !oltages is shown below using the I0OQ1. & couple of resistors + and +y are introduced at the input in a potentialdi!ider #ode at the nonin!erting pin V6 of the I0. The !oltage to be #easured isapplied across the resistor +1 and ground. Through proper selection of $% and$y, the range of the #eter can be !aried and different !oltages can be

#easured.

In case you want to #easure alternating !oltages then the circuit illustrated belowcan beco#e useful. The wiring is si#ilar to the abo!e wiring, howe!erthe positions of $% and $yha!e changed and also a coupling capacitor co#esinto the scene at the in!erting input of the I0. Interestingly the #eter here is nowconnected across a bridge network enabling the #eter to display the rele!ant &0potentials correctly.

&nother circuit to #easure Cirect current or ps using the I0 OQ1 is shownbelow. The configuration looks pretty si#ple. Mere the input is applied across theresistor +L i.e. across the nonin!erting input pin V6 of the I0 and the ground.The range of the #eter can be si#ply !aried by changing the !alue ofthe resistor $&.
http://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://images.brighthub.com/67/4/6740af5242a312b1b083a7e796b7277e33ba2258_large.jpghttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspxhttp://www.brighthub.com/hubfolio/swagatam-majumdar/blog/archive/2010/12/27/homemade-multimeter-parts-list.aspx


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+esistors are one of the #ost i#portant passi!e co#ponents which ine!itablybeco#e an integral part of e!ery electronic circuit. & circuit #ay be !irtuallyi#possible to build without acco#panying these a#aLing current controllingde!ices. ith so #any resistors in!ol!ed, a possible fault can always be on thecards. Identifying the# re"uires a #eter W an /h# #eter. & si#ple design usingthe I0 OQ1 is shown below ust for the purpose.

Unlike #ost of the analogue designs which tend to ha!e a rather nonlinearbeha!ior, the present design !ery efficiently tackles the proble# to produce aperfectly linear response with the corresponding #easure#ents. The range ispretty i#pressi!e, it can #easure !alues of resistors right fro# 12 up to a
http://images.brighthub.com/8e/6/8e6b960603f299bb7e2a2df6501925538e6c6dff_large.jpghttp://images.brighthub.com/05/3/053761844c2404c4b7d5f2c31018bacf50bcc6c6_large.jpg


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staggering 17 . Fou #ay go on to #odify the circuit for enabling the#easure#ent of #ore etre#e !alues.The range is selected by #o!ing the rotary switch switch into therele!ant positions. 0alibreating th instru#ent is si#ple and is done with thefollowing points:

&dust the selector switch to the X172Y position.Tri# the base preset of the transistor until its e#itter !oltage shows eactly 1 !olt(#easure using a digital #ulti#eter.*%et, 8i an accurately known 17 2 resistor into the #easuring slot.

&dust the tri##er associated with the #o!ing coil #eter until the #eter shows afull scale deflection.

&ll the circuits discussed abo!e use dual supply !oltages. The #eter used is a#o!ing coil type and is specified as 1#& 8SC.The preset across the pins 1, Q and of the I0 OQ1 used for this ho#e#ede#ulti#eter is used for adusting the initial condition #eter to eactly Lero.

$ele'ant Val#es of $% and $y

The following are the !alues of the resistors re"uired for !arying the range of therespecti!e #eters.C0 9olt#eter++yeter 8SC17121 29171721779171772179J772177219%I317727.19

C0 &ATA++Leter 8SC7.11&1177#&1717#&1771#&12177u&17217u&17721u&

&0 9/3TATA++y+eter 8SC

172171291772171779117179111911772177#9117217#91121#9


96/99

& reuest rom one o the -een ollo)ers o this blo/:

Hi Swagatam

Is it possible to design a small circuit module which can be used with amultimeter to measure minimum/maximum voltage of a uctuating signal at any

point of a circuit under observation.

For example, we can switch a toggle switch in our module at I! position andmeasure the voltage at point "#$. %he volts shown by the multimeter would be&'()S% voltage of the signal. #nd when the toggle switch is positioned at #*,and the voltage is measured again at point "#$ the meter will show the HI+H)S%voltage of the signal.

The Desi/n

This is a simple ohm meter that is small to build. The circuit operates

with a constant current around T1. The current depends on the emitter

through S1 can be chosen. These are the ranges of the meter. The

constant current through T1 causes a voltage drop across Rx, themeasured resistance. This voltage is measured by M. D2 is a

germanium diode. You can try a 90 or OA 118 using AA. The resistance

value is the range times the meter reading in volts.visit page.
http://www.circuitsonline.net/schakelingen/27/meten-en-testen/ohm-meter.htmlhttp://3.bp.blogspot.com/-Arhe1p_iSyg/UfiY6PtD4CI/AAAAAAAAE6Y/foXMxMpJFGI/s1600/741+voltage+tester.pnghttp://www.circuitsonline.net/schakelingen/27/meten-en-testen/ohm-meter.html


97/99

R1 = 1.5 kOhm R2 = 820 kOhm R3 = 82 kOhm R4 = 8.2 kOhm R5 =

820 ? D1 = 9.1 V zener D2 = germanium diode T1 = BC 547B M =

meter 0-15 V

/h# eter

he circuit diagram of the ohm meter in this site is very usefull for measuring

the low resistance range form 0 to 1 and and 0 to 10. Eou can ad+ust the range

according to your wise. he circuit for a low *hm meter described here is

simple and has the following advantages over other meters4

1. 5o donFt need to see it again and again +ust set it once and forget it forever).

(. cale reading capacity of this circuit is from ;ero to a fi7ed value rather than

infinity.

. his meter is low power consuming as it uses a 1.8-volt penlight cell, two

scales &0-1 ohms and 0-10 ohms) over a dial and a push-to-on switch large

power consumption by the circuit.

=art Gist


98/99

RE#$#TOR#

1 ('B

( .B

.B

$ 0B

61 100 *H/

D$ODE#

51 1$001

5( 1$001

3I3I 5IAJA/

*hm meter 3ircuit

3I3I 53I=I*
http://www.electronicsproject.org/wp-content/uploads/2012/04/11i.png


99/99

he circuit diagram that you can see below is the ohm meter which can

measure the resistance for 0 to 10 ohm . Eou can see the selector switch over

there the circuit diagram which can select the measuring rang form 0 to 1 ohm

and 0 to 10 ohms. ransistor 1 works as a constant current generator which

passes a know current through the resistors which resistance is to be measured.

If the ma7imum drop of the voltage across the emitter of the transistor 1 willbe more than 100 m6 and the ground is displayed on the meter whose internal

resistance is much higher than the testing resistance that is 10 ohms. 2ecause of

which this ohm meter can not load the circuit.

here is a diode 5 across the micro ammeter which is use to protect the oh

meter form the overload during the the absence of the testing resistor which

resistance is to be measured.

esistors 1, 61 , ( , , 51 , 5( and $ are biased by the transistor 1.

5iodes 51 and 5( are use for holding the bias level constant inspite of the

decaying battery.

he scale of the meter in this pro+ect should have 0-800 KA . he shunt

resistance in this pro+ect can be any general propose meter. ransistor 1 is the

silicon npn with a high gain factor.

ow the meter should be ad+ust by shorting probes A and 2. If the meter is

ad+ust before the it shows a ;ero resistance. Eou only have to ad+ust in 0 to 10

ohm scale first and other ad+ustments will be follow automatically. his can be

easily built within a few minutes. his is the very useful pro+ect for the

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