Oletko ajatellut koskaan, mitä sähkön liikkuminen materiaaleissa oikeastaan merkitsee? Miksi jotkin asiat johtavat sähköä ja toiset eivät? Tässä harjoituksessa pääset tutustumaan sähkön johtumiseen tarkemmin. Sähköä johtavillakin materiaaleilla voi olla erisuuruisia resistansseja eli kykyjä vastustaa sähkövirtaa.

Tehtävä on aika lyhyt mutta kätevä: saat käyttöösi anturin, jolla voi mittailla monenmoista mullan kosteudesta banaaneihin. Jos muistat mitä teit edellisessä tehtävässä (hämäräkytkin), pääset tosi helpolla: käytännössä kyseessä on sama kytkentä. Valovastus vain korvataan kahdella johdolla.

Ohjelmointivaiheessa näytetään myös, kuinka mitatut lukemat voi muuttaa ohmeiksi - tällä systeemillä voi mitata vaikka Mehackit Maker Kitin vastusten, veden tai perunan resistanssin.

Alla olevassa videossa poltetaan kuvioita puun pintaan johtamalla suurta sähkövirtaa puulankun läpi. Puulankun pinta on sivelty suolavedellä. Näin vaarallisen suurilla jännitteillä ei tällä kurssilla pelata, mutta sähkön johtuminen näkyy videossa hyvin konkreettisella tavalla!

doublereno: Lichtenberg Wood Burning Blue Epoxy White Oak


okay so then I've got another one of my old cut-offs from the sawmill this is a piece of white oak and it's a it's a really thick piece not very big but it had a cool shape to it it's got really nice grain to it and it still got bark on the backside and my plans for this one I'm gonna burn it just a simple microwave oven transformer burn and I'm gonna burn it hot I want to get some good deep burns and then I'm gonna come back and I'm gonna fill this one with a colored epoxy and I'm gonna try something new I struggle with this the getting the excess epoxy off and cleaning up without having to clean up the excess epoxy without taking detail out of the burns so I got this pretty flat I got it sanded smooth and then I'm gonna try this this time I this is just a it's a stainless steel squeegee I got off Amazon so I'm gonna try that an actual squeegee to see if that helps me get hopefully the vast majority of the excess epoxy off so I'm gonna get set up and we're gonna burn it first and we'll clean it all out and then we'll put the epoxy on and we'll see how well the squeegee works okay I'll pretty soak this and I'm gonna put one more coat on it I'll be ready to burn you see how it's already discolored the wood so the microwave oven burner I can get it pretty wet and it's still burnt okay so I'm gonna just start out maybe from the corner to the middle there okay so I'm part in transformer on got the very icon and low turn the very icon start cranking it up real slow and once I've got a good bite on both of them I'll turn it back up to full speed I'm selling this what I want to I want a pretty deep burn okay go from that corner over to here so what we get very act back down low cranking it up okay let's see if they'll reach from all the way over there to there start cranking the very echo once I get that going I'm cranking it at the full power like so I want this to be relatively hooker [Music] okay when we get that wet I'm gonna get a little bit more down in the middle that's gonna be a little too less too little for what I want that suckers hot I'm gonna keep power a little bit lower out here I just won't kind of fill in that open area a little bit make one more adjustment there very good ended up in here and one less area you're up through they're gonna do okay that'll be good I'm gonna take it and clean it out and I'll bring it back we'll take a look at it okay here it is all cleaned out I think I've got some pretty good burns especially I got some deep ones it'll be good for that epoxy and you see that the baking soda solution really changes the color of the wood it brings the tannins out in the wood so I've got my wood bleach oxalic acid I mixed that with a teaspoon of the oxalic acid and one cup of water get it mixed up good and I just take an old disposable brush and just paint it on here and let it soak in and this will bring it back pretty pretty then close to the natural color of the wood he'll take that stain you know there remove some of it I guess the tannins that it brought up it will help reverse some of that bring it back to a more natural color of the wood give it more of an even look this white oak really changes colors with it it's not gonna bring it back to the white white color that it was before but it'll it'll help even it out and I'm gonna go ahead and hit a little bit on the tips here there's some of that stuff ran down the side so I'll go ahead and hit the tips in the sides nothing really to see there I'm just get it on all of that so anyway so I'll let that soak for a little while and I'll put two or three coats of this on here till it looks pretty even in uniform you can already tell it I mean it looks better it's already a dramatic improvement but I'll let it soak for maybe a minute or two I'll put another coat on there let it soak for a minute or two do that maybe three times and then I'll go wash all the oxalic acid too off real good we're just with clean water you want to be sure you get it all off after you put it on here because sometimes it will react with whatever kind of finish you have so anyway I'm gonna apply another couple coats on it I'll go wash it out and let it dry and then we'll bring you back and look at it before we get ready to put epoxy on it okay I got this thing all cleaned up I let it dry I came back with a 400 grit sandpaper sending it early smooth but didn't take any detail out not that I can tell so I'm ready to apply the epoxy to it so I'm gonna set this off to the side real quick I've got my crystal clear epoxy two-part epoxy that I always use I've got my I'm gonna use a pearl X paint pigment the turquoise color I got my little measuring cup so I can measure out equal parts of my epoxy and a couple mixing sticks so this is not going to take much epoxy at all so what I'm gonna do I've got increments on here of one ounce to ounce so I'm gonna mix two ounces which should be more than enough to do this with so I'm just gonna fill this up you can't see this on the video you said to trust me okay so there's an ounce of the hardener and this is the actual live resin so I'm gonna go and this is really thick here so you gotta just takes a little bit longer before so I'm gonna put announcer this in these little measuring cups are great because they let you get an accurate mix and they're cheap enough that when they're done you can just on boy you don't have to worry about trying to clean them out or anything like that as opposed to a regular regular top measuring cup so I'm gonna tell you one of my paint sticks stir this around a little bit get a kind of halfway mixed where I can see what its gonna look like and then I'll take my paint pigment and there is no exact science on this paint pigment I'm just gonna take that much and we'll dump it in there and mix it around and see how it looks if it looks dark enough to me then that's where I'll go with yeah I think that's plenty you can see that that's and it's pearlescent stuff is really cool let me start here for just a second you know show it to you better you see it's got that little like a pearl like a pearl paint on an automobile or something's got those little streaks and there that pearl look to it so it's pretty cool this stuff it needs to be stirred for a couple minutes to let at least begin to firm up so I'm gonna go off video and I'll let that get set up I'll get my board back here ready to start applying it get my heat gun my squeegee that we're gonna use on this one I'll get all that stuff ready and then we'll come back and we'll be ready to start applying the epoxy okay I got everything set up now I've got my epoxy mix and I stirred it for off and on for probably about five minutes it's still pretty thin it's not super warm outside so it's not gonna set up super quick but so what I'm gonna do I don't want to use my squeegee right off the back because I want to use that just basically for the final cleanup so I've just got this little stick I couldn't find my little yellow cards like I usually use so what I'm gonna do I'm gonna just pour a little bit in here and I'm gonna take this stick you know what else I'm gonna do I've got a good flat spot over here I'm gonna use that for my scrape off point and I'll show you that that's why I'm gonna do that a little bit later I'm trying to level this thing out just a little bit yeah I got a stick under that just to level it out a little so I'm gonna take this and use that to kind of spread it I'm gonna try my best to keep from running this off the sides cuz it's gonna get in that bark if I don't if I let it get off the sides it's gonna get in that bark and I don't want it in the bark the ends are not quite as critical as the sides sorry if I'm blocking your view on this I know that's kind of defeats the purpose a little bit when you can't see what I'm doing I'll try my best to not block the camera okay so that's pretty good at that field I'll just slowly scrape this off to the other side this blue is gonna look really good on this wood I think I am going to move around to the other side where I can see that a little bit better okay I'm just very gently trying to get that in all these holes piece of grass there this epoxy is easy to apply it's cleanup is the booger that's why you gotta kind of be as careful as you can to not get it on something you don't want it on because it does not come off easy once it's on there I'm just moving this excess down to the other end do a little bit more here that a fly that is biting my leg another piece so using a piece of wood is not a good idea because I'm getting splinters out of the wood in it I'll just be careful that I get all those cleaned up in one of the other things - then we have talked about this before you know I'm trying to be careful not to get it down the sides but I do want to cover the entire surface the top surface with the epoxy because anywhere this epoxy touches it does change the color of the wood a little bit so if you leave a spot like if I leave this spot right here uncovered it'll be a different color than the rest of the board even after I get it cleaned up so you got to be careful to make sure you cover the entire board at least with a thin layer other stuff just to keep that uniform look across the whole board this thing developed a few cracks in the end after I washed it and you know cleaned it off with the water and so those are going to end up with probably a little bit of epoxy on them too but I think that'll be alright I don't think it will cause me any problems some of these deep deep burns will really suck this again so you got to be careful when double-checked all the deep spots and make sure you didn't get a bubble in there that pops and leaves a void with the epoxy that one right there did that a little bit okay I think I've got good coverage here go back over it and look real good and a little bit of runoff right there on that one does the burn goes off the edge but that's okay okay all right I think I've got good coverage there now so come back around the other side here and I'm gonna take my little rubber squeegee and get off good okay now let's see how well this works I'm just gonna take that old painter's rag here you just swipe I take I'll go and wipe that excess off it's gonna need to be a disposable right because you're not gonna get that off it feels like it's really taking it off I mean it feels it's flexible more so than the things I've used in the past every swipe I'm getting a little bit more of it off almost nothing that time so I must have the majority of it and my goal here is I'm hoping we'll go a different direction well I'm hoping I can do here is get all of this excess epoxy off would it be such a micro thin layer the sandpaper will remove the rest of it easily that's pretty much it the squeegee worked really good I got a little bit of epoxy over here on the side but I think I can sand that off got very little on the ends the only thing I see I'll get a little bit down in the grain throughout here and that may or may not send out but if it doesn't I don't really care it actually looks kind of cool excuse me the blue looks great on this board so what I'm gonna do I'm gonna let it sit and dry overnight and then tomorrow I'll come back and we'll send it and see what it looks like and then we'll put a finish on it I'm thinking seriously about putting the like or finish on this because of that bark I think I can spray that bark on the other side with a lacquer and it'll help preserve that bark and keep it from falling off so anyway we're gonna let it dry and we'll come back tomorrow and look at it and sand it off and clean it and start putting the finish on it okay this thing is dried overnight it's pretty warm today you know but letting it sit in the Sun for a little while so I think it's dry enough that I can go ahead and try and remove the excess epoxy off came out looking really good I mean I really like the blue I definitely have a little bit of the blue tint you know in the grain outside of where the wood is too so I'm gonna give this a try normally if you watch my videos before I use a scraper it scrape the excess off but I mean I can see through here this is very very thin very few spots to the epoxy is really outside of the burn so what I'm gonna try we've got a 400 grit sandpaper and so I'm gonna try I'm just gonna pick a corner here first and you had to excuse the shadows or the shadows get bad I'm just gonna try standing them a little bit and see how well that takes that excess off I should have charged up my air hose before I did this that's the only way I'm gonna really kill I need to be able to blow it off I'm gonna pause the video for just a second and I'm gonna get my compressor turned on where I got a little bit of air and we'll see what that looks like blown off okay I got my hair now so let's see what this looks like still got a little bit down into the greens and I may not be able to get all this out of the greens but as long as I get the excess off I'm okay with a little bit chilling in the Greens I can sand it out and get it completely out but I'm gonna I'm gonna lose a lot of the detail of my burns which I don't want to do such a fine line on what you can do but you can't do to give this it you out this thing to the same paper it's gonna bind up really quick that epoxy will bind up your sandpaper yeah that stuff's pretty deep in those grains it's a relatively deep grain would you see like where that runs through here and here I don't think I'm gonna get all that out not without removing all the details on the burns which I definitely don't want to do and you'll see here what I'm gonna do I'm gonna come back and sand the rest of this off-camera it's gonna be boring for you just to sit and watch me hand sand this thing but uh right in here this a lot of that dust gets done in these fine burns I'll blow it out I get most of it up blowing it out but then I'll come back with some mineral spirits and wipe it out and it'll remove all the rest that dust and that detail rotted the very tips of these burns will come back so I'm gonna take it and I'm gonna finish sanding it down and then I'll bring it back and we'll put the mineral spirits across it let you see what it looks like with that on there okay I kind of sanded it down and like I figured I'm not gonna get all the blue out of the little grain marks right here but I think it's okay I think it's still gonna look pretty cool then I've got just plain mineral spirits here and then an old painters rag and I'll just go over this and that'll help remove out any more dust that's left down in there it actually gives you a pretty good little preview what its gonna look like finished now I'm gonna use the spray-on lacquer on this one and I'll use it on some other stuff I don't know if ever uses it on hearing burnings yet I've used on some other wood work that I've done you know like it takes forever to put on because you got to spray it so thin and it takes a lot of coats but you still end up with a really nice finish with it so I think that's still a pretty cool look so what I'm gonna do I'm gonna let that dry for a few minutes I'll go get my wife ready to go and then we'll come back we'll spray a coat of lacquer on it and see what the end result is gonna be okay I'll let it dry now got a nice smooth finish all the dust and stuffs removed and it's just a rust-oleum spray-on lacquer clear lacquer so I'm gonna lock coat of this on here okay let's think of the lacquer it's a easy to put on if you put any more than that on it you're gonna get run so it's not gonna look right so you just got to put multiple multiple coats on just like that so I'm gonna let that sit for a minute I mean it dries pretty quick I'll put two or three coats on and then we'll come back and take a final look at it I'm not gonna mess with finishing the bottom on this on the video because that's gonna take a while with all that bark but we'll mainly just be concerned with seeing what the top looks like then I'll finish the rest of it later and I'll post it on my Instagram account so there's links to my Instagram down on the in the description of the video okay there it is with I would say six or seven coats of the spray on lacquer use that right there and like I showed you the when I applied that first coat just real nice thin coats if you miss a spot you're better off just to leave it and catch it on the next round because if you get it too thick you make a mess but as long as you just stay with nice thin coats and apply frequently it comes out really nice I'm really happy with it I haven't done anything with the back yet but you see the blue looks great on that wood and you know I got the little streaks of blue through some of the grain in there but I think it actually looks pretty nice so I'm pretty happy with a squeegee idea the other combination that I'll probably try eventually is the the sanding sealer because the sanding sealer will eliminate this so if you use the sanding sealer first and then come back put the epoxy on and use the squeegee you'll have almost no cleanup I mean a real light sand with a 320 or 400 grit sandpaper will probably clean it up but I don't think I lost any detail it's hard to say after the fact but I mean they're still pretty fun detail on that so I'm real happy with it anyway I appreciate you watching check out my social media a Facebook page my Instagram page my twitter links to all thats down in the description of the video and then links to the products that I use in the video we're down there to appreciate you watching please like and subscribe and share my videos thank you [Music]


Mehackit boardin (tai Arduinon), USB-johdon, koekytkentälevyn ja johtimien lisäksi tarvitset seuraavat komponentit.

LEDPidempi jalka (kuvassa mutkallinen) eli anodi + -napaan eli Arduinon porttiin. Lyhyempi jalka eli katodi miinus-napaan eli maahan.
Vastus 330 ΩVastus vastustaa sähkövirran kulkua. Vastuksen arvo mitataan ohmeissa (Ω) ja ilmoitetaan komponentissa eri värisinä raitoina (tässä oranssi, oranssi, ruskea ja kulta).
Vastus 10 kΩ.Vastus vastustaa sähkövirran kulkua. Vastuksen arvo mitataan ohmeissa (Ω) ja ilmoitetaan komponentissa eri värisinä raitoina (tässä ruskea, musta, punainen ja kulta)
Sähköä johtavia asioita (esim. hedelmä tai kupillinen vettä)Tässä harjoituksessa testataan asioiden sähkönjohtavuutta. Voit kokeilla ympärilläsi olevien esineitä!


  • Banaanin tilalle voit laittaa mitä tahansa sähköä johtavaa ainetta - vaikka vettä, kunhan huolehdit että vain johtojen päät kastuvat!
  • Kytkentä on sama kuin aiemmin valovastuksen kanssa, mutta valovastuksen jalat on nyt korvattu kahdella johdolla.
  • Kytke toinen johto viiden voltin jännitteeseen ja toinen johto analogiseen porttiin 0 sekä 10 kilo-ohmin vastuksen kautta maahan.

Ohjelman tavoite

Harjoituksessa kokeillaan erilaisten asioiden sähkönjohtavuutta lukemalla arvoja analogisesta portista. Harjoituksen ohjelma on tuttu ja sisältää vain aiemmin käytettyjä komentoja.

Tuttuja komentoja

int sensor = analogRead(A0);

Ohjelmointi 1/2 - Sähkönjohtavuusanturi

Aluksi tarvittava ohjelma on hyvin samannäköinen kuin edellisessä tehtävässä.

Syötä ohjelma Arduinollesi. Vastaa sitten monivalintaan ja siirry eteenpäin testailemaan sähkönjohtavuusmittariasi!
int sensorValue;
int kirkkaus;

void setup() {
  pinMode(5, OUTPUT);

void loop() {
  sensorValue = analogRead(A0);
  kirkkaus = map(sensorValue, 0, 1023, 0, 255);
  analogWrite(5, kirkkaus);

Jos yhdistät virtapiirisi kaksi irtonaista johtoa sähköä johtavaan asiaan (esim. banaaniin tai itseesi) ja tarkkailet Serial monitoriin tulevia arvoja, niin mistä ne kertovat?

Sähkönjohtavuuden testailua

Kokeile johdoilla, kuinka hyvin alla olevan listan aineet johtavat sähköä. Voit myös kokeilla muita aineita. Sähkönjohtavuutta voit päätellä Serial monitorin arvoista ja ledin kirkkaudesta. Saat mittarista langattoman käyttämällä 9V paristoa ja paristonepparia, jolloin voit irroittaa USB-johdon Arduinostasi.

Esimerkkejä testattavista asioista:

  • vesi - älä kastele muuta kuin johdon päät!
  • vesi, johon lisätään hitaasti suolaa
  • ihminen (purista johdonpäitä molemmissa käsissä peukalolla ja etusormella, testaa myös kuinka sormien kostuttaminen vaikuttaa)
  • hedelmät
  • metalli
  • mikä vain mitä löydät käsiisi!
johdot vedessä johon lisätään suolaa

Ohjelmointi 2/2 - Resistanssimittari

Seuraavaksi tehtävä ohjelma ei varsinaisesti muuta Arduinon toimintaa, vaan muokkaa pelkästään Serial monitoriin tulostettavia arvoja. Näin voidaan tutkia, kuinka suuri resistanssi ohmeina irtojohtojen välisillä aineilla tai komponenteilla on.

Syötä ohjelma Arduinolle ja tutki Serial monitorin kautta saatuja resistanssiarvoja. Mikä on suurin resistanssi, jonka pysyt havaitsemaan? Avaa Maker kittisi, ja vertaa tätä arvoa niiden vastuksien arvoihin, joita paketissa on. Arvot lukevat Maker kitin kannen alla vasemmassa yläkulmassa.
int sensorValue;
int kirkkaus;

// muuttujat, joita tarvitaan resistanssin laskemiseen
int Vin = 5;   // jännite, jonka Arduino syöttää virtapiiriin (5V)
float Vout;    // jännite, jonka A0-portti mittaa piiristä
float R1 = 10000;  // kiinteän vastuksen resistanssi (10 kilo-ohm)
float R2;     // mitattavan aineen resistanssi

void setup() {
  pinMode(5, OUTPUT);

void loop() {
  sensorValue = analogRead(A0);

  Vout = sensorValue*Vin / 1024.0;  // muunnetaan anturilukema jännitteeksi
  R2 = R1 * ((Vin / Vout) - 1);     // lasketaan mitattavan aineen resistanssi
                                   // tunnettujen arvojen perusteella
  Serial.print("Vout: ");
  Serial.print("R2: ");
  kirkkaus = map(sensorValue, 0, 1023, 0, 255);
  analogWrite(5, kirkkaus);
Alla on selitetty ohjelman laskutoimitusta, jolla analogiseen porttiin tulevasta arvosta päätellään virtapiiriin lisätyn kappaleen sähkönjohtokyky. Vastaa selityksen pohjalta sivun alalaidan monivalintaan.

Jos Arduinon virtapiiriin syöttämä jännite (Vin, 5V) tunnetaan, niin anturiportin A0 mittaama arvo (sensorValue, välillä 0-1023) on mahdollista muuntaa jännitteeksi (Vout).

Myös piirissä olevan kiinteän vastuksen arvo (R1, 10 000 ohmia) tunnetaan, joten sisääntulevaa jännitettä ja A0:sta saatua jännitettä hyödyntämällä voidaan laskea myös irtojohtojen välinen resistanssi (R2).

Mittari ei ole tarkka, jos mitattavat resistanssit ovat paljon pienempiä tai suurempia kuin piirin kiinteä vastus. Jos mitataan pieniä arvoja, myös kiinteän vastuksen pitäisi olla pieni, esimerkiksi 100 ohmia. Suuria resistansseja mitattaessa myös vastuksen täytyy olla suuri, jotta tarkkuus säilyy.

Tiesitkö että...
Kun resistanssia mitataan yleismittarilla, täytyy ensin valita mitattavan resistanssin suuruusluokka. Tällöin mittari kytkee virtapiiriinsä samaa suuruusluokkaa olevan vastuksen, jotta mittaus on tarkka.

Mitä arvo R1 edustaa?

Arduinon analogiportit mittaavat jännitettä. Tekemässäsi kytkennässä anturi on kytketty 5V jännitteeseen sekä maahan kiinteän vastuksen kautta. Irtojohtojen välissä olevan aineen resistanssi vaikuttaa analogiportista mitattavaan jännitteeseen. Valovastuksen ja kehittämäsi anturin lisäksi muuttuvaan resistanssiin perustuvia antureita ovat esimerkiksi potentiometri ja puristusvoimaan reagoiva anturi (pressure sensor).