Introduction: Quickie PCB Production (with Bonus NiCd Battery Charger)

There's a lot of very good resources on Instructables for doing laser printer toner transfer, and for complicated circuits or fine-pitch work it's probably the way to go.

But what if you're stranded on a desert island with no laser printer and you still want to work out a super-quick printed circuit? What then?

(OK, honestly, I just find it a bit of a hassle to fire up Eagle, etc just to make a quick and dirty battery-charger circuit.)

Here's a start-to-finish home-etching-with-Sharpies mini-tutorial. It's basically "draw the circuit with sharpies, then etch" but the gold is in the details.

Step 1: Stuff You'll Need

From blank copper-clad board to working prototype:

Copper clad board
Ruler and matte knife to cut it
Scrubby pad to shine it

Three (3!) Kinds of Sharpie: Ultra-Fine, Fine (Regular), Chisel-Tip (Wide)
An empty piece of protoboard (the secret ingredient)
Etchant

Drill and bit
Components
Soldering Iron

Pen and paper for working out the circuit layout

Step 2: Design the Circuit and Lay Out the Board

Here we're building a constant-current battery charger with almost no bells or whistles.

The circuit's straightforward enough. Put a resistor (or group of resistors in parallel) between two pins of a LM317T voltage regulator and it spits out current equal to 1.25 V / R ohms.

The only whistle this cicruit's got, in fact, is the addition of a bunch of sockets which'll enable variable charging currents, which is handy because I've got a bunch of different NiCd/NiMH batteries all with different capacities and want to charge them all.

So first I Googled around a bit, then sketched the circuit out on paper. Next I looked up the pinouts for the LM317 on the datasheet and did a quick mockup drawing on paper with actual-spacing pin-holes, just to see how much room I need on a board.

Step 3: Draw Out the Circuit in Mirror-image (optional)

It's nice to have a mirror-image version of the circuit layout to help you with the on-copper version you're about to make.

If your digital camera's around, photograph your drawing and view it mirror-image on your computer. If you're doing the mirroring by hand, remember to flip all the pins on ICs left-to-right. Double-check the orientation of your diodes and transistors.

For this circuit, I didn't do either. I got away with it because the LM317's only got 3 pins and so I could just stick it in upside-down on the topside, and it would be the same as the mirror-image. If you've got anything in a DIP package, this trick won't work -- upside-down is only equivalent to mirror-image if there's only one row of pins.

Step 4: Cut the PCB Down to Size

Using your to-scale drawing, figure out how big the board needs to be.

I left a little more room for the power cord to enter on one side.

Cut by scoring repeatedly (light pressure is fine) with a matte knife. Flip board, mark carefully, and repeat on the other side. Then you should be able to flex it a few times and it'll fatigue and snap.

(If you watch the video, I didn't really score it enough. Still made a fine edge anyway.)

Step 5: Clean Up the Copper and Lay Out the Parts

Scrub up your copper with the green pad -- it's time to get drawing.

Here comes the first "trick."

Using your perfboard section and the ultra-fine point sharpie, lay out the holes for all your components as you worked them out on paper, In the mirror-image if you made one.

Ultra-fine sharpie just barely fits through the perfboard holes. If it doesn't work on one side, try the other. Worst case -- widen some of the holes with a drill bit.

Putting down evenly-spaced 0.1" dots makes sure that the pads you're about to draw are nicely centered so that your components will fit.

Step 6: Draw Pads and Connect the Dots

Time for the fine (regular) sharpie and some fine-art freehanding.

Draw wider dots around each of the fine-point pin centers. These'll be what you solder your parts onto later.

Then draw the circuit out. Here the usual width rules apply -- make the traces as wide as you can without compromising your design. Thin, ultra-fine point traces may be necessary if you have to snake in between pins. Otherwise, regular sharpie is a good width for most traces.

If you've got any close trace-to-pin places, feel free to draw out the traces first and make asymmetrical pads to work around it. You know where the pin-centers need to be because you marked them with the perfboard.

Step 7: Fill in the Ground Plane

It's easier on your etchant (and sometimes has beneficial electrical properties) if you keep a fair amount of copper on the board to use as a very-low-impedance ground path.

And it gives you something to do with the Chisel-tip Sharpie.

If you feel like it, you can do split power/ground planes. For some circuits you may want bypassing capacitors between the two planes scattered liberally around.

Step 8: Let the Ink Dry, Inspect, Do Small Touch-ups

Sharpie ink holds up better in the etch bath if you let it dry for a while. Not sure how long "a while" is, but I tend to wait 5-10 min or so.

This gives you time to double-check the circuit, look out for accidental connections, and scratch some kinda design into the ground plane just for fun.

Here, I ended up scraping a little ink from between two pads -- not because they connected, but because they were a little close for pads which would be carrying high current.

Then, thinking about high current, I widened up some of the traces.

Finally, I scratched a transistor symbol into an open part of the ground plane. (It's hard to think up a good logo on the spot in the 5 min it takes Sharpie to dry.)

You can use a component lead, sewing pin, paper clip, or something similar to scrape off the ink. I used the non-business end of a small drill bit.

Step 9: Etch

Etch the board using the etchant of your choice.

Shameless plug: Use a cheaper, re-usable etchant instead of ferric chloride. (Either of which, BTW, you should not be throwing down the drain. Look in phone book for hazardous chemical disposal companies if you're not re-using etchant.)

Step 10: Drill Out the Holes

Another couple tricks here. Since you don't have drill-holes made for you, an easy way to get them in the right place is to use your perfboard template again.

For a long row, like the resistor socket header, I'll put a dot in the middle of the top pin, then one in the lowest pin, then line up the two dots in the perfboard and fill in the rest. That way the whole header is on exact 0.1" centers.

The ultra-fine tip sharpie goes right through perfboard holes.

Have a look to make sure it all looks ok, then once you've dotted up the whole board, get to drilling.

The perfboard again, except this time as a drilling jig. Line up all the holes so that you can see the dots in them, then drill out the top and bottom pins, just like when marking them. Now you can stick something in the holes to keep the jig and board aligned. I used some solder that was lying around. Thick component leads or hook-up wire work well too.

Once you've got the perfboard jig set, the other holes take only a few seconds. This trick is huge for multi-pin ICs.

I drilled 6 more holes in the board to accomodate the wires from the power-supply plug so that they can loop through the board as a measure of strain relief. (Think I learned that trick here somewhere. Can't remember where. It's a good one.)

Step 11: Populate the Board and Solder It Up

Nothing special to say here. They're all in a row because you used the perfboard jig. And the pads you drew give you a nice surface to solder to. This part is smooth sailing.

If you're not interested in the battery charger circuit, you're done.

To recap: Practice on paper first, use different widths of sharpie, mirror-image with camera if needed, and use the perfboard as template/jig whenever you can.

Otherwise, just draw the circuit.

Bonus points if you can make it look kinda modern-arty.

Step 12: Particulars of the Charger Circuit

Two cautions:

Because this super-simple charger has no turn-off mechanism, be very careful and be sure to pull the batteries out when they're charged. I stop charging my batteries early. They'll last longer if you under-charge slightly anyway.

And don't use it for LiIon or LiPo batteries -- they need constant voltage instead of constant current.



The LM317 chip got very hot with a few hundred milliamps current, so I built a simple heatsink by taking a copper clad scrap, drilling a hole in it, and bolting it to the LM317. Now it's pretty hot all across the copper, which means it's working -- pulling heat from the LM317. Is it enough heatsinking? Not sure without doing some math. If it burns out, I'll redesign.

Note also that the resistors are high-wattage ones. Since only 1.25v is dropped across the resistors, you should be able to get away with regular (1/4 watt) resistors for most reasonable currents, as long as you're not using single-digit resistance values. Still, mine get kinda hot. Definitely use power resistors if you've got 'em.

Most of the connectors on my battery packs are standard 2-pin female sockets as far as possible. This makes it easy to hook them up to the charger, or make an adapter you need. For instance, I took the top off an old 9v battery, soldered a wire to it, and now I can use it for rechargeable "9-volts" (they're actually 7.2v). Modular plugs/headers are your friend. I think some of my robots are 1/2 connectors by weight.

And speaking of 9v. The datasheet for the LM317 shows it needing about 2-2.5v more input than output. That's ok for me, b/c my highest-voltage battery pack is 7.2v, which wants to charge up to around 9.6v. But I'm pushing it. It might be better to use an 18v supply. Or maybe it's a good idea to slow the current down when the battery's almost fully charged?

If you want to use this type of charger yourself, go give Battery University a look. They talk a lot about good charging currents, times, discharge cycles, etc.

Because you can regulate the current very easily with this design by swapping out resistors, one set of resistors can make it a trickle-charger for charging up overnight, while a different set of resistors can charge up your batteries in 1 hour for when you're impatient.

A smarter charger is a project for another day.

Comments

author
ecellingsworth (author)2013-07-10

Looks like someone might be getting some fan mail, Mr. Elliot D. Williams of Apt 407, 1851 Columbia Rd NW, Washington DC 20009, but it won't be from Pottery Barn.

author
simplebotics (author)2013-03-28

What reusable etching solution were you using. Currently I used ferric chloride.

author

Ferric's great, and simple.

I actually use a homemade etchant that I'd recommend for the long run: https://www.instructables.com/id/Stop-using-Ferric-Chloride-etchant!--A-better-etc/

Oh, and tthe shirt-folder bot is awesome!  I'm going to have to make one for my girlfriend.

author
simplebotics (author)2013-03-28

Nice robot walker!

author
hosam_eldin (author)2007-09-06

nice work, but you should monitor the battery temperature to avoid over charge wait for my Instructable for a PC based smart charger

author
tytower (author)hosam_eldin2012-12-13

I'm surprised at this The more usual charge for 850mAh rechargeables would be about 85mA for 10 hours . At 600 mA odd the life of the battery would surely be reduced markedly wouldn't it ? Guy above says nice work and he is going to do one too . Will I wait for that ? Don't think so.

author
The Real Elliot (author)tytower2012-12-18

It's true that there are tons of chargers that charge at 1/10 C (85mA for an 850mAh battery) over 10 hours, and these are mostly the cheapest, simplest chargers. The reason for using such a low current is safety -- even if you leave it on the charger forever, it probably won't vent or blow up. Overcharging is _not_ good for the battery, but at least it's not a safety hazard.

According to
http://batteryuniversity.com/learn/article/ultra_fast_chargers, 1C (for a roughly one-hour charge from empty) is a "gentle" rate. According to the wikipedia article on NiCd's, 1C is the standard fast-charge rate. I've charged and discharged old NiMH phone batteries hundreds of times at 1C with no noticeable ill effects. My drill's battery charger gives 5C to the cells (15-min charge), and they worked for five or six years before I had to replace them.

What you _don't_ want to do is overcharge. And charging faster makes that easier to do. So if you use this charger at 1C, set yourself a timer and check on it again at 1/2 hour if you're not sure that the battery was fully discharged to begin with. Be conservative -- a NiCd or NiMH battery lasts longest when neither fully charged (avoid overcharge) nor fully discharged (which is bad for NiMH).

And of course, the best thing to do is get a smart charger that ends the charge by time, temperature, and/or voltage. But in that case, there's even less reason to avoid charging at full speed.

And _do_ note that none of this goes for lithium batteries, which want a totally different charging schedule.

author

Agreed. One absolutely should monitor temperature increase (or the slight voltage drop that NiCds get) when they're at 100%. This was just a quickie to get a bunch of batteries up to working levels pretty fast. And as I was doing it, I thought to myself that the only other tutorial on hand-drawn sharpie PCBs was one with non-standard pin-spacings. So... Someday, I may get motivated to make a proper voltage-drop-based charger. (I got some ideas.) Probably not until I leave this one one and fry a few batteries, though. Love to see your charger design. Feel free to post up in the comments here when you get yours running.

author
tytower (author)2012-12-13

I'm surprised at this The more usual charge for 850mAh rechargeables would be about 85mA for 10 hours . At 600 mA odd the life of the battery would surely be reduced markedly wouldn't it ? Guy above says nice work and he is going to do one too . Will I wait for that ? Don't think so.

author
Jugfet (author)2012-12-02

With the 317, as with most V regs, it's best to keep the input voltage as low as possible and then the chip will not generate as much heat.
If for instance you want 6V out - that would be 6Volts + 2.5V offset so 9 volts would be good 12 volts would probably be more practical 18Volts and your heat will be much higher making an overall less efficient circuit.

author
Unit042 (author)2011-04-29

I have been using the battery charger circuit here for a while. A variable constant current supply is quite handy.
Anyway, the PCB techniques you present here are excellent ideas, thank you for taking the time to post them online so the rest of us can learn from it.
I am impressed by your creative use of perfboard, both in marking and especially drilling. I was half expecting you to say "okay, now guess at where you thought the pins were," but no, you give us a very good tactic instead.

I may not do PCBs by hand anytime soon, but now I am armed with a proven plan of attack when the need arises. Hooray for The Real Elliot! :)

author
amando96 (author)2009-12-04

you sir are awsome, i'm making a barebone arduino board with this, will i regret it?

also dude down there `´ `´ dont drill it, cut the pins off and solder like they are smd's, takes some practice, but driling is a pain without the proper tools

author
raykholo (author)2009-06-02

you use a hand drill for the holes? what size bit? i ve heard that the tugsten carbide bits will break unless used with a drill press. So if ur using like standard 1/16 in. bits (not made specially for pcb use) then is it still possible for those big holes to accommodate the component leads? ... yeah, its a lot of questions but i like your method and am trying to be able to do this on the cheap ( i dont want to buy a drill press...) :) thanks

author
The Real Elliot (author)raykholo2009-06-04

I use PCB bits, the small ones. I've never broken one, but they do seem a bit fragile. I bought 3 on sale from All Electronics or Electronics Goldmine ages ago. Sometimes, if I'm freehanding it, I'll steady the drill on my leg to make it kinda like a press. (You'd have to see it.) Using the protoboard as a drilling jig probably also helps a lot.

author
raykholo (author)The Real Elliot2009-06-04

so those 2 sites are ones that i use so that helps and i can just add the bits in my next order... the perfboard jig idea is pretty clever the pcb bits u use seem to be tugsten carbide and i was just wondering about using them freehand... other methods i have read advise using them with a drill press at 2000+ rpm otherwise they risk breaking so nice to know thats not exactly true and thanks 4 answering that thanks again and any other advice u got 4 someone just getting started with this stuff would be great... im good at electronics... just havent worked with pcbs yet...

author
The Real Elliot (author)raykholo2009-11-16

Update:  I bought some super-tiny carbide drill bits and discovered that they are brittle after all.  Be careful not to apply much side-force to them when drilling.  At least they're only $1 each from the surplus guys.  But, buy a few more than you need. 

The non-fragile bits I have been using look like regular 1/16" bits, but read 0.039" in the calipers (5/128"?!?).  They seem to be stronger than the carbide ones, and are a good size for DIY holes. 

Or skip the drilling and the mirror-imaging entirely, and do everything top-mount.  (I smell another instructable coming...)

author
crazyromanian (author)2009-07-20

The Real Elliot, where did you buy the etchant? thx.

author
twenglish1 (author)2008-07-22

this is awesome i just made something today using this method and it works great and i also used your etchant (5 stars)

author
osgeld (author)2008-05-11

the 317 wont burn out, it should be limiting current to prevent it also that might be enough of a heat sink, ive used alligator clips and pulled an amp but they are solid metal and not just a foil coating

author
The Real Elliot (author)osgeld2008-05-12

Yeah. I've been using this very charger since I posted it up, and it's going strong. The "heatsink" gets pretty hot, which I take to be a good sign. I've started putting it in the outflow from my power supply's fan for extra cooling power. It also does double-duty as a LED tester/meter. Put a 100 ohm resistor in it and sources ~12.3mA which is good for most low-power LEDs. You can then measure the voltage that the LED needs to light up. Handy. Everyone needs an adjustable constant-current source! :)

author
osgeld (author)The Real Elliot2008-05-13

warm = good
hot = might as well not really be there because its saturated with heat

for a heat sink to really work it has to dissipate more heat than its absorbing, that way its wicking the heat away into air

try replacing that copper pcb with a flattened down "slot cover" from the back of a pc and see if you notice any difference, i bet you will :)

author
osgeld (author)osgeld2008-05-13

btw ... if you use one that has some sort of "paint" on it sand it down to bare metal first, where it contacts the regulator (use a faint amount of compound of your choice too, the silver stuff is mine ) this goes for anodized parts and plated parts!

author
andy60 (author)2007-09-07

excellent idea, bravo! lol

author
Myself (author)2007-09-05

In the schematic presented, I don't see the big bank of resistors. What am I missing?

author
The Real Elliot (author)Myself2007-09-05

It's not overly clear in the schematic, but the parallel resistors (or at least the headers for plugging them into) take the place of "R1" and "Rv" in the original circuit diagram, or the one just labelled "R" in my quick sketch with components. That is, there's only one ( thevenin equivalent ) resistor in the whole dang design. In the one from the web, it's a resistor + variable resistor combo. In my design, it's a bunch of resistors all parallelled for a) lower resistance (higher current) and b) greater wattage handling.

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