Sunday, April 25, 2010

found a board / sketch-a-matic for the power section

Over at general guital gadgets I found a perfect amp circuit. What I like about it is the common ground design and the usage of the 10ohm/coil.

When I worked at DAK they bought the brand of a very high end amplifier company, A common theme in all the amps was that all the ground traces went to a common point/solder point that was tied to the chassis. Prevents ground loops, and in a high gain chip power amp ground loops spell disaster. Who can hear the 1mz oscillation of a poorly designed board? That is until the magic smoke appears. This is the only circuit I found with the recommended 10ohm with a few turns of wire to prevent oscillation.

Another theme was torrid transformers - much better regulation of the voltage out, and quieter too. However i fear i will have to forgo this as a luxury. I am on a strict budget. I have some 25v ct transformers in the junque box, and the lower voltage means less power, and a smaller output heat sink - the chip is capable of ~68w - I am only looking for 10w or so. Even at that level the wife will probably yell - even if i could hear her :-)

Speaking of cheap I went yard saleing and found for $.50/ea a pair of 6" car speakers. I thought that they would do quite the job for this amp. They looked like they can handle the power, and with the full range - the acoustic guitar I am planning to hook up to this mini beast will probably sound more like and acoustic and not a buzz saw electric.

... off to google a pre-amp/tone controls for the project....

..... Googleing, Googleing .......

Found it - - a banaxle opp amp based design. Kewl...

Monday, April 19, 2010

Finishing up the PCB

Finishing the boards what i did was:

Drilled the holes.

The Last time I tried to drill a board I used up the entire kit of bits I got on eBay, using a genuine Dremel tool in an expensive genuine Dremel drill stand. The Dremel screws into the stand and it must have not been as perpendicular as it looked. Most of the bits broke just as the drill went through the board completely. Too much flex - plus the Dremel sounds way too much like a dentist drill for me to like them much. I had even purchased a regular looking chuck adapter that didn't help matters any. Not recommended.

Since then I got a real drill press at Harbor Freight - I got it for around $50 on sale - Nice bench top unit I have used quite a bit for regular sized projects. I figured that the bits had 1/8" ends , so I mounted up the bits, set the speed as high as it would go, and drilled all 64 holes without one broken bit. Considering I spent less on the big drill press than I did on the Dremel tool, I can't recommend the bench top unit enough. Plus while I was there I spied a grab bag of small drill bits and picked up a set that I used this time.

Tinned the traces.

I was initially to intimated to try surface mount soldering, looking at the Sparkfun tutorials and then actually doing it is sometimes very different.

For those of you out there contemplating it here are a few tips. Especially if you made you own board and it's gleaming copper traces are staring you in the face.

Buy one of the flux pens. I initially put a little dot of solder on each pad to solder down the chip, however when i tried to get it aligned these little dots got in the way - the pins wanted to fall to either side. I tried to get the solder bump a little smaller, but the iron wouldn't pick up as much as I needed - then the little over the head light bulb went on and I just covered all the traces liberally with the flux (you may need to push the pen down a few time to get fresh flux flowing I did). Then all I had to do was was drag the soldering iron along each trace, trailing a small blob of solder and leaving a nice thin coating of solder. Only once in a while did i need to refresh the blob of solder on the tip of the iron.

I brushed the Flux pen over the pads again for a fresh coat. Then I heated one of the corner pins, dropped the chip down on it, checked to see that it was stuck, and did the same to the other corner of the chip. After that it was just a matter of touching the iron to each pin , and inspecting after all were touched - they all had a little meniscus of solder from the IC pins to the pads. Touching with a pin I tested each to see that they were well and truly stuck.

I have to mention the soldering station I got. A Weller WES51 Analog Soldering Station, Power Unit, Soldering Pencil, Stand and Sponge. The analog is quite sufficient, who cares if it is 600 or 611 degrees, I don't. It is a absolute joy to use, and one of its best features is that it turns itself off after no use in 99? minutes. It cost close to $100 on sale at Amazon. As they say - you remember the quality long after you forget the price. Highly recommended.

Cutting the baoards.

Well i could have done it many ways, as soon as I get a real paper cutter I will try one of those, but I have a scroll saw, and it does the job quite nicely. Wish they made saw blades designed for cutting FR4 for the saw.

Lessons learned and notes for next time

  1. Use real tools - drill press and soldering station.
  2. Hook up a the hose from vacuum to the drill press to catch the drill dust.
  3. Look for real paper cutter at the yard sales.
  4. Get a new prescription for the glasses I use in the shop, all these tiny precision tools, parts and soldering gives one a headache.

Sunday, April 18, 2010

Fun in the shop etching Printed Circuit Boards

I started a project of documenting and organizing all the various IC's I have acquired over time. Up till now they were in a little plastic box that I consulted time to time.

I stopped by Staples and picked up a handful of report binders with pockets - they conveniently had them on sale for $.50. The binders I was looking for had a insert for three hole punched paper and pockets. I printed out the data sheet for each part, 3 hole punched it and stuck the part into the pockets. Now I don't have to run to the computer when I pick up an IC I don't remember.

Being of an age where seeing very small parts is getting harder, I tended over time to accumulate a lot of soic parts, and they sank to the bottom of the sample box.

I finally purchased an inexpensive adapter board from dipmicro electronics that let me mount most of them for prototyping. I was suitably impressed of the quality of the board and @ a quarter a piece I got a bunch. One side lets you mount soic and the other ssop.

However there were a few parts that had wider spacing and no one seemed to have a break out board for them. Or should I say in my price range - I am, after all, a cheap son-of-a-gun.

So I decided to whip up one of my own. I bit the bullet and set aside the time to finally get proficient enough at Eagle CAD to make a passable design. After a few false starts I managed to get the libraries loaded and Eagle set up, thanks in no small part to a Spark Fun Tutorial.

First I could not find an outline for the part I wanted to use (DS2321 RTC chip). Google found a person that had designed a board with the part, and I set out to modify his layout. I got all hung up on making a library out of it for later use, after all this was just supposed to be a board for any so16wide parts. A forum post showed me how to make a library from the original. I was now drawing a hobbyist quality, wildly inefficient, but functional design. I had one in an hour or so. Now it was ready for toner transfer.

The last time I made a board I had purchased some wildly expensive coated picture paper. I remembered reading someplace someone using magazine covers, I grabbed a copy of Information week - the cover seemed a bit thin to me - but I was in a hurry and decided to try it. I found a back cover that was mostly white and it printed just fine in the laser. It took a bit to realize I was not enough of a pro at Eagle - I had managed to get some stuff on the top layer and some on the bottom - But after I set it to print both the top and bottom together, everything worked out swimmingly.

I now was on familiar turf ( I thought). I tossed a pieced of freshly scrubbed PCB stock onto an old re-purposed waffle iron to heat up - and ran around to locate that darn roll of masking tape. Finally taped it to a very warm board and ran it through the laminator a few times.

Looking through the cover at the toner - there were some supicious spots that I though had not stuck sufficiency, grabbed the Clothes Iron and cranked it up as far as it would go - folded a paper towel to made a pad, and pressed hard as I could to make sure the toner was re-melted onto the board.

When I put the board into a hot water bath I was very pleasantly surprised, the last time it took almost an hour to get the photo paper soft enough to get it away from the toner and off the board. This time the paper just peeled away in a few minutes, and came away very cleanly. Just a few wipes with a sponge was all it took to rid the toner traces of any paper residue. I guess it had to do with its thinness.

I guess photo paper is meant to last and was considerably thicker. It took forever to get soft enough and stuck perniciously to the toner. I guess I will renew that Information week subscription they keep hounding me about - if for nothing than to have lots of cover paper :-)

I could see where I had pushed too hard with the iron - it looks like the toner had smeared onto the board, a few pokes with a Xacto knife and and some touch up with a fine point Sharpie and it was ready for the etchant.

Then came the fun part where I get to dress up like a serious dork. Donning lots of protective clothing, chemical safety goggles, breathing mask, and nitrile gloves. I got out some Hydrochloric acid/Peroxide etchant that had to be almost a year old, and dropped the board into the cold solution. It went slowly, I had seen in some video where someone used a little sponge brush to rub off the disolved copper and let fresh etchant at the board. So I brushed and brushed and swore to next time I would take time to heat the etchant and I would flood fill that unused part of the board so that not as much copped had to be removed.

Pretty soon the etchant was so dark it almost looked like ferric chloride. Just about then, the door bell rang and I went to the door worrying what the caller would think with me a dressed up like a toxic spill worker. Just my Brother-in-law and my sister. The board sat there in the etchant for the better part of an hour as we discussed pleasantries.

When I got back nothing seemed to have happened to the board much, so I quickly whipped up a new batch. WOW. A new batch of etchant really works fast - like in another minute the board was completely etched.

A quick wipe with some acetone and the bare copper was exposed, and the family was suitably impressed by the little lines of copper running all over the board.

This morning when I got back down to the shop - the dark murky old etchant was a bright green and almost clear. I goggled around and found this weighty tome explaining how, just by exposing the etchant to air, it re-generates it given enough time. This was about 3/4 cup in a closed plastic container overnight- not much air - but evidently enough. I grabbed an old coffee container, for it's opacity, and poured both batches together for next time (have to get an acquarium pump)

Now its time to clean up all the spills and wash down the bench-top so I can get out the drill press and Scroll saw to do the necessary machining. Etchant is very, very, very corrosive and it will rust up any tool that it touches - last time I managed to spill some unknowingly onto my favorite set of needle nose piers - that will never happen again.

Time to go break lots of little expensive carbide drill bits.

So lessons learned, and notes for the next time
  1. Spend some time with Eagle cad futzing around - just to get comfortable.
  2. Use magazine covers to print.
  3. Don't press so hard with the clothes iron, or try not using it at all.
  4. Make sure to regenerate your etchant with a cap full or two of Hydrochloric acid before starting to etch.
  5. Heating it up first wouldn't hurt.
  6. Look at the yard sales for an aquarium or other small air pump to regenerate the etchant.
  7. Find really, really, cheap place to get all those tiny carbide drill bits that are so fragile.

Sunday, April 11, 2010

Photo Circuit Boards

Custom Photo Circuit Boards

I have been investigating how to make precise homemade circuit boards. I have tried toner transfer and have been disappointed more time than not.

Back in the day when I had a full blown dark room setup – making boards was easier, partially because you could actually see the parts, and you could run down to any number of local distributors and get chemicals and boards. Plus a 1/16” drill did the job for most off the drilling.

25 years later I am in the mood to try my hand at it again. When there is no PCB for sale, there is usually an eagle cad brd and sch available.

So I installed Eagle Cad from here. Impressive package, a steep learning curve to make you own boards, but since I had the artwork already I just opened the files and looked at them. Printing them yields a positive – perfect for toner transfer but I am a little more adventurous.

To convert the positive to negative – I installed ghostscript and gsview. Eagle cad can output Postscript files so these applications interpret them into a printable version.

How I did that

Open the brd file in Eagle – then from the File Menu select Cam Processor. The layers ( in this case there was only one side so I selected bottom, pads and vias. Some people recommended that the fill pads be checked – but I find that it makes drilling easier if the holes are there to help position the drill.

On eBay there was a place that sold 6”x8” sheets of resist that could be applied with a laminator or with a hot iron – the critical piece of information that I gleaned from their page was you could use velum or tracing paper to creative the negatives.

A United states source on ebay (jaded_caveman) Dry Film Photoresist Sheets+Developer

Here is a link to a great video – using positive pre-sensitized boards. Circuit Skills: Circuit Board Etching

A lot of good information here : PCB Processing Manual