May 2014: BenchBuddEE

May 1, 2014

I got my scuba photo rig together with the macro adapter and took a better shot of the 0Ω soldering…

Surface Mount Resistor Closeup

This is a transcript of the build log forum I maintained during the Contextual Electronics Session 1B class. I didn’t log the steps taken during the circuit design and layout portion, but since most of that was rip-up, re-drawing and time spent searching Digi-Key, you’re not missing much, I promise.

The power switch is connected on both boards and both are working!

Power Switch

Working on completing the power circuit: soldering job on the inductors and capacitors on my custom board, soldering job on the thermistors on the official class board, custom board in the PanaVise, connected to the PSU and displaying the funky +5p rail on my Rigol DS1102E, how I rigged up the oscilloscope to test the board, and the display on the scope showing the +5p ripple at 20ns…

I pulled up the schematics and PCB files on both my custom board and the official class board and checked +3V3, +5P, +5V, +/-12P and +/-12V. All of them were powered and displaying relatively good values on my meter. The -12V was in the -11 something range. I didn’t see a significant difference between scoping for noise between the +/- 12 P or V rails but chances are I just don’t really know what I’m looking for.

Also should point out that I tried shorting the 5v and 3v3 rails to ground and the thermistors kicked in as expected. Sweet!

May 4, 2014

Installed the potentiometers… I tried using some files I had to hack down the mechanical posts and then wondered why I was spending so much time on it and just snipped them off with the side cutters. The action on the pots I got is really easy and smooth so I’m not worried about stressing the solder joint at all. It’s not like it’s a radio tuning knob that’s going to get adjusted all the time. Put the rest of the adjustable voltage section together on both my custom board and the official board.

May 14, 2014

Got the first bout of travel out of the way and spent the last two days finishing up through to the completion of the relay circuit for both boards. The fixed voltage regulation works as shown in the video on the class board for both +/- FE_V. On the custom board though, there is no regulation happening for FE_V+, it’s just going through as straight +12v. I haven’t had any time to play, troubleshoot, etc, so I don’t know if the problem lies with my schematic, my layout, a shit soldering job, bad component or d) All Of The Above.

The relay circuit worked perfectly for both boards and clicks nicely as expected, and the LED lights on both boards too. Soldering that LED was a gold plated $%#@!!!!

Haven’t watched any of the OpAmp videos yet. Don’t know what lies ahead.

Photos that follow – relay indicator LED, soldering job on the optoisolator, relay circuit overview for both boards, soldering down of the SOT-23 BJT Transistor, and the bypass jumper area on both boards.

May 15, 2014

Finished installing the opamp on both boards. I guess it’s working on both boards, but really, the “testing the opamp” video was hard to follow as it seemed disjointed. It railed when I expected it to rail though, and since the only signal generator I have is my iPhone with a frequency generator app playing through a mono headphone plug that I repurposed from an old TiVo IR Blaster cable, I’d say my testing equipment is about as disjointed as the methodology. Signals were created… signals were amplified. I think that’s all that’s particularly important at this time.

May 16, 2014

(In response to a member’s question, “What iPhone app did you use to generate the frequencies?”)

It’s called “Tone Generator!” by Peter Deelstra. I’m sure it’s not better or worse than any of the others, but I’m not a fan of “free with in app purchases available” so I just spent the 99 cents for something that just does what I want it to out of the box. The 100Hz signal I had it outputting registered as exactly 100Hz on my Rigol, but it was incredibly noisy. My suspicion is that it was mostly the incredibly janky copper in the cable I was using, which seemed pretty corroded even when I snipped and stripped it half way along it’s length. I didn’t want to spend the time trying to figure out how to do it with stereo cables though.

It’s funny hearing, tonally, the difference between a sine wave, a triangle wave and a square wave. Sine waves are smooooth.

May 20, 2014

So a couple of updates…

  1. I have both boards installed through and including the constant current circuit. I sort of wish I had bought one of those mega-parts-bins just so I could have all the parts ready to go instead of having to stop / extract individual resistor / solder resistor / repeat / do all over again for second board.
  2. The FE_V+ fixed regulator on the custom board is still not doing any regulation at all, just pumping 12v through. I haven’t had time to troubleshoot.
  3. The constant current test on the official class board does not work, but works perfectly on my custom board.
  4. I verified good solder joints through the entire schematic of the current portion. The next step is to try to figure out how to test each component individually. I figure if I go back and check if the opamp is still amplifying my cell phone signal gen that means it’s still working, and if all the resistors check out as their value, then I’ll be forced to figure out how to test the mosfet. If the mosfet is bad, then having some suggestions on how to desolder that bastard will be greatly appreciated because I can’t figure out how I’d do it without a heat gun or chipquick.

May 27, 2014

I haven’t gone back and done any extra work on the DAC or even popped an ADC out of the packaging. I did order a bunch of Arduino specific shield headers from SparkFun, they are precut to the specific pinout and sold as a pack for 45 cents, not bad. Unfortuntely, since we didn’t populate all the pins from the Uno through to the BenchBuddEE, I had to cut down and sand a few pins off anyway to make it work on both boards.

It’s nice having sockets on top of the board so that I can plug test leads in and not have to hold them the entire time.

I also used a spare set of those headers to work as a stand off between the Arduino and the board, works brilliantly.

I wrote and uploaded a relay test program and it worked first time out on the official board, but behaved oddly on the custom board. It would turn the relay on, and leave it on, while the LED blinked away merrily. To make matters worse, as soon as I would connect my DMM or scope to test, it would start working perfectly.

Turns out that before I put the headers down for the arduino, I went through and populated all the 0Ω resistors around the pins so that I wouldn’t have to try and awkwardly go in later to populate them. When I did it for the custom board, the footprints to connect the arduino to ground were moved up a bit for cleanliness so I didn’t populate them as I was going down the board on either side. As soon as I popped those on, the relay circuit worked as expected.

Here was the rig I used for the initial testing of the current source. The phone is connected to the FAN_OUT+ and FAN_OUT- and is running a signal generator app at 100Hz which wasn’t the cleanest signal ever. I bought the xminilab as a kit and it’s horrible, featuring several missing components, so this is as close as I could get to an AWG. I also bodged on the diodes to both boards so that the backflowing voltage into the Arduino wouldn’t occur as it did in the videos.

Here is a shot of the multiple stacked shield headers I got from SparkFun that help to keep a decent spacing between the USB and plug port of the Uno and the bottom side components of both the official and the custom board. The top set that looks like it should be soldered to the benchbuddee is, in fact, soldered in place, but the middle set of headers are just stuck in there as 0Ω spacers.

Arduino Shield Headers

I hacked this together to provide mechanical stability for the pots. I’ve noticed Chris has the same problem with even gentle manipulation of the pots causing them to twist laterally around their pins, which can’t be good even in the short term. This happened to me, because I snipped the mechanical posts off the bottom so that it would sit flush against the board.

Step 1: Grab some old trimmed off LED leads and bend them into little hooks at the top.
Step 2: Thread it through the plated mechanical hole on the board with the hook grabbing onto the body of the pot. Tack it in place, then use a pair of pliers to pull it snugly down while you quickly reflow the solder to allow you to pull it tight.
Step 3: Flood the entire thing with oodles of solder. Oodles I tell you.

Once those are soldered in place, the whole action of the pots becomes far more solid.