Background

I used the original CBL with my students since the devices were first marketed. Over the years we endured some of the hardware weaknesses I always felt were inherent while attempting to program around others which students themselves brought to the devices. Some of these weaknesses caused some colleagues to abandon the CBL altogether but I felt the gains outweighed the losses, and because I used them for so many different things in the Honors Chemistry course (and we made what I considered a significant investment in them) I tried to patiently persevere.

In 2006 my then colleague, Chris Dartt, and I decided to make a few modifications to the CBLs which we felt would make our lives much simpler in the lab. These modifications address two major issues I had with the CBL almost since we first took one out of a box:

            1. The “link” jack is incredibly flimsy, held on only by surface solder tabs (no pins through the
                circuit board). The early right-angle link cable plugs were a disaster for this weak design,
                constantly breaking solder joints as students were encouraged to be sure the plug was inserted
                firmly. The fit of the plugs in the jacks also simply became "loose" with many years of use.

            2. Power is supplied to devices only intermittently unless the programming is set for continuous
                reading. For most devices this is not a huge issue but it affects the pH amplifier in a
                significant way. A continuous power supply enhances the stability of a few other probes such
                as the colorimeter.

The first issue had been discussed many times among the Honors Chemistry teachers. Early on we toyed with the idea of getting rid of the jack and hard-wiring a link cable into the CBL. “One less connection to go bad”, was our reasoning. Our experience with the early right-angle plugs, however, made us hesitate. Anyone who used those cables frequently will surely recognize the picture at the right. Not only were the right-angle plugs a bad design, there were poorly manufactured. We had too many plug tips come off inside calculators and CBLs. Thus we feared that a CBL would be out of use if this happened to the plug permanently attached to it (at least until we could replace the cable). So we did nothing and tried to keep after the solder joints.

Meanwhile different style link cables became available and we never saw the broken tip problem once we changed to the straight-line connectors (current choices from Vernier seen at the left). Teachers came and went on the Honors Chemistry team and I just learned to endure the vagaries of the CBL and hope for good luck (which I usually had).

The second issue I took up with both TI and Vernier immediately. I knew from experience that Vernier’s pH system was much better than the CBL made it look because I had used it in a lab interface system I had designed years before for the Apple II. The problem was clearly the power. TI, in its questionable wisdom, had elected to make battery life a priority, hence the intermittent power to probes. Since pH was the main problem, I first thought of altering a class set of pH amps for my AP students as an experiment. My “solution” was to add a power jack to the amplifier box, snip the wire inside that brought the intermittent power from the CBL and instead supply power directly through the new jack from a CBL AC power pack. This worked like a charm but was tedious to do and somewhat hazardous as several of the boxes reached near-escape velocity on the drill press.

My second approach to this problem was to avoid altering another class set of amplifiers and instead construct a kind of “cheater cord” which could be used on any CBL and for any probe. This cord had a power jack and plug at the two arms of a “Y”. The AC power pack connected to the jack and then the plug connected to the CBL. Another wire from the jack went to something resembling the grey BTA-DIN connectors that many probes use to attach to the CBL. The DIN jack on my setup had the wire which delivered intermittent power from the CBL snipped and the wire coming off the power jack connected in its place. This effectively bypassed the power coming from the CBL and provided constant power to the probe. This approach, if somewhat inelegant, was effective and versatile but fragile in the hands of students who tended to pull on cables before releasing BTA plugs and so cables were needing occasional repair work.

          

The Crisis

Some of the most complicated CBL work my sophomores did was pH or conductivity titration. There is calibration to execute correctly, set-up of the collection and graphing parameters, and so on. Doing all or some of this and finding that the CBL was not paying any attention because of a bad link connection was disheartening and soon generated a chorus of voices calling the instructors name. Meanwhile other non-electronic problems were taking place (as they always do). One can only take so many days like this and at the end of one particularly grueling one I found my colleague Chris Dartt in the lab looking intently at a CBL. I knew that look, having stared in frustration at the little beasts many times myself. Fortunately there was no hammer in the near vicinity.

We had discussed CBL problems many times before and I had gradually brought him up to date on the history behind the things we had tried. As we started to talk again about the problems we were still having I casually said “Well, we could just remove that jack and add a cable coming out of the case. One less connection.” He agreed this was a worthy idea and added that the jacks on the calculators, although possibly just as flimsy, did not see nearly as much action as a CBL which was potentially used by five different students in a day, many times a year. We talked a little more about it and he offered to convert one of the CBLs as a test case.

In a few days we had a working model and every time a student had to make up a lab or we needed to use a CBL we grabbed that one. It behaved well and gave us a chance to consider optimum cable length for a mixture of TI-83/84 calculators. When Chris offered to do the entire class set over Spring Break I proposed to look at the power issue and see if it would be possible to eliminate one more headache in the form of the “cheater cords” I had designed. If I could find a simple way to solve this problem internally I would do the entire class set.

The plan was to use these altered CBLs for a year and then decide whether to similarly “improve” the set used by the AP students.

The weakest link

Opening up a CBL can be quite a challenge without the right tool. A very few of the 22 units we worked on had small Phillips-head screws in the back. Most had screws with little star-shaped slots that require a T10 “Torx” drive. My local hardware store knew exactly what I needed when I showed them. Removing the batteries first makes opening the case easier.

The entire circuit board should be removed and this is accomplished by removing the Phillips-head screw in the middle of it. Sometimes this screw holds down a postage-stamp sized circuit board that sits on top of the main board. Sometimes not. Just don’t break any wires. I found a surprising amount of variation in the innards of the 22 cases I opened, so it’s not possible to predict what you will find. You'll have to slip the battery clips off their little posts to remove the board from the case.

Owing to its poor design, the link jack is easily desoldered. For a cable Chris settled on the long (16”) link cables available from Vernier. Apparently at one time the long cables were only 12” because we had a mixture of lengths in our backup stock. Chris felt the 16” ones gave a better fit when used with either model of calculator (83/84), allowing the calculator and CBL to sit side-by-side with no strain on the cable. One plug is snipped off the cable and the wires stripped and soldered into the position from which the jack was removed.

          

Chris used a small cable tie to restrain the cable on the circuit board and hopefully prevent students from pulling it loose. He used a 7/16” drill to make hole in the side of the front half of the case (about 4” from the bottom) and then gently pulled the drill sideways to make a slot for the cable. This size slot makes the cable fit snugly and the hole is drilled fairly close to the edge so that when the case is reassembled the back presses gently on the cable as well. It’s not going anywhere.

We stored our CBLs in their original cases so Chris also sliced a slot into the foam so that when the CBL is “laid to rest” the cable slides down into the slit. He guesstimated that he was eventually doing about three conversions per hour.

All power to the probes!

The AC power packs and the battery supply both are 6 V while the power requirement for the probes is 5 V. In my altered pH amps and in the “cheater cords” I used a diode (based on a suggestion from folks at Vernier) to cut down the voltage from the power pack. I knew there had to be something similar inside the CBL so I began to trace the power circuit, looking for a diode and a place where the voltage was a steady 5 V. The transformation actually happens rather early in the power trace and once I had the point on the circuit board to tap 5 V from, I only needed to find the place where the intermittent power was delivered.

There are three analog channels on the original CBL. I examined the BTA jacks and looked at the circuit board traces. The CBL manual describes the pin-outs so I was able to locate the pin that supplies 5 V to the probe. At that point I decided to cut the trace on the board that delivered the power and then to solder a jumper from the constant 5 V supply point I had discovered to the pin on the BTA jack.

          

The only question was whether to do all three channels or just channel 1. After a discussion with Chris I decided on channel 1 only. In the Honors course we never used more than one probe at a time and while the AP students occasionally used two, only one was ever a power hungry one like the pH amp. Also, this would leave open the unlikely scenario that the TI engineers probably envisioned: students tramping through the underbrush somewhere, measuring something far from an AC power source. To conserve battery power channels 2 or 3 could be used instead of channel 1. This also simplified my task.

I used an Exacto knife to slice through the circuit board trace. The space is sort of tight and it’s difficult to see exactly when the deed is done (at least with my bifocals) so I began to use a pH amp/electrode combination with the CBL in multimeter mode. While the trace was intact there was some kind of voltage reading between 1 and 2 volts (depends on the pH of your storage solution). Once I had succeeded in severing the trace the reading became something negative or a small positive millivolt reading. I also tried using a multimeter to simply check for power at the jack but lacked sufficient hands to easily get everything where I wanted it in close quarters.

I decided to route the jumper wire around the board because the places where I could easily solder were on opposite ends of the double-sided board and the LCD screen was in my way on the flip side (keep away from that screen; the electrical connections are held together by enchantment alone). It’s not quite as elegant as the work Chris did, but it is also not subject to student tugging or any other stress. When I was having luck with the Exacto knife I was doing about 4 surgeries per hour.

           

Survey says…...

The following year, pleased with the improved stability and performance of the altered CBLs we decided to modify a second class set for the AP students. Mission accomplished.