Superprobe meets vintage multimeter

One of the first tools I built myself many, many (decades!) years ago was a logic probe. At the time things like yellow LEDs were still a bit rare and cool, but this baby had both red, yellow and green LEDs to indicate TTL logic levels. It was a true rat’s nest of wires – and everything stuffed into one of those plastic cases for one-time travel toothbrush that you (used to) get at some hotels. A stiff copper wire was glued to one end of the case. Ugly as h-ll, but it worked quite nicely.

Years have passed, and I actually found that probe a year or so ago. It still worked, but with access to things like multi-channel logic analysers and digital storage oscilloscopes, that probe was sent to rest at the place where electronics never return from.

Still, the basic concept of a logic probe IS kind of nice. And as some people have taken this concept and extended it to include a bunch of other tools in the same circuit, I thought I’d spend some time building a new probe.

It’s really just a Superprobe in a somewhat unique (probably not, but I like the concept..) case. The Superprobe exists in various flavours, the ones I’ve liked best so far are the original onethe MKII one, and the one from Dangerous Prototypes.  The one I built is a mashup of the three, dropping the voltage reg (as I feed my probe from a USB cable, which provide a stable +5V that the PIC can use. Adding programming headers á la the DP probe was a must-have. But dropping the display resistors, seems to work just fine without them.

The project really kicked off when I was cleaning out some boxes of old stuff, and found the first digital multimeter I ever bought, probably around 1985 or so. It is a total piece of junk, was probably the same back then, but still expensive at the time. Interestingly enough, the main ADC chip of the DMM was a MAX131CPL – which is still available for purchase today!!

Off to work then.
After ripping apart the DMM and stripping away all components, LCD, daughter boards etc (all through-hole components, of course. This is pre-SMD times.) I was left with some space in the DMM that should nicely handle the Superprobe circuitry. It might even be possible to squeeze a Dangerous Prototypes Part Ninja in there, and then multiplex the display between the two tools… Nah, one thing at a time, I’d rather finish the probe first.

It all comes together quite nicely, with a mini USB jack providing power to the probe, the probe’s two push buttons are hot glued to the upper left/right sides of the case, making it easy to operate them both when the meter is on a table, and when it’s held in hand (in that caseit’s actually possible to operate the whole probe with a single hand, using thumb and index finger, while the probe rests in the palm. Nice!).
The input jacks from the original DMM are re-used and soldered to the input and Gnd of the probe, with one of the original mechanical range switches wired as on/off for the probe.

Works like a charm, the pictures below show the probe while in use.
Only one small glitch, not sure what’s going on. When the probe is turned off, it needs a minute or two before it agrees to turn on again. Some capacitor that need time to discharge, I guess. Thinking about adding a reset button.. should be an easy thing to add, just hooking pin 1 to ground through a small push button.

All in all – nice little project that is likely to be quite useful up ahead.

Smoke tester from Dangerous Prototypes

A while back I got a free PCB from Dangerous Prototype’s Free PCB program. It’s a nice little board designed to provide easy current monitoring during prototype stages of a project. Features include over-current tripping with visual indication, as well as dual 5V and 3.3V supplies.

So, a week or two ago I was about to reverse engineer a laptop camera (built with very small components…). The camera’s attaching cable has unknown pinout, but system block diagram indicate it’s a USB 2.0 device. The cable is a 6-wire variant, so it shouldn’t be too hard figuring out which wire does what. It would however be nice to have a controlled power source feeding the camera during the work.

Enter the smoke tester… I figured I would build it, to the extent possible, with parts already found in the lab, if needed maybe even scavenging some old computer or similar..

The result is pretty nice:

smoke_tester_20130227

Some comments/feedback on the design:

  • Input power screw terminals don’t have +/- marked on the board.
  • Holes for bana jacks too small for any of the jacks I’ve tried.
  • Really nice with the three parallel pads for INA 138 load resistors. They give good flexibility for selecting shunt resistors for the INA 138. For example, I used a 0.1 ohm (instead of 0.075 ohm as suggested in the schematic) shunt resistor, which means the load resistor should be 50 kohm. Easy – two 100 kohm resistors in parallel does the trick.
  • Documentation is very limited, basically just a forum thread and schematics (that don’t include all component values, e.g. R13 and R14.

A few components are missing on thew board:

  • USB out connector. I had a hard time finding an SMD connector that would work. Figured I’d mainly be using the screw terminals anyway.
  • Banana jacks. Going through the parts bins here, all the banana jacks were too big to fit in the PCB’s holes. Those wholes could be a bit larger, IMHO.
  • Input power jack. Couldn’t find a suitable SMD one, but as I will be taking power from a wall wart with USB output I will be fine.
  • R13, R14. There is no value for these in the schematics (as far as I can tell), leaving them out thus. But as the USB out connector is unpopulated anyway, it’s not a problem (right now). Edit: They should be ca 20 ohms.

Links:
Dangerous Prototype’s forum page