So this idea started with my need for reliable power I can use for each calc my school has me fixing, so I can test them without the batteries being a factor, and without buying tons of batteries. I bounced a couple of ideas off the fine gentlemen here at Cemetech, when KermMartian suggested I use USB instead of a wall-wart, which would provide clean, reliable power. With this idea in mind, I went and made a quick model of what I wanted to do:



I shared this model(which has the - line incorrectly colored for aesthetic purposes) with the group here, and Kerm signed off on it. So about an hour ago, me and my dad got to work prototyping it, and this is the result:



The wires go through the lego battery and are soldered to the contacts at the ends, which are in turn hot-glued to the lego rods.

However, after plugging it in, I was unable to get any response from the calculator. Both the calc and the computer I plugged it into are still working fine, but the calc won't run off the power from the USB cord. I still need to test it with a multimeter though, so I'll keep you posted on my progress once I have a chance to get to radio shack.

Point to note, for the last 2 years my 84+SE ran off of USB power entirely. I just stripped a USB cable and stripped the wires and twisted them around the contacts

Allow me to share credit with my esteemed colleague Ben Ryves, who though he does not have the formal training that I've gone through, has more practical real-world experience and surpasses me handily in knowledge of many things, especially microcontrollers. I look forward to your results with the multimeter. It's possible that your computer is being smarter than you want it to be with power. Do you happen to have a powered USB hub or something that you could test it with?

I tried it with a USB hub first, but it didn't have an external power source, it just got it's power from the old computer it was connected to.

*its power. Gotcha, fair enough. And you checked the polarity etc, I assume?

Red is positive, black is negative, right?

EDIT- whoops! it looks like when I was gluing the contacts, I accidentally pulled the black wire. Time to go back to the workbench...

Ouch. Yup, red is positive, black is negative. Your diagram has the correct placement of each.

I just tested the bare wires on the battery contacts of the calc, to see if my line was good, and it worked. I accidentally shorted it and overloaded my USB hub in the process, but at least I know it works, and as a bonus, I now know there are fail-safes built into my computer in case I mess up.

EDIT- fun fact, your tongue makes an excellent wire tester for low voltages...

True story, I've used that trick for many many years. Also great to hear both that the power is sufficient and that you have an intelligent hub.

This looks like an interesting project, but I'm not fully sure I get how it works. Is this just using the +5v from the USB? Doesn't the calc show up as self-powered instead of Bus powered, though, essentially making this useless as it won't use the +5v?

Notice that he's not plugging USB into the USB port, though, he's just using the +5V from the USB host in the place of batteries.

From your diagram, it looks like you are only connecting the USB power to two batteries, but the calculator requires four batteries to run. I could be wrong though.

The four batteries in the calculator are in series. Think of it like this:

+ {|||] - + {|||] - + {|||] - + {|||] -

The three intermediate -+ connections don't really matter; it's between the leftmost + and the rightmost - that you get a 6V differential. Therefore, the calculator only really connects at those two points; the other metal contacts are just to make the - + connections.

I'm surprised no-one has yet commented on my choice of materials for the battery stand-ins...

I think LEGOs are the natural choice there from many of our points of view, so no one really thinks it strange.

Oh yeah, I use legos all the time to model stuff.

For my Digital Logic Design (DLD) course I almost made a final project that was a PoV (Persistence of Vision) display using LEGOs for the mechanical bits.

Update: while I haven't been able to finish this yet(my dad works a lot) we have decided on what we're going to change for when we re-solder this. First, we're using nice round contacts stripped from dead AAA batteries. Second, I'm adding a notch to the stud at the end of the last cylinder in each rod, with an equal notch in the bottom of the small 1x1 cylinders that will cap them off, with the wires running through the notches so as not to be pressed against the spring or contact of the calc and worn down. I hope to get this done later today, as my progress report for the calc project is due tomorrow, and I at least want to have this and a few diagnoses done to show what I've learned so far.

Superb, thanks for sharing. I hope you were careful about the caustic contents of the batteries when you stripped out the contacts. I'll look forward to pictures when you get a chance!