Who says you can’t live on the edge as a geek?  This next building block is really in there for safety more than anything else.  What am I talking about?  LiPo batteries.

LiPo batteries have an excellent stored power to weight ratio, however, they are contentious beasts.  You can’t overcharge them, or they’ll explode.  You can’t drain them past a certain point or they’ll become damaged and will burst into flames the next time you charge them.  Check this out:

YouTube Video about LiPo Dangers

One of the most common things I’ve seen in my research about LiPo batteries is that people tend to forget to disconnect the batteries after use and the battery silently reaches the critical discharged state over time while no one is looking.  It’s safer to just toss the pack than try to revive it, although there are many accounts of folks doing just that.

Well, I plan on having an on/off switch on the scoreboard, so I don’t expect to run into any issues of discharge over time when I’m not using the battery, however, I do want to have some sort of protection from excessive discharge while I am using the scoreboard.  I’d rather not have my scoreboard burst into flames when it runs out of battery.  What I’d much rather do is warn me of a low battery, and then have it simply turn itself off if the voltage gets critically low.

Luckily, there are a number of IC companies that make chips that do just what I want.  I decided to go with an ICL7665 from Maxim because their datasheet was very detailed and included quite a bit of information relevant to my particular application.

The design concept is simple.  The chip, which runs off the full 12V, has a couple comparators internally.  These comparators monitor the voltage on their inputs and toggle an output when the voltage is less than the reference.  The first comparator will be used for the Low Battery Indication LED.  The second, with a bit of hysteresis built in, will drive a MOSFET on the 12V input.  When turned on, the MOSFET will allow 12V to enter the scoreboard from the battery.  When the voltage reaches a critical level, the ICL7665 will turn off the MOSFET and the scoreboard will go dark.

I’ve put the design into a schematic and laid out a SMD version of the board.  I’ve not tested this, but I’m expecting to have this built in the next few days.  I will report on my success or failure.  Here are some pictures of the design:

[Note:  I’ve removed the schematics and board layout because there was an error in how I had the MOSFET connected.  I don’t want to post things that don’t work, so hopefully I’ll build this thing, test it, and then be able to repost the design.  Sorry folks.]

An interesting concept that I thought of and that I will be testing on a prototype board is whether I can have the ICL7665 be powered AFTER the MOSFET.  Why would this be a good idea?  Well, the IC still takes power, even when it’s cut the power.  If you put the IC after the MOSFET, then it will be powered down when the power is cut.  The problem there is how do you power things up initially?  Well, I was thinking a momentary switch across the Source/Drain of the MOSFET will allow me to temporarily bypass the MOSFET to allow 12V into the circuits to turn them on.  Not sure if I explained this clearly, but if this works, it’ll be really cool and I’ll have to write about it in more detail.

Cheers!!

It’s pretty funny how my neighbors view my different projects.  They’re always asking, “so…why don’t you just buy one of those on the internet?”, to which I reply, “none of those on the internet do exactly what I want them to do.” 

This project is no different.  I wanted an open scoreboard hardware platform that I can control from common wireless devices (mobile phones, netbooks, etc.)  I also wanted a scoreboard that didn’t cost me $1000 and that I could actually repair myself.  Well, I haven’t exceeded the $1000 mark, but it hasn’t been a cheap project either.  Subsequent versions (i.e. those made by fans of the blog) should be significantly cheaper without all the trial and error (and error, and error, etc.)

When I began to plan this project, in earnest, I quickly decided that the best way to pull this off would be to cut the project down into building blocks.  Mass produced products benefit from a single monolithic PCB that does everything.  For me, that simply isn’t the case.  This thing needs to be modular so that I can build the smaller building blocks, test them, and then move on to others.  I’m going to detail each building block in a series of posts along with the files I used to create them.

The building blocks I am working on now are score digits, inning digits, balls/strikes/outs, microcontroller, and finally the wireless interface.