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 [ 3 posts ] 
 Making A System More Portable 
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Joined: Sat Aug 03, 2013 11:02 pm
Posts: 43
I have been messing around with zx81's, and I want to know a few things. Can I create a (somewhat) portable power supply using a 9 volt battery. If yes, about how long would that last during a normal operation. Second off, the display, what small portable display could I interface to the zx81.

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Fri Dec 20, 2013 1:19 pm

Joined: Tue Dec 11, 2012 8:03 am
Posts: 285
Location: California
I had one of those small Timex Sinclaire computers (I can't remember which one). I don't think it used CMOS parts, so the power consumption would have been too high for economical 9V battery use. Do you know how much current it takes? If it's 250mA for example, you might get as much as two hours of use from a 9V alkaline battery, and a lot less per charge from a rechargeable one.

I don't know what kind of TV you could get today that would be tiny. There were pocket TVs (I remember Radio Shack selling them) before smartphones and digital TV, but there probably weren't enough sold and they probably weren't taken care of well enough to still be operational so you could find one on eBay.

What might be a good goal would be to see what you can learn from the packaging of computers like the ZX-81 and apply it to a computer of your own build. You'll want to use CMOS parts of course, not NMOS, and not 74LS. There are lots of shapes and sizes and colors of ABS plastic project boxes available from gobs of manufacturers (just take a look in the Mouser catalog, or even Jameco which is much smaller than Mouser). ABS is marvelous material for milling with a Dremell tool and filing and drilling and hot-glueing. Some come with battery compartments for the 9V or AA batteries. I'm sure you won't be spending the tool-up costs for a custom membrane keyboard like the Timexes had, but there are some smaller off-the-shelf ones, and you can use something like the Grayhill series-87 keyswitches which fit neatly into standard prototyping board and their transparent keycaps can be removed and put back to put your own custom-printed labels under them. LCDs come in a gazillion sizes and types, and there are some tiny VGA adapters like I show on the displays page of my 6502 primer.

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Fri Dec 20, 2013 7:27 pm WWW

Joined: Wed Jan 09, 2013 6:54 pm
Posts: 1690
A quick search for [zx81 power supply] gives us "The ZX81 requires 420 mA of power at 7–11 V DC"

But you should know that dry cell batteries have a substantial internal resistance, so for high currents you get a substantial voltage drop within the battery. Another search [9v battery max current] suggests 7 or 8 Ohms. Another page suggests nearly 30 Ohms: ... hp?t=75345

You can use physically bigger 9v batteries or run many in parallel if you need more current, or you need a lower voltage drop from the internal resistance.

Sat Dec 21, 2013 9:13 am
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