Hi, how about using an 7805 linear regulator?
It can be used without external parts, but a small heathsink, a diode and two caps might be recommended. Another diode on input side against accidental reverse polarity is a nice extra (if it was a PSU, I'd add a fuse too).
78xx needs input voltage being a few volts about output voltage (2-3v?).

Edit: Back in 20th century, I would just have used an 4,5v lantern battery. Like the 3LR12.
It was the spritual predecessor to 9v battery (two of them replaced a 9v battery).
https://en.wikipedia.org/wiki/Lantern_battery#4.5V

Edit: Protecting the LPT port is a good idea.
The parallel port is sensitive against shorts and overccurent/surge, I think.
Adding some resistors for current limiting might provide some level of protection, maybe.
If it was an one way road (some pins output only/some others input only), adding diodes for protection might help.
Though some people may argue that diodes can cause fluctuations in voltage somehow.

Be very careful about which SD breakout boards you use. A lot of them simply connect the SD signals directly to the breakout pins without any regard to voltage conversion; connecting them to 5V I/Os can fry the cards.

Jo22 wrote on 2025-01-10, 20:19:

Hi, how about using an 7805 linear regulator? It can be used without external parts, but a small heathsink, a diode and two caps […]
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Hi, how about using an 7805 linear regulator?
It can be used without external parts, but a small heathsink, a diode and two caps might be recommended. Another diode on input side against accidental reverse polarity is a nice extra (if it was a PSU, I'd add a fuse too).
78xx needs input voltage being a few volts about output voltage (2-3v?).

Edit: Back in 20th century, I would just have used an 4,5v lantern battery. Like the 3LR12.
It was the spritual predecessor to 9v battery (two of them replaced a 9v battery).
https://en.wikipedia.org/wiki/Lantern_battery#4.5V

Edit: Protecting the LPT port is a good idea.
The parallel port is sensitive against shorts and overccurent/surge, I think.
Adding some resistors for current limiting might provide some level of protection, maybe.
If it was an one way road (some pins output only/some others input only), adding diodes for protection might help.
Though some people may argue that diodes can cause fluctuations in voltage somehow.

Linear regulators don't seem quite ideal as I understand they tend to float high until you pull a couple of hundred mA from them, and this is gonna be ~50mA... then also with battery use, it's not really ideal having 30% of you energy warming up the 7805. If I were pulling off a robust 12V line, I might say hell with it and give it a power light to pull enough to stabilise, or maybe "use up" some mA with a buffer chip for extra peace of mind but doesn't seem right for batteries.

Those lantern batteries, seemed in a "weird place" when I was young. There was this one thing in the house which used one, and they were either nowhere to be found in the stores, or findable and $10, or suddenly available widely and $2... unstable supply and price... so I was never particularly tempted to use one for a project, despite older sources recommending such. Anyway, 4.5V I know would be fine for TTL supply normally for something standalone, but I want to avoid the problem where if supply is at 4.5 and maybe still sucks off the port lines because they're a bit higher.

jmarsh wrote on 2025-01-10, 20:22:

Be very careful about which SD breakout boards you use. A lot of them simply connect the SD signals directly to the breakout pins without any regard to voltage conversion; connecting them to 5V I/Os can fry the cards.

Good point. I have the ones I bought when Universal Solder had a sale a while back and they have the proper converter on. Though I wish there was more info available about 5V tolerance on SD and older MMC cards, since I have a stash somewhere of full size ones that came free with cameras etc in the noughts and are all tiny-for-today sizes like 16Mb, 32Mb, and I would totally just wire some of those up soldered to 25pin plugs on the Ivan Drago "If he dies, he dies" principle... if I was sure they'd actually work for a while first... though those aren't at hand at the moment and have only the one plug to work with right now.

BitWrangler wrote on 2025-01-10, 23:53:

Linear regulators don't seem quite ideal as I understand they tend to float high until you pull a couple of hundred mA from them, and this is gonna be ~50mA... then also with battery use, it's not really ideal having 30% of you energy warming up the 7805. If I were pulling off a robust 12V line, I might say hell with it and give it a power light to pull enough to stabilise, or maybe "use up" some mA with a buffer chip for extra peace of mind but doesn't seem right for batteries.

Hi, I didn't think about this., my bad. 🙁
Back in the day, I've used the 7805 and 7812 for most projects and didn't think about this detail.

Hm. Does adding a power light help? A normal red LED draws 20 mA, that's not enough.
How about an old incandescent lamp? Bicycle lamps had used 3v or 6v?
The 6v lamp seems to draw 2,4W - that's 0,4A (400mA).
That might be enough as a basic load for the 7805.

Or how about the little 7805 models in TO-92 transistor case (LM78L05-C845)? They're rated 0,1A.
I have little experience with them, though. 🙁

BitWrangler wrote on 2025-01-10, 23:53:

Those lantern batteries, seemed in a "weird place" when I was young. There was this one thing in the house which used one, and they were either nowhere to be found in the stores, or findable and $10, or suddenly available widely and $2... unstable supply and price... so I was never particularly tempted to use one for a project, despite older sources recommending such. Anyway, 4.5V I know would be fine for TTL supply normally for something standalone, but I want to avoid the problem where if supply is at 4.5 and maybe still sucks off the port lines because they're a bit higher.

I understand. I must admit that the battery was slightly out of fashion already in my youth, too.
It was still readily available in petrol stations, though,
because those flat torch lights for cars had used them (signal lamps with red/green filter, too).

They looked like this:
https://www.rohling-express.com/rohlingexpres … henlampe-P.html

That being said, the 4,5v battery was very popular here in Europe in the decades before the 90s, though, so I think the Wikipedia article is right.
In my country, the 4,5v battery was used in school and in electronic construction sets, at least.

Probably because it had lots of current (mAh) but was otherwise safe.
Even if shorted, the 4,5v battery wouldn't catch fire so easily. It would simply discharge.
Doll houses used the battery, too. There was an optional plastic cap for 3LR12 with a switch and a few sockets, meant for miniature plugs.
Over there in former GDR, the coal (carbon) version was very popular and just called "R12" or so I heard.