Paul Evans

Perl, Electronics, General geekery.

Pronouns: he/him
Github: https://github.com/leonerd

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Last Notes

2026-02-07 21:42:49 UTC

Looking for software recommendations, for a Raspberry Pi.
Smallest/lightest way to redirect TCP port 80 over to some high-numbered port for running an http server as non-root.
I *could* run lighttpd or haproxy or somesuch; but is there anything even smaller?

2025-10-10 11:30:46 UTC

#postfix question:
My SMTP server has multiple IPv6 addresses. I have attempted to tell Postfix which one to use:
```
smtp_bind_address6 = 2001:8b0:3f7::100:2
```
My SPF record says "only accept from +mx":
```
leonerd.org.uk. 14400 IN TXT "v=spf1 +mx ~all"
```
And yet, I'm getting SPF fail bounces from google because postfix tried to send from a different address - namely, the interface's default.
What's gone wrong here?

2025-10-01 15:20:08 UTC

With no context whatsoever, I present this as today's hacky code snippet solved a small problem:
https://cdn.fosstodon.org/media_attachments/files/115/299/578/385/961/964/original/0a54093fb80e7c99.png

2025-09-23 13:17:04 UTC

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I was also pondering a `->\&` operator for this purpose. I don't know if it's too subtle.
my $mref = $obj->\&method;
would basically act the same as
my $mref = sub(@args) { $obj->method(@args); };
but possibly be more efficient to implement internally, not needing a full CV clone and closure.
But I don't know if the syntax is a bit too subtle.
Maybe a subject for someone to write up a PPC? ;)

2025-09-04 11:55:36 UTC

#makefile question
I can set a variable for only one specific target, by:
```
targetname: VAR="value"
```
I have a whole bunch of targets, and I wish the variable to have one value for one target, but a different value for every other target. Can I do that somehow?
There doesn't seem to be a target negation ability, or negative pattern match.

2025-08-30 09:56:10 UTC

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See also
(from https://www.reddit.com/r/mildlyinteresting/comments/h9lr64/the_toilet_paper_roll_in_my_hotel_is_put_sideways/)
https://cdn.fosstodon.org/media_attachments/files/115/117/112/935/035/584/original/c2a55c3df47b73fd.jpg

2025-08-24 13:33:11 UTC

"Oh, when you said you were a prompt engineer, I presumed it had something to do with LLMs"
"No, I just work very fast"

2025-08-18 17:16:25 UTC

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2/
Normally there's only one controlling node in charge of many others, but various "interrupt"-type situations call for any of the others to spontaneously emit a packet. These spontaneous transmissions cause bus contention if multiple nodes try to transmit at once. Since my protocol has checksumming and retransmission, I don't need to handle actual multiple access, I just have to detect this contention and have at least one side abort the transmission attempt and try again later.

2025-08-18 17:16:03 UTC

A thread on an #electronics problem, relating to transmitter contention on a RS-485 bus.
1/
I've been working a lot lately with a system based on an RS-485 bus, with multiple nodes on it. The bus talks async serial at 115.2kBaud, in short bursts of self-contained packets. Each packet has checksumming and sequence numbering, with a protocol of timeouts and retransmissions built on top of it for reliability.

2025-07-24 20:07:54 UTC

I recently worked out why there's a weird dip in the sunlight graph of my weather station every day. It turns out the atmosphere sensor shelter box is taller than the optics sensor, and casts a very particular shadow.
https://cdn.fosstodon.org/media_attachments/files/114/909/984/082/525/532/original/4c8e8c91bd7743cb.png
https://cdn.fosstodon.org/media_attachments/files/114/910/000/778/673/665/original/824ec6b357b4ce38.jpg

2025-07-19 16:49:39 UTC

> No, generative AI does not count as fair use, EU report finds
https://filmstories.co.uk/news/no-generative-ai-does-not-count-as-fair-use-eu-report-finds/

2025-07-06 11:00:10 UTC

#TodayIAchieved Much faster display drawing code in my frequency meter.
By the magic of making a second one, I can use one to measure the update time of the other. A total rewrite to more efficient code brought this from 14 fps to over double, at almost 35 fps.
https://cdn.fosstodon.org/media_attachments/files/114/805/923/214/642/185/original/ea56f4de7fbe7442.jpg
https://cdn.fosstodon.org/media_attachments/files/114/805/932/030/241/489/original/fd8ec0373c128e8e.jpg

2025-06-14 20:55:13 UTC

Looking to find some folks who know about the events system and TCB timers on the newer #AVR microcontrollers. I am encountering some strange behaviour.
Specifically, as far as I can tell, only event system channels 0 and 1 seem to be connected to event inputs on the TCB timers on my ATtiny1626 chip. Code which works fine on those two channels fails to do anything if all I do is switch it to any of channels 2 to 5.
This is troubling because in my current application, I need 3 channels.

2025-06-11 16:22:01 UTC

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I maintain that Windows 95/98 were the peak of usable UI design. It's all been downhill since then

2025-05-28 20:41:04 UTC

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Thanks.
Also, Ooof. Predictable yet still scary stuff. :/

2025-05-08 14:18:59 UTC

I've got myself a nice little portable USB-C soldering iron that uses T12 tips. It doesn't have a built-in battery though, so I'm in search of some sort of USB-C output battery pack that can output at least 15V, if not 20V. Are there any such?

2025-05-06 11:36:44 UTC

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What's EC or ECS in this context?

2025-05-01 09:41:06 UTC

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builtin::trim was new in 5.36.0, so congrats - you already have it.

2025-04-27 17:01:02 UTC

I forgot to design in some freewheeling diodes onto this 12V motor controller. Fortunately managed to find myself some DO-214-sized ones that fit nicely across the pin header for the connectors
https://cdn.fosstodon.org/media_attachments/files/114/410/972/407/189/887/original/7a68d1eec27ed260.jpg

2025-01-20 12:05:05 UTC

#electronics folks: I'm looking for some kind of panel-mount low-voltage DC connector, for doing controlled power *out* of a control box, into some cable-attached devices.
DC barrel jacks would be OK, except that's for power into the control box. So I'm looking for something else.
Have considered such things as GX12 or mini-XLR, but both of those have panel-mount male and cable-mount female connectors, which is the wrong way round for power output.

2025-01-08 16:25:20 UTC

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Hmm? perlfaq7 was written a very long time ago and isn't aware of all these newer things. In any case, I don't think it should be considered as a gatekeeper for what new ideas we can add.

2025-01-05 19:32:06 UTC

Ugh. Laid out a circuit board with this INA219 chip and somehow failed to properly read the data sheet on these two pinouts. The two different package sizes have mirrored footprints. Of course I made the wrong one.
Well, at least that explains why the circuit doesn't work.
https://cdn.fosstodon.org/media_attachments/files/113/777/409/189/627/902/original/c82f1e957907cd72.png

2024-09-09 18:24:01 UTC

I'm really not having any luck at all with USB-C docks. :(

2024-08-16 23:30:51 UTC

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Ahyes, "inf" and "nan", the unfortunate almost-a-number values