Jack of All Trades

Can we take a moment to remark on how wonderfully useful PdfJam is? Behind the scenes it’s just LaTeX. Thus you might be surprised to notice that

pdfnup --nup 1x1 --paper a5paper --no-landscape image.png 

Is an excellent way to turn an image—say, a bunch of screenshots of a page which you concatenated with

convert image1.png image2.png image3.png -append image.png

Into (say) an a5 pdf, ready to be turned into a full-length pdf with

After a few months of balancing my bicycle between two walls where once, it would seem, was a pantry, it got too annoying. Ideally one would like some kind of stand to hold the bike out the way, vertically. The internet turns up many such designs. After a trip around the rather poor selection of wood available at B&Q (the only place open), I settled on something slightly different—an L-shaped half-lapped frame with wooden chocks to space it, and the bicycle balanced with just a small inward turning moment counteracted by its own weight.

One expects a student’s life to have a certain ascesis to it. One, for instance, does not buy rings for one’s napkin, one makes them. And so I procured some cord (the seller considered me carefully and then said that I had a face which permitted me to indulge in sailing, which in turn justified buying cord to tie knots—‘Sailors fiddle’, she said approvingly; though I cannot play the violin). And I procured a helpful video on the tying of 3-lead 8-bight Turks Heads, because I can never remember any of these fancy knots. Yet being a student is no cause for shoddy workmanship. It seems I cannot recognise cotton, for I tried a butane backsplice. But lo! cotton does not melt. So the question is, how small a sailmaker’s whipping can you make?

I’ve always thought of those clap-activated switches as a bit of a gimmick—after all, you can always just move the lightswitch if it’s that hard to get at. But currently my mother is hobbling around on crutches, and it suddenly struck me that turning the uplighter on from the doorway would be handy. A rummage through the junkbox found a board—I think from an old washing machine—full of relays, and 5v relays at that, so no need for a voltage multiplier to turn the relay on. I also found an old ‘usb charger’ rated at 1A 4.2V (?!), which is thankfully enough to turn on the relay.

On a rainy day here I wondered exactly when I should book a ticket up to Durham for another term. Most of the booking websites will help you, but not much: it’s tiresome to check a few days to see what the price variation is—and it can be fairly enormous. I was also curious as to what the long term trends might be: they don’t just seem to go down. All of which suggested some code to screenscrape all the tickets from somewhere and analyse them. It seems the national rail website encodes the query straight into the url (though not as an http query) and then sticks the results, in json, in the page served, presumably for some javascript to parse later. So all we need is to generate the url:

Since this has, alongside other things, occupied a whole day, I thought I’d put it up here in case anyone else tries. Back in the day KiCAD had a python scripting console. Currently that would be very useful: but it’s implemented with gtk2, and everything ships with wxwidgets compiled against gtk3. So we have to compile wxwidgets, which is fairly straightforward: get the sources compile—except it’s not a ./configure, make, make install job: rather a python script calls other python scripts, and so on. And of course there was a problem: a function whose argument might not be a char buffer throws a format-security error, and the whole thing won’t work. Normally we’d just edit the GCC flags and set -Wno-format-security, but how to do that here? I spent ages ag-ing around the sources trying to find out where the python code actually called gcc, and was about to give up. Then I thought of environment variables. In fish (which is not quite bash-compatible) we do:

Now that we have a working counter and display the next thing to look at is the timebase. The timebase I built when first working on this counter is odd. It uses, for some reason, a 3.2768MHz crystal divided by (2^16 * 10n) to get a gate pulse which is a multiple of 1s, then further divided and decoded by a 3-to-8 decoder to generate pulses which latch the counter onto the display, and reset the counters. This much was copied from EMRFD but without the intelligence that excellent book expects: obviously, this solution is fine for a quick gate but highly tedious for the long gates I decided to add, as well as being useless for pulse-width-counting, which I wanted to make it do like G0UPL’s counter—the other project I copied rather blindly. I’d be much better off with a 10 or 20MHz crystal and drop the needless 2^16 counter. But more on that later when I finish it.

The internet in Durham, which was allowing me to use the workstation there remotely, has gone down; support tell me to ’turn it off and on again’. But obviously I can’t do that remotely. Ah well, it wasn’t really needed. But what if it were? I’ve run servers over unreliable wifi links with good uptime before, with a failsafe script which rebooted everything if it couldn’t ping the gateway for too long—and looking at the logs, sometimes it was needed. But I can’t reboot the router (and though I’ll replace it, there’s little opensource DSL firmware, so I’m never going to be able to reboot the modem). So what we need is the remote equivalent of pulling the plug out and putting it back in again. I have a few of those Poor Man’s Rasberry Pi boards lying around; one could surely be made to pulse a relay if it can’t ping after a while? But the only relays I have (mainly rescued from a faulty boiler) are designed for industrial 24v DC. I hunted for a while and then gave up on finding any 12v relays. Isn’t there a circuit called a voltage doubler? As usual, SM0VPO has some information. I was going to use a 555 timer, but that shamed me: I can still build a discrete multivibrator—it was one of the first circuits I ever built, after all. But first off the relay inline in an extension cord. Here I made a completely daft error: the box is about 100% too long, because I thought the cable exit grommets went the other way round. But anyhow, here is how to make a box out of hardboard, that most intractable thing:

I started this peice of test equipment when I was about 15, I think, built the timebase with enthusiasm, the counting chain with a bit less, and then wondered how on earth I was to attach the 7-segment displays. Surely not one wire each?! So the project languished, and eventually got relegated to the shelf without ever being finished. All I had was the breadboard and two schematics which nearly corresponded with what was in front of me:

This is a very simple project indeed: add a line-in input feeding the amplifier in my sister’s CD/Tape/Radio, which has much better sound than the ‘device’ she mainly plays music from. How hard can it be to find the amplifier IC and feed in directly?

Taking the thing apart was none too hard, and it’s cheap ’90s hardware so all through-hole and single-sided PCB. I found the mode switch, traced it back to the amplifier, paid careful attention to the pinouts and soldered a small cable onto one input, the other end going to a 555 ‘signal generator’ kit I had lying around. Power up: the signal generator exploded and the lcd backlight died.