...but I make up for it with loads of stubbornness!

I've got a small homegrown music box which sports a 20x4 LCD and an infrared receiver for interaction. When I set it up with lcdproc, I realized that their wiring diagram for controlling the backlight asks for a pnp transistor (as it uses the nSel parallel port line which is active low). I didn't and still don't have any of those, so I simply soldered in an npn with the result of the backlight logic being inverted.

Of course such matters can be resolved in software. But on taking a peek at the code in question, I decided that I don't want to get my brain dirty fingering this messy stuff.

So, how do you cleanly invert a TTL signal using only npn transistors? I had no idea, as my practical electronics-fu is piss-weak, too. The play-hookey site as well as allaboutcircuits were very helpful, while this lcdmodule schematic simply didn't work as I found out after some laborious messy experiments with tons of test leads. I won't bore you to death with the details, but in the end I actually managed to understand how an RTL inverter works and how I can switch the actual current-controlling transistor with it. Soldered in 1kohm and 10kohm resistors, minimizing space usage as you can see from the photos below (but maximising the mess) and it works.

 2006_04_02-lcd-backlight-v2.jpg  2006_04_02-lcd-backlight-v2.1.jpg

SC1959 transistors are tough beasts! I had salvaged two of them from something else, and managed to fry one pretty badly: sent over 1A through it and apparently not across the junctions where it should have gone, so it got stinking hot and emitted a faint pop sound...I thought it would be an ex-transistor after that, but careful measuring revealed that it still worked. Sweet.

The reason for all this mucking around was that the LCD backlight had been on for a whole year now, continuously, drawing a good 250mA. It lights the living room up a fair bit which is annoying at night and attracts all kinds of flying bugs. Now I've overhauled the software-side (lcdraptor and a homegrown jukebox in perl) to go dark when not doing anything useful and added all kinds of things to them and lirc-xmms on the way.

Update (Sat 08.04.2006 02:42):

Well, looks like my solder-fu is improving. Just ripped the dead Ultra1 power supply apart, to look for bad or dead caps - not that I expect a 8-10 year old Sun PSU to have used crappy caps, but something must have died and electrolytic capacitors do have a limited life time by design. Found two somewhat seedy looking small capacitors (100uf and 22uf) in the secondary section, replaced them and contrary to my expectations the PSU works again :-) Amazing tech, this thing; prodding it with the voltmeter showed that it has two primary stages, one that sports a healthy 400V DC. I was somewhat careful not to touch these solder posts...

[ published on Thu 06.04.2006 13:08 | filed in brainfarts | ]
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