A couple of weeks ago, I decided to learn how to program and use microcontrollers, and today I turned a new 7 segment display (single/10-pin) into a 1 segment display in a couple of seconds. I say we’re off to a good start.
it all started with hackaday.com. although most of the recent posts aren’t that good (it’s not their fault, it’s the hacks that are getting lamer and lamer, bleh!), some of the older projects posted at hackaday.com that were exceptional and down right amazing got me thinking about doing my own projects. since all the complicated projects required uCs, i needed to get my hands on one.
Almost everyone who knows what i do knows I’m an old fashioned TTL guy. I worked a lot with both 74XX and 74LSXX families, and know there attributes (fan out, current sinking, …) like the back of my hand. aside from 74*s, 7805s, 555s, and the required components required to get the circuit running, i never dealt with anything else.
anyway, I searched for a uC. i looked into PICs, and Atmels and ended up choosing an Atmel ATMEGA168. these are inexpensive (they go for 4.11$ at sparkfun.com) and powerful. but best of all, the ATMEGA168 can be programmed in circuit using high level languages (C). when i decided to buy some, I chose sparkfun. this is because of one simple reason: they provide a very well written tutorial. of course, i couldn’t just buy the uC by itself, i had to buy other *stuff* including the ill-fated 7 segment LED display.
I unpacked the package and inserted the 7SD into an old populated (555 astable circuit) breadboard i had laying on my desk since god knows when. without even referring to the datasheet I started experimenting to determine the pin layout (mistake #1). after doing a quick image search, i found the pin layout digram. i connected what seemed to be the OUT pin of the 555 to a segment’s electrode. the segment lite for half a second then went out. being the person i am, i tried another, which ended in a similar fashion. i know i blew something and that the display was probably history but still, there were five left :). anyways i knew that the 555 OUT wasn’t connected to the power rail directly, so exceeding the current rating was not a possibility. i continued on
burning testing the segments one after one until only one remained. then i realized what had happened; I connected the jumper to VCC!
There were two lines side by side. one was OUT which was buffered and could not drive a lot of current. the other, however, was connected directly to the power supply and could supply at least 800mA (upto 1A). no wonder why the thing wouldn’t light up.
bottom line: don’t drive 1000mA into a device that’s rated for 20mA or things might get hot and some get well.. destroyed.
edit: it turns out that one other segment survived along with DT (the dot). i guess i have a 2.5 segment now 🙂