WayBack: Discrete 74xx Project

Posted by FirmWarez on 1 Sep 2012 in Blog | 2 comments

I’m still geeking out over this. So, some twitter discussion got me digging through boxes of old lab junk for an 80186 SBC I built for a class while working on my undergrad. In the same box was another project, a timer gizmo done for a design lab, again, back in the old college days, designed around 74xx series discrete logic.

Let’s see what all is here. And whether you believe it, I’m going to do my best to say what the ICs are from memory. I promise not to cheat. Anypony want to look things up and see if I’m right?

Qty Part Description
1 555 Timer (duh, you’d have be a zombie to not know what a 555 is)
1 7404 Hex inverter
3 7490 Decade counter
2 7447 BCD to 7-segment LED driver
1 74121 Monostable multivibrator
2 7485 Ok, I’m fuzzy on this one…
1 8 position dip switch
2 red 7-segment LEDs

So, if I remember what my sophomore college self was up to, and keep in mind this was for a design class…I think the idea was it was an event timer and time recorder for some model rocket idea. Yeah, like that huge board is going to fly, but remember, for a class. Design had to work, for a grade, and be built be the me I was WayBack. Still I used some way cool uber elite “surface mount” resistors on it. Yeah, just stand there and stare in awe. Anyway, power up, to zero on the timer. First switch (launch detect) is released, starting the timer. Upon reaching the time set by the (dig it, programmable!) dip switches, yeah, in BCD, a relay would fire for some time. When the second switch was released, it would stop the timer. I think I had this idea of launching a small parasite rocket off a larger one after some time. The timer halt function was to record when the recovery system was activated, recording the time of the boost and coast phases. The little red LED is to indicate the relay firing, which in actual use would be the igniter of the parasite bird.

The rockets were never built however, probably for two reasons. 1) there are too damn many trees in Arkansas and it’s hard to find a good place to fly big rockets, and 2) I probably spent that grant money on beer. Oh well. Here’s a pic of the board and a video of the thing running, yeah, it worked wayback, and it still works today.

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