I'm kinda new to EE, however. I can solder, but have no PIC programmer or anything similar, so I'd be buying protoboard and LEDs and the rest. Also (Just to add to the difficulty), budget is kinda limited on this one (Hence why I'm wanting to self-build rather than buy a completed unit as G-Gnome did for WMD), and I need to count down (And display) DAYS, hours, minutes and seconds. It should also have battery backup (For the time circuit, at least, so it only needs to be set once, and will keep counting down even if the power (and display) are off). So, does anyone have such a beast around? I've tried searching the 'net but all I keep coming up with are JavaScript countdown timers, which isn't much help. Thanks in advance.
hmmm what sort of output would you like, lcd, 7-seg? Theres an 7-seg count up timer thread, with some small modifications it could be used to count down easily enough. The current draw is fair though, however in the battery backup mode you could just turn them off. In terms of cost it really might be cheeper to buy a prebuilt. Your gonna have to pay for componants, when I built my 7-seg clock for AS electronics I had to use a 555 timer, assorted resistors, capacitors, BDC's, display drivers, 7-seg outupts, DIL chips. Then if this is gonna be a permanent thing then you might want to have a board etched. It would run into a fair bit, still i'd be a good learning experiance
I tend to agree, or a kit, but the 'days' requirement could be a stumbling-block on commercial kits. There are a couple of circuits here, one using a PIC which greatly reduces parts count, but only minutes and seconds.
Ideally, I want to drive LEDs to make this nice and big ad readable. I should hope it'd still come in at less than $360US (http://electronicsusa.com/ck3000.html) Not going to be permenant (One use only, another reason why I don't want to shell out for a pre-made one) and ideally I'd like to be able to customise the display some (Change the shape of the numbers, as much as is possible with a 7-seg-like display). If push comes to shove, I'm sure wherever sells PICs around here also sells programmers, if someone can furnish me with designs for a circuit and program to use. I was thinking of having the "Time Circuits" (Back to the Future, anyone) running from a 9v PP3, and the "Display" running from mains power.
Which is the main problem with all the ones have found. None seem to be for anything longer than 99 minutes/hours (At tops).
I've found something I think I can get my head around: http://www.interq.or.jp/japan/se-inoue/e_counter1.htm This is only for minutes and seconds, but it uses a 555 timer to generate clock pulses, and a BCD Countdown timer (Per digit) to handle the counting and display. All I need to work out for it is how to get the minutes to re-set to 59 at the top of every hour. I was thinking of using a NAND gate (Like the seconds do) to pull the "Load (LD)" line and the required data lines high on 9 (Which is what the counter reads after being counted down from 0) to re-load the counter to 5. The problem I think I'd have is that the re-set would interact with the pre-setting function ('cause I'd need to be able to set the value(s) before starting the countdown). It's also got seperate power drives for the display and circuitry (And I'm pretty sure I could use Diodes to have a battery backup on the +5v side, meaning the whole system is driven from mains (+12v) when present, and falls back to batteries (6v regulated down with a zener) when not. Comments? Suggestions?
Use 4 2x1 MUXs on the input of IC1 to switch in between loading a value for preset and loading 5 in during counting.
