The circuit should not use a PIC, and an IC is preferable.
The circuit needs to countdown from 180 minutes to zero.
Intended use: A start button to begin counting A red Led lit while counting At the end of the 180 minutes the red Led should go off, green led on Hit a reset to turn all leds off Circuit sits idle until start pressed again.
I also need the "countdown" displayed on 3 seven segment leds.
Why? If this is homework you should say so. If you mean "an IC" literally, then you are going to need a microprocessor of some sort, or programmable logic.
sci.electronics.basic is the place for this question.
Since this problem has "homework" written all over it you should make that clear, point out what part of the assignment that you don't understand, and perhaps give a short, one-sentence explanation of why you can't go to your professor/tutor/grad student and ask their help.
You should _not_ expect anyone to answer it for you. Most denizens of these groups take a fairly dim view of doing someone's homework for them.
Unless the class is being very poorly taught you should have been given all of the necessary bits of knowledge to make this work: you just need to pull them out of your little bag of tricks in the correct order to make the circuit.
I trust your assessment of my mental stability in this area, but I do not have PIC programming capabilities, nor do I have the resources, or time to learn how to program a PIC.
Is there really now way to do this using off the shelf parts? Justin
(missing context says:) I need help designing a timing circuit.
The circuit should not use a PIC, and an IC is preferable.
The circuit needs to countdown from 180 minutes to zero.
Intended use: A start button to begin counting A red Led lit while counting At the end of the 180 minutes the red Led should go off, green led on Hit a reset to turn all leds off Circuit sits idle until start pressed again.
I also need the "countdown" displayed on 3 seven segment leds.
Any help appreciated!
cjdublu wrote:
For easy reliability from a 555 you want to run it no lower than about
10Hz. You can get reliability from it at lower speeds, but "easy" goes out the window.
I'd do this with a timebase, perhaps a 1-second one, and a chain of dividers. So:
Make a 1Hz clock. I'd consider a 4060 and a watch crystal, or a 555 divided by 16. The 4060/crystal combination will be more accurate.
Divide it by 60 with a couple of dividers to make a one minute clock. Hopefully you can still find 74xx divide-by-10 and divide-by-12 counters to do this, or you can find a loadable counter. A loadable down-counter would be best, because then you could have a seconds readout if you were so inclined.
Take the one minute pulse and divide by 180. I'd do this as a minutes counter and an hour counter, and I'd look to using a loadable counter or divide-by combinations.
Figure out how to make it all stop counting when it reads 0:00:00. If you go this route you'll be looking at using at least six 16-pin packages for the counters, plus goodness knows how much glue logic.
Or you could get a Basic Stamp and do it all that way, and only have to learn how to 'program' in Basic.
Keep in mind that there is probably a PLC out there that would do this job for you easily. It would require programming in it's own peculiar ladder logic, but it would have all the hardware and switching that you need built in, and you could probably induce it to show you the down-count as well.
The count down 7 segment LEDs is the simplicity killer here. It is not going to be a one or two IC solution. I know you don't want to use a Pic but using the kit at Radio Shack for and three hours of time and you would be 25% of the way to making this circuit work.
If you don't want to use the Pic go to Radio Shack anyway and pick up Volume I: Timer, Op Amp, and Optoelectronic Circuits & Projects by Forrest Mims.
Or just keep googling. Buy one of these and reverse engineer it:
If you use something like a BASIC Stamp [microcontroller], I can pretty much guarantee that you'll spend far less time _overall_ getting the design done than if you do it with "jellybean" logic ICs. Picking up BASIC is really easy with one of the "development" boards and a tutorial. You'll also have learned a useful new skill...
There is... something like a 74HC925 or '926 does a lot of it... it's a 4 digit counter/multiplexer/7 segment display driver all in one IC (available -- at least at one time -- in F, ALS, LS, HC, HCT, and even LVX). You can probably still find one somewhere, but people just don't do *new* designs with chips like that when using a microcontroller is typically no more expensive and far more powerful.
Well, a typical PIC capable of doing this sells for under a pound, as does a simple AVR (ATMega series). I don't think I could do this in ordinary logic for that nowadays.
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