Countdown timer question

Thread Starter

Adrastos

Joined Jan 5, 2009
17
Hello all,

I'm very new to electronics, but right now I need to make a countdown timer for a game (based off "Celebrities" in the show "Sportsnight"), involving various components. Most of what I need to do I already know or can figure out on my own, however my question relates to the actual counting / display ICs.

The timer has to count down from 120 seconds, to 0, and when it hits zero, it should beep / buzz / whatever. Since I don't have a microcontroller programmer, and know very little about them, I figured for now I'd do it the good-ole-fashioned way, rigging up a bunch of IC's together.

What I've come up with is to use three cmos 4510's (up/down decade (0-9) counter with preset) in series (one for the hundredth's place, one for the ten's place, and one for the one's place), then feeding the outputs of each of those "decimal places" into their respective 4511's (BCD to 7-segment display driver), and then into the 7-seg displays.

Am I correct to assume this is how these counters work? The one's digit will count down from 9, to 0, triggering the ten's digit to count down one, which when *it* reaches 0, will trigger the hundred's digit to count down one? My only question would be - how to set the timer to initialize at 120 seconds? (resetting each IC puts it's outputs at 0).

A couple other ideas I had:

  • Use a standard up counter, but then using a full adder, subtract the "up" count from 120, then displaying that. So, when the up counter is at 0, the output is 120, when it's at 30, the output is 90, etc. Only problem with this is it's somewhat hokey, and I can't figure out how I'd then convert the 8-bit BCD required for that into the 3 7-seg displays.
  • Use a microcontroller & multiplex the output to the 3 7-seg displays. The internet has a lot to say about this, so I'm sure I could do it, but I don't have a programmer, and no working knowledge about microcontrollers, and a somewhat limited budget at the moment.
Thanks for your time and consideration!
 

beenthere

Joined Apr 20, 2004
15,808
I would suggest using a 4029 instead. The 4510 is a binary counter, and will have several illegal inputs into your 7 segment driver. The 4029 can be controlled to be a BCD counter. Get the data sheet to see how to cascade these counters, plus the inputs to make them count down.

You should be able to set the counter to 120 with a pushbutton (4029's have a parallel load function) and start a logic clock running with another to count the 4029 string towards zero. The last carry when it hits zero will be a signal to sound the buzzer.
 

Thread Starter

Adrastos

Joined Jan 5, 2009
17
Yes, right, I forgot. Also, how I have the massive amount of pull up / down resistors, is it acceptable to combine for example the resistors on the 9 input pins on the 4029s to have one resistor, such that the 9 pins all connect, in parallel, then one giant pull down resistor (in series with the 9 pins in parallel) takes them all down to ground?

Thanks!
 

beenthere

Joined Apr 20, 2004
15,808
The current limiting resistors are only necessary in the drive lines to the displays. With 5 volts, 330 ohms would be a good value.

CMOS logic inputs may be tied directly to ground or Vcc. No resistors are necessary.
 

Thread Starter

Adrastos

Joined Jan 5, 2009
17
Two things:

  • Should not the current limiting resistors be on each input line to the display, so that all the digits will have the same brightness (one resistor would change the brightness depending on how many portions of the display are lit up)
  • I was told CMOS logic inputs must be used with pull down resistors?
 

LesterPhil

Joined Feb 25, 2009
10
there are a lot of option that you can do.... using a 555 timer, you need to bias it to get an aprox. 1 second delay and the ad-on cuirtcuit which is the counter you can use 74xx 193 and 74xx08... and a relay cicuit...

But there is more simple. You need only one IC(microcontroller) it cuold be PIC and it could by done by PIC16f84.....
 
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