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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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Old 11-17-2011, 01:32 PM
dandeman360 dandeman360 is offline
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Post A Digital count down Timer Schematic

Heya guys

i was wondering if some would help find a schematic for a Digital timer for my project. i need the timer to have six segments which are counting in seconds and milliseconds. the resolution is (1/1000 per sec) i dont know much about times or how they work but would appreciate it if someone could help me.

thanks
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Old 11-17-2011, 05:10 PM
elec_mech elec_mech is offline
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Welcome to AAC.

Do you mean six digits? How do you want your display to look?

Example:

SS:mSmSmS (five digits)

where S is second, so SS is either 60 to 00 or 99 to 00 and mS is millisecond 999-000 or you could make it two digit instead 99-00 where 10 = 100mS.

You could use a microcontroller for the least number of parts or go with discrete logic (no programming). The gating factor is the time base. If you opt for three digits for mS, then I think you could use a 32kHz crystal with a CD4060 for a discrete logic solution. If you're okay displaying two digits for mS (1/100 second) then a 25.6kHz crystal with the CD4060 might do it.

Since you want to count down and not up, I assume you want to set the starting value? To do this, you'll need a presettable IC, such as the CD4510. Feed the clock to the CD4510, then feed the CD4510 to a CD4511 for each digit. You'll need to cascade them, but that's easy enough to do.

What is this project for? Is it part of a school project?
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Old 11-17-2011, 05:34 PM
dandeman360 dandeman360 is offline
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thank you very much.
i meant a 7 segment counter using 6 digits. the format i need it in is SSS.mSmSmS [999.999]. The counter itself is supposed to count up from 000.000 to 999.999. i am planning to use a micro-controller but i'm still trying to understand how timers work. i am currently looking for a schematic that i can use. its for my final year project, i am designing and building a Timing system which has two gates. each gate has a infrared sensor with it and as soon as the beam is broken, a signal is sent to the counter to start counting and then when the second gate is breached, the counter stops automatically. so the system is like a lap timing system.
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Old 11-17-2011, 05:57 PM
elec_mech elec_mech is offline
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The microcontroller (uC)can handle the timing for you so long as you have an accurate clock. This is best done, in my limited experience, with an oscillator. You connect the oscillator to the uC following the guidelines of the manufacturer or you can cheat and look at a demo board made for the uC you are using and see how it was done or just use the demo board. Then you tell the uC in the define section what size oscillator you are using, e.g., 4MHz, etc. and program your uC to display SSS:mSmSmS. Next you get into the nuts and bolts of figuring out how best to connect the uC to the six digits. There are many 7-segment driver ICs on the market using SPI or I2C and multiple 7-segment digits with multiplexed pins.

If you want to get your hands dirty, you should easily be able to do this without a uC, although, again, it will require more parts. If you only need to count up from 000:000 and don't need to preset a value, then a crystal, a CD4060, and six 40110s with some passive parts would do it. Well, plus a circuit to interface between the sensors and counter.

As far as the timing goes, to get 1/1000 of a second, you need a 1kHz time base/clock. The microcontroller will figure this out for you once you tell it what oscillator you are using. Most include an internal oscillator which will probably work okay for your project, but an external oscillator will be more accurate. On the discrete logic side, the CD4060 divides the input crystal frequency by 16, then gives several outputs with different frequencies divided by multiples of 2, e.g., divide by 2, 4, 8, 16, etc. So, to get 1kHz, we grab a 32kHz crystal, pop it into the CD4060, its first output divides by 16, giving us 2kHz, its next output gives us 1kHz. At least as I understand it in theory. I've used Bill's 1Hz/1second clock which uses a 32.768kHz crystal to the same effect: http://forum.allaboutcircuits.com/sh...2&postcount=12.
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Old 11-24-2011, 01:13 PM
dandeman360 dandeman360 is offline
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thanks very much for your help. sorry about the late reply, i have been away from my office. i will take into consideration the info you have provided me with and will go do some more research on the oscillator and other things you have mentioned. thanks again. i will keep in touch
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