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Hello everybody,

one of the forum members gave me this forums name to become a member. He thinks it will be helpfull for some of You having difficulties putting together an electronic device of a german kit concerning the language. I will be glad to help everybody out with translations and if necessary illustrations.

Just let me introduce to You briefly. I am Hans, a retired GAF captain, having flown all kinds of aircraft for 23 years, last RF4-E Phantom. Since I handled my aircraft allright it always handled me right and I had no accident during my flying time.

I have a problem concerning a timer schematic. That is what it is supposed to do:

Count up and down, must be presettable for up and down count and stop counting after the preset time has elapsed thereby triggering a sound signal and a relay. I've got the schematic so far to count up using  M74HC190 and the appropriate BDC to 7-segment decoder and LED driver but I have no idea about the "count down" part and how to switch back and forth. May be somebody has the circuit in his drawer and can help me.

Thank You all for Your attention

Kind regards

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Guest 2old4digital

You want to use a 74HC190, ok.  If you can find The Book Digital Fundamentals  By  Floyd    the  9th  edition, you will find some help on or around page 445. This section covers up / down counters, also if you can find from the manufacture of the chip the spec. sheet that will help also. I'm glad the someone other than me at my age is doing Digital.  Good luck and lots of success.

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Hi Herman,

I do not have Eagle installed in my computer; you can (while Eagle displays the circuit) make a screen dump (Prnt Scrn key) and post it here.  8)
For the future (wouldn’t have helped in this case but); if you have a file with a suffix not allowed (like .sch) just ZIP or RAR it and it’s possible to post it.  ;)

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You can Also build it with just a Microcontroller very simple and with lots of abilities.
Of course building such circuits with TTL Ics is much more enjoyable.

HTH - Shahriar

A very good idea. maybe I can handle the hardware but not the software. The other way with wired logic is easy to understand and also easy to trace in case of malfunctions. I have made a lot of control units for industrial purposes, but never used a Microcontroller. Wired logic is just my thing! Even using a full size 19" rack with up to 17 PCBs  doesn't bother me at all.  Boole's algebra is logic. A small piece of (SMD mounted plastic or ceramic) is not logic and will become as intelligent as the programmer is.


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ِDear Herman

For Counting down the UP/Down pin should taken low.
if you want when counting is finished, the counter resets, you should connect the Max/Min output to Load Input via a NOT gate or a NPN transistor While all Outputs are pulled down via Appropriate Resistors (If IC doesn't have Internal Pull down Resistor, Check it). you can also use this Transistor to trigger your Buzzer or LED.
If you want your Buzzer make sound for some seconds, you can build a Monostable by 555 and connect the buzzer to 555 output.

I don't know if this is your Problem. ???

HTH - Sahriar

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  • 3 weeks later...

Thank you for your help. I checked it and just found a timer. This is not what is on my mind. The timer should have the following properties: Count up and down 999.99 seconds and use optoelectronics to start (car races for instance), switch off the microwave oven after the exact preset time and work as a photimer as well to expose PCBs.

I just have difficulties with the pins clock in, clock out, borrow and carry since they are not described thoroughly enough in the data sheet.



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