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Wiring a count down clock to time events

Other Parts Discussed in Thread: SN74LS247, SN74LS193

I am building a timer that goes from 9 - 0 and then stops. I am using a 555 chip for my timer running at 1 pps. my countdown chip is a SN74LS193. My bcd to seven segment decoder driver is a SN74LS247. It is driving a seven segment display that is common cathode. I am having difficulty making my 193 chip countdown. Could you please show me the pin outs and where they go for the 193 and the 247 to allow my display to countdown from 9 - 0 and stop. I also need to be able to reset my 0 back to 9 for another countdown operation. Your help will be greatly appreciated. This is a project for the youth of our church that I am putting together. God bless, George Keesee

 

  • Hi George,

    The counter will continue to cycle through as long as clock pulses are applied.  In order to stop them, the clock will have to be either disabled or disconnected when the countdown reaches zero.  There are a number of solutions for this.  Can you give me a better idea of what your end goal is with this device and perhaps I can provide a full solution?  My guess is that this is some sort of game timer -- pressing a button starts a 9 second countdown.  Is there some sort of output when the unit reaches zero?  Or should it just stop with 'zero' displayed on the 7 segment?

     

    Any details, block diagrams, or schematics that you can provide me would be helpful!  In the meantime, I will work on making a diagram for you just for the counter/driver circuit.

  • George,

    This diagram shows the appropriate connections for these two parts.

    I labelled all pins with their pin number and name just in case I made any errors they would be easier to catch.  Please double check pin names/numbers match before wiring this up.

    Note that if you want to stop the counter at zero, additional circuitry will be required, but the BO\ (borrow, active low) pin provides a signal when the counter has reached zero which could be used as part of a stop signal.

    Also, the decoupling capacitors should be placed as close as possible to each device.

  • First of all I want to thank you for taking my question and second please excuse my typing errors. What I am trying to build is a count down simulator, kinda like NASA used on the terminal count of the Space Shuttle. At certain times during the terminal count down different things would occur. That is what I want to do during this 9 second countdown, using gates on the B C D output, lite an LED, turn on a relay and things like that. I will be running my 555 at less than 1pps so the kids will have time to see what is going on. The output of my 555 will be gated into the input of my SN74LS193N so I can stop the count at zero. My problem is I can not get my SN74SL193N to count down. I need to know which pins go where. I have tried several ways with no luck. If I can get the UP/DOWN counter to work I will be in business. I hope you can help me to get this demo working so I can show the kids some of the things that helped the USA put Men and Women into space.

    Thanks again for your help
  • Mr. Maier I have one more question. If the SN74LS247N has an open collector output, will I need pull up resistors on the outputs of the device? If so what size should I use.
  • I am happy to help :)

    For your first question - the above image I posted covers every pin, so if you duplicate it there should be no problems. Can you get me a schematic and/or pictures of your current setup? If I can see what you are doing, I am more likely to be able to help!


    As for pull-ups on the ls247 output, the short answer is 'no.'

    The outputs for the SN74LS247N should be connected directly to the anode connections for the seven segment display. You can include a limiting resistor on each if you want to reduce the current draw (I believe the max is 4 mA for this part, so it should be fine without them). The common cathode connects each of the segments to Vcc, so when an open collector output drives low, the circuit is completed and the LED segment lights up. When the output is 'Hi-Z' then no current will flow and the LED will be off.
  • Hi Mr. Maier

    This is George again. The countdown clock works great, but the problem I am having now is the stop count at zero. On the 193 chip, at zero all outputs go low, so I ran them thru a Hex Inverter to get them all high. All highs on the input of the 4 input NAND gate gives me a low on the output. I know that I need to remove the inverter on the output of G2 because I need  a low on the input of G1 to stop the count. Here is my problem, I understand the Logic but when I try to bread board the circuit for the stop count, my 7 segment display shuts down. I don't know what I am doing wrong. I will also tie the output of G2, thru more gates to turn on an LED that indicate a launch. This is simplified countdown clock that will give the kids a good idea of how NASA counts down their launches. Any help will be a blessing.

    Thanks George        

  • Hi again George. Can you post a picture of your setup and a full schematic?
    -Emrys
  • I am tr ying to, but when I send the reply,  the diagram disappears. I am using the "paste from word" button with no luck.

  • I have found the only way to post images on here that works is to save the image to your hard drive, then choose  , browse the the appropriate file, then 

    I don't know why the other options are so problematic, but I just don't use them.

  • Thank you, I will try that.

    Diagram01.pdf

  • I had a lot of trouble posting my drawing to you. It is labeled Diagram01.pdf in my last reply. The whole idea of this project is to simulate a launch countdown. Durning the 9 second countdown, I want to operate LEDs at certain times durning the count. I want to gate the output of the 193 countdown chip to preform these events. When the count reaches zero I want the clock to stop on zero and light another LED to simulate a shuttle launch. A teaching tool for the kids.
    Thanks
    George
  • George,

    I apologize for the delay in my response -- for some reason I could not access E2E from my home this weekend.  I believe the primary trouble in your circuit is with the inverter I indicate here:

    Personally, I would do away with the feedback loop logic and use the BO\ signal with an SR latch to turn on/off the 555 timer, but your way works, too.

  • Thank you for all your help. I have one more question. Can you show me the S R latch configuration on my drawing. Are you talking about a Quadruple Bistable Latch? (N7477)
  • I believe this will work for you.  The two NAND gates make a latch -- pushing 'BO\' low will change 'RESET\' to low, turning off the 555 timer (which happens when the counter hits zero).  Forcing 'LOAD\' low (via the shown push-button) will do two things - it will change 'RESET' to high, enabling the 555 timer, and it will load 9 into the counter - so one button will load and start the countdown.

  • That should do it. Thank you so very much. I will write if I run into any other issues. T I service is great and you guys are doing a wonderful service to the community.
    Blessing
    George
  • Thank you George!  We're always happy when we can help.