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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.
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
I am trying to, but when I send the reply, the diagram disappears. I am using the "paste from word" button with no luck.
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.
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.