Hi there folks!

I have a problem I've been trying to solve for a week now. I'm trying to design a presettable 24 hours countdown timer in Circuit Wizard simulator but I'm stuck with the seconds display. I'm using an 555 astable to generate the clock pulse, a 4510 as the bcd counter and a 4511 as the display driver. Given it's an hour/minute/second based countdown timer it is counting down from 59 seconds to zero and when the seconds reach 00 it sends a pulse to the load pin of the 4510 to reset itself to 59. Problem is that as soon as it hits 00 it immediately resets itself to 59, so 00 is displayed only for the fraction of a second. This generates two problems: 1.) It is not accurate because 00 is not displayed for a second, and therefore one cycle from 59 to 00 does not take 1 minute to occur. 2.) I would like to use the outputs of the 4510 with nor gates and AND gate(s) to make LOW the enable/carryin pin of the minutes display and therefore the minutes display counts back one digit when it receives a clock signal. However, as 00 is displayed for too short, the enable pin is not LOW long enough and by the time the clock impulse reaches the minute's 4510 it is HIGH again and does not count back. My question is how can I make the display to show 00 for 1 second like it does with the other number combinations? Or if this is not possible would it help if I would keep the oscillator freq 1 Hz, but I would change the length of its outputs High and LOW component (i.e. T1, T2) (they are currently 500 ms each)? I have restarted it from scrap after my painful 1 week, so I can only post the seconds display and its driver "mechanism". If needed I can post the circuit wizard file too, so you can fidget around with it.

 

You let so many input pins to keeping floating when they separated from +9V, they will getting the noise and causing oscillations, you should using a 10K resistor connecting to GND for each input.

 

Instead of detecting 00 for the Load signal, detect the value 9 for the tens digit (since the tens digit does not normally go to 9). This should work because in the count-down mode both counters roll over to a value of 9 after 0. Thus the count would be 59, 58, ---- 02, 01, 00, 99(instant Load to 59 occurs here so is not visible in the display), 59 58, etc.

Edit: Below is a sim of the ones and tens digit counter using the above technique. Note that you only need to detect the MSB bit for the 00 rollover PE signal.

 

You let so many input pins to keeping floating when they separated from +9V, they will getting the noise and causing oscillations, you should using a 10K resistor connecting to GND for each input.

You are right Scott, there are two reasons for this floating. I'm still building it and it is not finished. And because the simulator is programmed to think that a pin which is not connected to anything is connected to ground. Therefore there aren't any noise in the simulator.

 

Instead of detecting 00 for the Load signal, detect the value 9 for the tens digit (since the tens digit does not normally go to 9). This should work because in the count-down mode both counters roll over to a value of 9 after 0. Thus the count would be 59, 58, ---- 02, 01, 00, 99(instant Load to 59 occurs here so is not visible in the display), 59 58, etc.

Edit: Below is a sim of the ones and tens digit counter using the above technique. Note that you only need to detect the MSB bit for the 00 rollover PE signal.

View attachment 72641

Thanks crutschow! It actually worked... at least in the simulator. I'm not completely sure if it would cause any problems (delays, noise) if I'd built it in real life following your suggestion. Who knows, maybe it would show 00-99-59. However, given that it would be a fairly expensive project to build I will stick to simulating it.

 

By the way. Scott mentioned this floating phenomena. I read once in a forum or an article that ICs made with CMOS technology are internally pulled up/down and it is not necessary albeit recommended to utilize pull up/down resistors on floating pins. The article also mentioned that chips made with TTL technology must have their pins pulled up/down. Can anyone confirm if this is true?

 

By the way. Scott mentioned this floating phenomena. I read once in a forum or an article that ICs made with CMOS technology are internally pulled up/down and it is not necessary albeit recommended to utilize pull up/down resistors on floating pins. The article also mentioned that chips made with TTL technology must have their pins pulled up/down. Can anyone confirm if this is true?

TTL inputs go high with no input and must be pulled low for a logic zero. But most CMOS inputs are floating and must definitely be pulled high or low as desired. If they are left floating, they will go to an undefined state and can even cause a high current draw if the input drifts to a voltage halfway between high and low.

And I'm not sure that all simulators put a floating input in a defined state so it's always good policy to tie unused inputs high or low, even when simulating.

 

............................. I'm not completely sure if it would cause any problems (delays, noise) if I'd built it in real life following your suggestion. Who knows, maybe it would show 00-99-59. .......................................

I don't see that the circuit would be prone to any delays or glitches. And it can't show 99 for significant time because that generates the Load signal which rapidly (within the circuit IC delay time) sets the count to 59.

 

I don't see that the circuit would be prone to any delays or glitches. And it can't show 99 for significant time because that generates the Load signal which rapidly (within the circuit IC delay time) sets the count to 59.

TTL inputs go high with no input and must be pulled low for a logic zero. But most CMOS inputs are floating and must definitely be pulled high or low as desired. If they are left floating, they will go to an undefined state and can even cause a high current draw if the input drifts to a voltage halfway between high and low.

And I'm not sure that all simulators put a floating input in a defined state so it's always good policy to tie unused inputs high or low, even when simulating.

Thanks for your reply. So that article or comment I saw is a complete lie. Or maybe I was the one who did not comprehend what it was saying.

 

One last question (I hope) for this project. I am trying to make the counter stop at 00:00:00. Iwas so far successful with the design in the attached picture. The 8 input NAND gate's output goes low when the displays show 00:00:00, at other times the output is high and therefore the clock signal reaches its destination on the 4510s. However I noticed that when the clock signal is at low or when the display has been set to the desired starting time but the oscillator has not been started with the SPDT switch(i.e. when the NAND output is at high) there is some microamps and millivolts leaking from the transistor base through the collector towards the output pin of the 555. Isn't this harmful for either the 555 or the transistor or the machanism altogether? I know that the 555 can tolerate some current input into it's output (this is called a sinking connection on the output pin) but when the oscillator is on it outputs current while there is current input on the very same pin (from the NAND). And if harmful how can I get rid of this phenomena?

 

I presume you are attempting to use the NPN transistor as a logic gate.
Replace the transistor with an actual logic gate or use the RESET function of the 555 timer chip.

 

If you could post all your circuit or block diagram, maybe it has a better solution.

 

Scott, I have tried to copy and paste the circuit diagram from the simulator into a png file, but it became a bit messed up and hard to read. I will put up that "messed up" png, maybe it's not that horrible for eyes trained in the field of electronics. If anyone needs it I can send the circuit wizard file in e-mail (I can't upload it because it's expansion is not allowed here). The circuit is fully operational in the simulator file.

 

I presume you are attempting to use the NPN transistor as a logic gate.
Replace the transistor with an actual logic gate or use the RESET function of the 555 timer chip.

Thanks MrChips for the clue! I will try replacing it with the proper gate.

 

Scott, I have tried to copy and paste the circuit diagram from the simulator into a png file, but it became a bit messed up and hard to read. I will put up that "messed up" png, maybe it's not that horrible for eyes trained in the field of electronics. If anyone needs it I can send the circuit wizard file in e-mail (I can't upload it because it's expansion is not allowed here). The circuit is fully operational in the simulator file.
View attachment 72650

Using the original circuit and make it as *.zip file and upload it.

 

By the way. Scott mentioned this floating phenomena. I read once in a forum or an article that ICs made with CMOS technology are internally pulled up/down and it is not necessary albeit recommended to utilize pull up/down resistors on floating pins. The article also mentioned that chips made with TTL technology must have their pins pulled up/down. Can anyone confirm if this is true?

About the CMOS input, you can see tutorials.
Basic gate function.
CMOS gate circuitry.

 

All thanks to everyone who replied in this thread! You have been a great help with this simulator project. Now the timer works as I would like it to work. I used the reset input on the 555 to make it stop at 00:00:00. I will upload the zipped simulator file as Scott suggested as a token of my gratitude. You (and everyone else) are free to use, modify, distribute it as you like. It is easy to use. One button resets all displays. You can input the desired digit by setting the 4 SPST switches of each display and pressing the corresponding LOAD button (don't forget to reset them after pressing the LOAD button to xx:59:59, so it can automatically reset the minutes and seconds display after a 00 display output). You can start/stop the oscillator with the SPDT switch above the 555. If you think that I missed something (pull up/down resistors) or used something unnecessary please post it here, so I can fix my circuit. The two 2-input AND gates and the 3-input AND gate that they feed into at the hours display are only required if you want it to reset after it has reached 00:00:00. If you want the resetting vesion just remove the 8-input NAND gate from the lower-right side of the 555, and the 4-input NOR gate from the "tens" hour display and connect the 555's reset pin to the 9V source. Have fun with it! And thank you again!

 

Always nice to hear about successful outcomes on AAC.

 

Thanks for uploaded the file.
I think you should make or convert your circuit to the gif,png,png files, and they should be clear and compress it to the *.zip, then when we open it, we can see directly, not necessary to find another software to open it.

 

That's what I originally intended to do, but circuit wizard doesn't allow me to save my circuits in any other extension than cwz or cwt, circuits can be exported from it but only on CAM/CAD. I can't find any converter online to convert a cwz file to a picture image. I will upload that "slightly unreadable" png image I created of it by copy/pasting. There are a few hints on it too.