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MultiSim Schematic

I apologize for the schematic. I know it is not the prettiest to look at. My clock runs slow. I know this is because of my 555 timer. I was off somewhere in my calculations and ended up with a 70hz frequency rather than the 60 target. I will revisit that later. My issue is that I have added in comparators trying to simulate an alarm clock. ( I intend to make better with a rotary BCD setup later but for now I was making sure that it would work.) The problem: I can set any BCD value in to match the clock. Voltage will prove that A0 and B0 are equal, so on and so forth all the way through, however no signal ever makes it through the comparator. Any of them. I am stumped. Is it me or the comparator. Am I not set up correctly? The goal is to light the probe when the BCD switches match the clock. There are no pins to power or ground the comparator in the simmulated version. Any help is greatly appreciated.

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  • \$\begingroup\$ Set this aside for a minute and build up a simple new project that just has one 4-bit BCD comparator and input sources for the two values it compares. Play with that until you have a sense of how to get them to work, only then try to apply that knowledge to your goal project. \$\endgroup\$ – Chris Stratton Aug 1 at 22:04
  • \$\begingroup\$ if you are submitting the MultiSim schematic as part of your assignment, then remove the blue bus crossings on left and also in the middle ... lines should not cross unless necessary \$\endgroup\$ – jsotola Aug 1 at 22:20
  • \$\begingroup\$ Okay, I realized I needed to input a signal to the equal pin on the first comparator. Now somehow I am getting 5pV through the low side. I tied my low side to ground to pull low. I would use pull down resistors but student addition only allows 50 components. Is there a way to eliminate the noise without additional components? \$\endgroup\$ – Vincent Ledbetter Aug 1 at 22:34
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In the end. I needed to tie the equal too pin high on the first comparator in order for any of the comparators to work. Reading through the data sheet, I am not sure how I would have found that information. (Thank you Chris Stratton for suggesting to start small.) As for the ghost pico volts, I am still unsure. I originally used two pole switches so that I could tie the pin high or low. (I again can not use pull down resistors due to component restraints.) All signals were tied together as were all grounds. I initially thought this is were the noise was coming from somehow. I deleted all grounds but left the double pole switches. The 5 pico volts disappeared but 5 volts appeared on the empty latch. If memory serves correctly, fundamentals of DC Circuits would explain that. (Potential). However, My A pins read 0 volts meaning they were not equal to the 5 volt potential. Now the ever loving confusing part to me. I switched the switches for single pole and the problem disappeared. Circuit worked the first try. So my newest confusion. Why did it matter? I thought components in simulations were to be ideal. I cannot fathom how changing from a two pole with only one pole used to a single pole worked, or where the five pico volts came from in the first scenario. Thank you for the help. This is my major, any suggestions to personal simulation software would be greatly appreciated. Also, scrolling through a built schematic for diagnostics is not so bad. Scrolling through a schematic while designing, not so fun. Any one use like a TV for circuit design or am I doomed the nature of the beast?

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Find a CMOS manual for what you need to learn and avoid.

Look for rules on Switch pull-up debounce, Decoupling cap, Shoot-thru and ESD awareness.

It won't work without Pullup R's on the Switches. 10K or 100k or so. BCD THumbwheel switches are handier than toggle switches or DIP switches.

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