# Problem with making a combinational circuit and breadboarding it

Discussion in 'Homework Help' started by Circuiteromalaguito, Apr 24, 2015.

1. ### Circuiteromalaguito Thread Starter New Member

Apr 24, 2015
11
1
Hello.

I'm new at electronics and this is my first year at a vocational training school. I'm ADHD, I have been years without studying and the school is a bit of a disaster. This combination made me to have tons of pending homework and having problems understanding concepts, I also lack incomes and unable to pay a private tutor too.

No more crying, here's my question.

We have some exercises about making combinational circuits. They are described and we must generate the truth table and make the functions, then use Karnaugh Map and the logic circuits to use. Finally we mount it on a breadboard and the teacher checks it works.

The truth table and circuit seems ok, but something fails here.

The 9V really gives me 6.12V, I'm not sure if that can be the problem. I have no money to get another battery until next week. I have a 7805, 7806, 7812 and L1084 lying around (not sure if I can use one of them to power the circuit and get it from a computer power supply).

I used some CMOS logic gate circuits:
- 4011 (Quad 2-input NAND gate)
- 4082 (Dual 4-input AND gate)
- 4049 (Hex inverting buffers)

Plus LEDs, resistors, cables and a dip switch.

Inhábiles= Days the circuit doesn't work.
Salida=Output

Here are the Karnaugh Maps of each function.

Refrigeración = Refrigeration
Calefacción= Heating
Ventilación=Ventilation

Here's the schematic, that I finally do in a breadboard. I did it for reference an as a way to show the logic gates of it.

Here's the breadboard, it's quite old but I tried to check the connections and seems to work fine.

I know my style is quite crappy, but writing this stuff is still difficult for me.

I can provide extra detail, even translating the problem if necessary.

• ###### untitled.sch
File size:
291.2 KB
Views:
8
Last edited: Apr 24, 2015
2. ### MrChips Moderator

Oct 2, 2009
12,623
3,451
Are you sure that you have pin-7 on CD4011 and CD4082 connected to GND
and pin-14 connected to Vdd?

Circuiteromalaguito likes this.
3. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
If your 9V battery is only delivering about 6V, then either it is being loaded WAY too much or it is basically dead.

Instead of building up your entire circuit and then seeing if it works, how about first seeing if you can get a switch, and inverter, and an LED (with appropriate pull-up and limiting resistors) to work? Then implement just one of the functions (and perhaps even do that a couple inputs at a time)? Incremental design, construction, and testing.

Also, be sure that you do not leave ANY inputs to CMOS logic floating. I can't tell if you are doing that or not.

Circuiteromalaguito likes this.
4. ### Circuiteromalaguito Thread Starter New Member

Apr 24, 2015
11
1
Hello.

I hand made this schematic before mounting it...

(notice the voodoo, scratches and tons of wearing. I got insane checking it again and again)

I checked it tons of times, but I'll check it again

I suposse a 200-220K resistor is enough good for voltage limiting, right? I'm sorry, my brain is rusty these days and need to remember that stuff. Don't worry, I'll check it tomorrow.

I'll try a simple circuit and see if I can buy another 9V battery. I wish I had an adjustable power supply...

I'll check about the inputs, maybe I did put some of the gates floated because unused. I didn't know it.

Thanks a lot for your replies, they are kindly appreciated!

5. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
Uhm... look at the diagram you just posted. You have Pin 7 connected to Vdd and Pin 14 connected to Vss on the '11 and '82. That's backwards. You may well have destroyed your chips if you hooked it up as shown in that diagram.

Circuiteromalaguito likes this.
6. ### absf Senior Member

Dec 29, 2010
1,493
374
Just looking at your breadboard, MrChips was able to see that your 4011 and 4082 were having wrong connections at Vdd and Vss. I was able to confirm that Mrchips was right after seeing your handdrawn diagram on post #4

I was thinking why dont you just make a simple logic test probe on the empty space on the bottom right side of your bread board as attached...

You can use this probe to test the inputs, intermediate outputs and final outputs of any point in your schematic. Once you're not getting the correct logic level as in your schematic, you can just concentrate on that stage to locate the problems.

Allen

Circuiteromalaguito likes this.
7. ### Circuiteromalaguito Thread Starter New Member

Apr 24, 2015
11
1

I did a little fix to the schematic, so that empty gate is connected as the IC requires. Is it okay to connect it to GND/VSS?

The battery was part of the problem. I used another 9V battery and the circuit worked, but not as expected.

I'll check all your suggestions. I have another set of the same chips, I'll get some more extra and try it again to hand drawn the schematic.

Regards.

File size:
73.7 KB
Views:
11
File size:
292 KB
Views:
6
8. ### Circuiteromalaguito Thread Starter New Member

Apr 24, 2015
11
1
I did a hellish mess of wires. I promise to do better wiring next time.

The four switches are OFF, so something must go wrong. I'll check it this week

Thanks!

9. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
You want to develop better breadboarding skills (and it IS a learned skill).

Take the time to make the wires lie flat on the breadboard making 90 degree angles and to not cross other wires except where absolutely necessary. Never route wires over the top of a component. Yes, it takes more time to lay in a wire than just grabbing a lead and plugging in both ends, but I found that it usually saves me time overall. First, you are more deliberate about what you are doing. But far more importantly your work is maintainable. You can track wires much more easily if there is a problem. You can verify your work if needed. You can replace components without disturbing your wiring. You can take a picture of your work and post it and people can make heads or tails of it.

Circuiteromalaguito likes this.
10. ### Circuiteromalaguito Thread Starter New Member

Apr 24, 2015
11
1
I just reformatted to print it later. Thanks for your suggestions, they are very appreciated