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Breadboard Grounding

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anukit

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Hi,

I'm new here, so excuse me if this has been discussed before, but I couldn't find a search feature.

Here's my problem. I have a breadboard (JE27 from Jameco), and I have a DC supply plugged into it from the wall, which gives me about 6.5 volts. The breadboard has a metal base plate, and the actual breadboards connected together are attached on top of it. I've been trying to use some logic gates such as AND, OR, NOT, and so forth... Something strange has been happening though.

First of all, I can't seem to get anything to work right besides my NAND (CMOS 74AC00) gates. And they only work when I am touching the metal base of the breadboard. Does this seem strange to anyone? If I'm not touching the metal, the LED I have attached to the output of the gate, won't light up. I should be plugging everything in correctly. I have the Vcc and ground connected on the chip, and the inputs and outputs are correct. Could this be something wrong with my board, or does this normally happen?

Also, the other gates do not work correctly at all. For example, when I try an AND gate, it will act like an OR gate. A NOT gate just doesn't work, and the OR gates are acting strangely. Again, I'm supplying the Vcc on the chips with the 6.5 DC voltage, which says it should be within the limits (-0.5v -> 7.0v) of the chip. All these chips are CMOS 74AC series that I bought from jameco.com.

I'm fairly new to using cmos chips at all, so all of this is probably something I'm doing wrong, so any help at all would be greatly appreciated. If any more information is needed on my problem, I'll gladly supply it.

Thank you.
 
CMOS logic is very easily damaged by static electricity. Your metal base plate should be grounded (connect it to ground thru a 1meg resistor for safety). Also, touch the metal base plate before touching any CMOS logic. The ones that don't work may be blown already.
 
Just a guess, sounds like you have floating grounds on some the various chips. If you are not sure of the layout of the bread board look again. You might have ganged together some grounds together without a return. Also are You running the CMOS chips in a bipolar PS configuration? i.e. +15 -15, ground. The resolution of the problem is based on the understanding of the setup any info is helpful.
 
Thanks

My problem was with the grounding, that was it. I attached a wire to the metal base, and grounded it, and everything seems to be working fine now. It turns out also that I didn't short-out any of the chips, I just had too much power flowing through the AND gates, so they were outputting 1's all the time. Thanks a lot for the help.
 
Do you have the unused inputs connected to either gnd or Vcc? This is vital with CMOS because of the very high input resistance. If inputs are left open, the voltage at the input will be unknown and is the result of the PCB and IC resistances and thus may be between 0 and Vcc. If it is about half Vcc, then significant current will flow through the gate.

Also, you need bypass capacitors.

The 74AC series is very high speed and therefore needs a carefully designed PCB with gnd and Vcc planes in order to prevent unwanted effects. It may not work properly on a breadboard.

If you simply want to learn about CMOS gates etc. I suggest you use the slower 4000 series.

Len
 
Basically what I'm trying to do is learn about CMOS gates. I bought a bunch of them, heh, so although the 4000 series may be better to learn with, I only have the 74ac series.

Anyway, I'm trying to make something simple, like a half-adder. All the gates work fine on their own, but when I connect them together to create the circuit, something goes wrong.

What I think is the problem is that the outputs are floating high when I don't have the switches turned on.

I've tried on single chips to connect the inputs to ground through LEDs (since I don't have any actual diodes), and also send wires from the inputs to switches which control the +5v. To my surprise, this seemed to work when I used one chip at a time, and the outputs wouldn't go high when the switches were turned off.

I tried that solution with my half-adder setup, and unfortunately it didn't work. I'm not sure what I'm doing wrong, but I truly believe that I can make these ICs work, but probably only with a little help.

If schematics or a better description is needed, just ask. Thank you.
 
CMOS is unlike TTL in that ALL inputs AND outputs must be at ground or supply potential (or interfaced to another component that provides that path). You must provide pull up or pull down resistors ( TYP 10k ohms) or a load, or a direct path , if not they will be as flighty as a 14 year old girl : ) You cannot float any pins.

Look at this article , it states some design conventions and considerations : **broken link removed**
 
You did not mention my second and third points re bypass capacitors and a properly designed PCB. As I said, the 74AC series if very fast and needs careful design.

The 4000 series is not as critical but still needs bypass capacitors to avoid odd effects.

Len
 
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