# Voltage of Logic Low on ICs

I'm currently trying to create a 4 bit ALU using the CD4011BE quad 2-input CMOS NAND gate ICs. However, I'm having some trouble when creating the full adders. Sometimes, when the IC outputs logic low, I'm getting 2.5V, which causes the next NAND gate to also output something like 2.5V. This carries down to the sum, which is 2.5V when supposed to be under 0.5V. I've read that ICs need decoupling capacitors, but I'm not sure if that would cause this problem

I'm testing this on a Breadboard, and running 5V rails.

EDIT: I've tried testing the voltage of each pin. Strangely, the ground pin had 1.5V while only being connected to ground. I tried replugging everything in, and it worked perfectly now.

• You might want to add a schematic, so we can spot an error in your design (maybe) rather then guessing what might be wrong. – Arsenal Aug 17 '15 at 11:24
• What Arsenal said, also do you have an oscilloscope? – Spehro Pefhany Aug 17 '15 at 11:29
• The fact that your supply is 5V and that the circuits are CMOS logic means that any well defined output should be either 0 V or 5V. If you get 2.5 V I think one or more of the inputs of the gates are not connected. With CMOS logic you must always define all inputs, so tie them to ground or +5V. – Bimpelrekkie Aug 17 '15 at 11:31
• Logic gate levels MUST always lie inside defined limits. If they do not SOEMTHING is wrong. There are only two things that can be wrong (with lots of subthings inside these two things. (1) One or more ICs are damaged. (2) YOU are doing something wrong. SO|| - Measure ALL supply voltage and ground pins - all should be 5 or 0 v within fairly tight limits. - Measure ALL input s that you have control over. ALL should be at ~= 5V or ~= 0V. All MUST be above Vinhi_min or below Voinlow_max. Fix silly mistakes. - Then measure Vin's that you have no control over .... – Russell McMahon Aug 17 '15 at 11:36
• .... (eg driven by outputs). Any wrong - work out why - fix. || If ANY lvels are still wrong IC'(s) are damaged or you have missed something. |||| Floating inputs -> death. | Shorted hi or lo output -> death. | Load resistor too small -> .... | Too many gate inputs on an output (fanout exceeded -> ... . | – Russell McMahon Aug 17 '15 at 11:38