IC-Circuit not working correctly.

[johnbox]

Hey there,

im currently doing my seminar paper, and my task is to do a bridge-combinational circuit to control the opening automatically just by 4 sensors, that check wether there are ships/cars etc.
So i created my boolean formulas, minimized them, and built the circuit in a digital circuit simulator, where it is FULLY WORKING.
Im still pretty new to this stuff, so don't hate me if I'm coming up with weird stuff. So.. whatever, my components arrived today, and after i tested my circuit on a breadboard, nothing worked. What went wrong..? I removed everything and tested the first IC, a simple 2 Input-AND IC. (MOS 4081)

Now.. I've got a video for you:
https://www.youtube.com/watch?v=hHPqMXBZfmI


Could it be, that as the LED with the supplied IC is darker, that the IC gives a lower voltage to the LED but not turning off the signal completely? And how do i work with that then?


Thanks,
johnbox

 

[E-design]

With CMOS, you can't leave any inputs floating. Pull all inputs low via a high-value resistor (100k) and it should work.

 

[johnbox]

so.. I need those 74HC IC's?
Thanks alot, almost went insane about this.

 

[godfreyl]

so.. I need those 74HC IC's?

No, you missed the point.

Every input of every gate must be connected to something. If you leave an input unconnected, it may be high, low or anywhere inbetween. This causes the output to be random.

 

[crutschow]

You don't need a resistor to tie the unused inputs low. Just connect them directly to ground.

 

[Audioguru]

The AND gate used in the video had pin 3 as its output, the input pin 1 was an input signal and input pin 2 was wrongly not connected to anything.

 

[E-design]

You don't need a resistor to tie the unused inputs low. Just connect them directly to ground.

The idea was that he will be able to toggle inputs and observe outputs without having floating pins while experimenting on the breadboard.

 

[crutschow]

The idea was that he will be able to toggle inputs and observe outputs without having floating pins while experimenting on the breadboard.

I understand. I was referring to the input of gates not being used. You should ground those, even if you are not interested in their output, because they can sometimes cause erratic behavior otherwise or even cause the chip to draw excess current.

 

[std_match]

The inputs have protection diodes connected to supply and ground. If there is no supply voltage connected and any of the inputs have a high voltage from a driver with enough power, the IC will work (with some restrictions). It will get the supply voltage from the input pin, with a forward diode voltage drop. That is why the output LED can be on even when the positive supply pin is disconnected.

As others already have pointed out, connect all inputs to a well defined "high" or "low" voltage. For the 4000 CMOS series, ground and the supply voltage can be used for this.

Some TTL families had a very strict maximum input voltage, so a series resistor was recommended if the "high" voltage was taken from the supply voltage. The supply could handle 7V but the inputs only tolerated 5.5V, so the series resistor was chosen so the input was protected for supply voltages up to at least 7V.