4081 gate output

Thread Starter

Nhubbard1

Joined Jan 14, 2020
13
Hi all,

I'm wondering if I have compromised this IC by static. I am using only one gate of an CD4081BE chip and even with zero level inputs into the gate, the output is high (essentialy the Vdd voltage). Can anyone explain this?

Cheers
 

MrChips

Joined Oct 2, 2009
27,641
As Eric alluded, you cannot leave input pins unconnected.
Also make sure you look up the pin configuration of the IC.
 

MisterBill2

Joined Jan 23, 2018
13,680
The reason thatall CMOS gate inputs must be connected to a logic level is that otherwise the devices will quickly drift into their linear region of operation and exhibit undesirable operation.
 

MrChips

Joined Oct 2, 2009
27,641
Off topic.
Interesting consequence of the internet that benefits sites like AAC.
We have four AAC members in conversation in three continents, four countries and four different time zones.
 

Thread Starter

Nhubbard1

Joined Jan 14, 2020
13
hi N1
Do you mean the gate inputs are open circuit, not connected , when you see a high on the output.?
E
No I am yet to try another chip. I only finished wiring this prototype about an hour ago. I'll add a pic although I don't think it will help much.
 

Thread Starter

Nhubbard1

Joined Jan 14, 2020
13
As a temporary test, try putting jumpers from pins 1 and 2 to GND.
Good idea. Thanks. Will test. Essentially I want each radar sensor output (12V) to be triggered simultaneously in order for the gate output to trigger an alert. I note the input at pin 14 is 12.45V and pin 3 is at 11.64V. The gate outputs for the other 3 gates is at zero V.
 

Thread Starter

Nhubbard1

Joined Jan 14, 2020
13
Good idea. Thanks. Will test. Essentially I want each radar sensor output (12V) to be triggered simultaneously in order for the gate output to trigger an alert. I note the input at pin 14 is 12.45V and pin 3 is at 11.64V. The gate outputs for the other 3 gates is at zero V.
Ok. Holding pins 1 and 2 to ground arrests the output of pin 3.
 

MisterBill2

Joined Jan 23, 2018
13,680
The CD4081 is a QUAD AND gate. That means that there are 8 input pins that must be at a defined logic state for correct operation. So if tthe plan is to use only one gate, then the other six inputs must also be correctly terminated to either a logic on or a logic zero. Leaving them float will certainly cause a problem with all of the gates, even though in normal operation they are separate.
 

crutschow

Joined Mar 14, 2008
31,091
The gate inputs are the outputs of 2 interal relays within radar sensor devices.
If those relays provide an open circuit (floating) for the "zero" signal, then you need to add a resistor (e.g. 10kΩ) from each gate input to ground to establish a 0V zero state voltage.

And you cannot reliably measure the voltage of an open-circuit with a voltmeter, since the input impedance of the voltmeter pulls the floating voltage to ground.
 

Thread Starter

Nhubbard1

Joined Jan 14, 2020
13
If those relays provide an open circuit (floating) for the "zero" signal, then you need to add a resistor (e.g. 10kΩ) from each gate input to ground to establish a 0V zero state voltage.

And you cannot reliably measure the voltage of an open-circuit with a voltmeter, since the input impedance of the voltmeter pulls the floating voltage to ground.
Thanks for the reply. If the gate inputs are tied to ground when the relay switches and the output goes high wont that track to ground instead of going through the gate?

Cheers
 
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