# Nonzero level detector question

Joined Apr 2, 2024
1
Using the Voltage Divider, not the reference voltage of 0V, we intend to configure the Voltage Comparator with the reference voltage desired by the user. Based on the reference voltage, it is intended to output Low when the input voltage is higher than the reference voltage, and High when the input voltage is lower than the reference voltage. High and Low are determined as follows.
 Min typical max High 4V 5V 6V Low -1V 0V 1V
In this case, I wonder why we should use LM358 instead of TL084.

#### ericgibbs

Joined Jan 29, 2010
18,988
Hi May.
Welcome to AAC.
Interesting problem.
Do you have a basic circuit idea you could post?
E

#### WBahn

Joined Mar 31, 2012
30,237
Using the Voltage Divider, not the reference voltage of 0V, we intend to configure the Voltage Comparator with the reference voltage desired by the user. Based on the reference voltage, it is intended to output Low when the input voltage is higher than the reference voltage, and High when the input voltage is lower than the reference voltage. High and Low are determined as follows.
 Min typical max High 4V 5V 6V Low -1V 0V 1V
In this case, I wonder why we should use LM358 instead of TL084.
Who is telling you that you should use the LM358 instead of the TL084?

Have you looked at the data sheets for both parts to see what is different between them and considered how those differences might or might not be relevant to your application?

Why are you using an opamp instead of a comparator in the first place?

Have you considered what might happen if the input is very close to the reference voltage such as noise keeps moving it back and forth above and below the reference voltage? Is that behavior acceptable? If not, what might you do about it?

#### Audioguru again

Joined Oct 21, 2019
6,757
The IC amplifiers are completely different.
1) The minimum recommended supply for the LM358 is 3V and its "input common-mode voltage range" is 0V to V+ minus 1.5V.
2) The minimum recommended supply for the TL084 is +5V and -5V. Its "input common-mode voltage range" is Vcc- plus 4V to Vcc+.