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# Basic voltage Comparator question

#### JonnyFrond

Nov 10, 2010
1
Dear Electronics wizards,

I am new to electronics, through the degree I am studying in Mechatronics, and I am trying to get my head around some of the basics.

I am currently learning about voltage comparators, and have come across a piece I need but cannot find anywhere, though I am loathed to harrass my lecturers.

Basically, in my assignment, there is a basic comparator circuit using a LDR and an led. Very simple light is shined on the LDR, and the LED comes on.

I am currently trying to calculate what resistor I need to use before the LED to get it to the optimum voltage.

What I need to know is:
With a comparator, I have been assuming that the main signal voltage going into the comparator is the same as the voltage at the output that is going to be used to light my LED.

I am now becoming aware that this is not a useful assumption.

I have never seen or used a real comparator as yet, but I am assuming it is similar to an op amp, in that it is powered with a separate feed from the power source. If this is the case, what will be the voltage at the output, is it the same as the main signal in, or is this signal input just used to switch the comparator on or off, and the output can then be assumed to be the same as the power supply.

Power supply 15V
Voltage at input has been calculated at 10.7V or 2.1V
Reference is 6V
Possible Output Voltage is it:
2.1V and 10.7V
or
0V and 12V

(or there abouts depending on the quality of the comparator of course)

It would also be useful for me to know what is standard practice so that in future I can add in a more accurate assumptions section to my answers to add clarity to my work.

Kind regards

Jonny

Last edited:

#### Laplace

Apr 4, 2010
1,252
An opamp is a differential amplifier that has been optimized for linear operation with negative feedback, while a comparator is a differential amplifier that has been optimized for non-linear operation and with logic-level output. It is generally possible to use an opamp as a comparator - it will work but not very well, especially in terms of response time. I've never seen anyone try to use a comparator as an opamp - probably a bad idea. Output voltage range depends on the particular output stage that was used - it could be rail to rail. Many comparators still use an open-collector output to be used with an external resistor. I would think that "standard practice" depends a lot on the comparator you have selected.

#### davenn

Moderator
Sep 5, 2009
14,297
have a look here.....

http://home.cogeco.ca/~rpaisley4/Comparators.html

it has a lot of practical info on comparators

What I need to know is:
With a comparator, I have been assuming that the main signal voltage going into the comparator is the same as the voltage at the output that is going to be used to light my LED.

The output of a comparator say a LM311 for example is not a voltage rather its a H or L state H = Off, L = On see the pic and comments down the page a little way where it starts by saying.....

Basic Comparator Operation
Input Vs. Output Results
1 .. Current WILL flow through the open collector when the voltage at the PLUS input is lower than the voltage at the MINUS input.
2 .. Current WILL NOT flow through the open collector when the voltage at the PLUS input is higher than the voltage at the MINUS input.

You can see thay have a diode connected to the output, Im picking in a similar way to what you want to do.

A bit over 1/2 way down the page there is a dual photocell cct driving a LED sounds like just what you want

cheers
Dave

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