Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Comparator Help Please

Comparator Help Please

Comparator Help Please

I'm trying to design a simple delay circuit for a Peizo Electric buzzer to be used on a 24 VDC vehicle, I bought 30 of these LM211-N Comparators thinking they would be up to the task.
On the data sheet it says the output is good for 50 mA but I don't seem to be able to get it to pull up more than 0.06 mA,
It won't pull down from the positive rail at all what am I doing wrong?

I also want to be able to get the delay to around 2 seconds, my RC theory is all gone, cam someone tell me what value of resistor to use with a 1 microfarad capacitor.
I figure the input impedance of the LM211 is very high

Sorry, having trouble getting the sketch on here.

Thanks a lot

RE: Comparator Help Please

Looks to me like you're applying +\-24V to the input of the comparator... is it rated for that on the input with a +\-10V supply? Can the input handle a 34V differential?

Dan - Owner

RE: Comparator Help Please

I can't see your schematic, but you say you can't pull up. This comparator only sinks current from positive for a start.

RE: Comparator Help Please

Thanks for the help smile

There's the problem, I don't have a ground, is it the same to tie 1 & 4 to - 10 which will end up chassis?
I have marked up the sketch to show how I intend to wire it to the machine

Is this how it should be wired?

Not I have added the front end, you can see it goes from around zero to near 24 Volts when the Proximity switch turns On.

RE: Comparator Help Please

That looks about right (to an ME).

Mike Halloran
Pembroke Pines, FL, USA

RE: Comparator Help Please

Thanks a lot, one more thing, I have forgotten how to calculate an R C network
What size resistor would I need for the 1 µF to charge to 16 Volts in ~2 seconds

I assume the comparator input is high enough impedance not to interfere with charging
The specs say
Input Offset Current 10 nA
Input Bias Current 100 nA

I assume I may have to switch the inputs to get it to switch the buzzer on after the Proximity switch closes.

I was a bit confused by capacitors, I thought originally of using something like a 100 microfarad electrolytic, Digikey don't list electrolytic type only Aluminium, is that the same thing, I ended up getting Ceramic.

RE: Comparator Help Please

The cap will charge 67% of the way to your charge voltage in 1/(R*C) seconds. That's called the time constant. It will be about 99% there in 5 time constants if I remember correctly.

DigiKey sells Aluminum Electrolytic capacitors. You'll want one rated for twice the voltage you plan to use it at.


RE: Comparator Help Please

Thanks, I knew the formula was simple but I'm getting a bit rusty
I take it C is in Farads

So I have 1µF ceramic capacitor charging through a 100k resistor
------------------ shouldn't that be a time constant of 10 seconds?
0.000,001 * 100,000

With the negative input set half way between the rails I don't see any time delay when I switch the resistor feeding capacitor from negative to positive.

What am I doing wrong?

RE: Comparator Help Please

Math is not my strong point
the 1 / (0.000001 x 100,000) gave me 10

So I need 100microfarad or 10Megohms for 10 Seconds

Never mind, I just found the right formula on Wikipedia

RC Time Constant T = RC

RE: Comparator Help Please

1.5 Megohms worked out pretty close to 2 seconds
I can fine tune it with a trimpot on the negative input
Thanks for the help

RE: Comparator Help Please

Doh! My mistake. The time constant is R*C. I usually work in the frequency domain so I naturally flip it. Sorry!


RE: Comparator Help Please

Ok, next question.

What's a Gerber File and is there an easy way, think cheap, to generate one.

I started to make my own circuit boards using the iron on etch resist. I soon realized I need professional help.
There is a company here who can turn them around in 24 Hours with a file.



Never mind, I found this explanation
I wonder if this old dog can be taught another new trick

RE: Comparator Help Please

A Gerber file is an archaic file type that was created to describe circuit boards. All board houses need them to "fabricate" circuit boards.

You normally use a "layout tool" to pictorially describe/draw/create your board's traces, holes, thru-holes, and pads.

Normally you use "schematic capture" software to draw the schematic on which each part is described electrically, part number wise, and package type.

The output of the schematic capture is used by the layout tool to assign each physical component and to "rat's nest" how the components are all interconnected.

The board designer interactively "places" all the parts using the apparent physical aspects, knowledge of routing issues, required physical constraints, and good electrical layout rules to "route" the board using agreed upon "design rules". Design rules describe "via holes" (thru the board inter-connects), board thickness, trace widths, trace thickness, and allowed minimum spacing between adjacent traces called "clearances".

Once the board is laid-out and approved by the person requesting it a "Gerber conversion" is done which is usually an automated procedure using the converter to produce the Gerber files.

The Gerber files describe the various aspects of the circuit board in layers. Typical layers are: solder side, component side, silk screen, solder mask, and drill layer. These "layers" are used by "light plotters" that paint photons on "resist" a chemical coating put on the raw "copper clad" material of physical board. When exposed to light the resist prevents the dissolving-off of the copper cladding in the board etching process - which leaves the traces on the copper-clad. The drill layer and a text description explains where and how big to drill the board's holes. The boards are then "isolated" from the matrix they're usually made in.

Asking for a one day layout is often suicide for the process - especially if this is your first time. It will also cost you perhaps 20X more than a normally paced process.

If you just need a simple 15 parts or less board you can sometimes draw it in a cad package as only islands-of-copper by drawing only lines of isolation. Then using "mechanical etching" by which is meant a CNC router, you cut off the copper to isolate the copper islands that make up your circuitry. Then you drill the needed holes and hand assemble your bonified circuit board.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Comparator Help Please

Thanks guys, more than I ever wanted to know about Gerber files.

I just hope the board manufacturer can write these for me otherwise I'll be back to the Laser printer and transfer paper.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close