low voltage sensor ckt.

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=90288
Printed Date: May 08, 2025 at 4:57 AM

Topic: low voltage sensor ckt.

Posted By: slofy
Subject: low voltage sensor ckt.
Date Posted: February 09, 2007 at 9:59 PM

I have a different thing that I am trying. I have been installing 12v power inverters in service trucks. I have installed remote starts for manual battery maintenance after a heavy load. I am pulling my hair out trying to find a relay or ckt to sense a low battery voltage to switch a 12v relay to trigger remote start automatically. does anyone have any ideas? thanks charlie

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(slofy) Charles Fols
Delaware

Replies:

Posted By: hotwaterwizard
Date Posted: February 09, 2007 at 10:18 PM

I like this idea . Lets see what we can come up with.

posted_image

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Posted By: hotwaterwizard
Date Posted: February 09, 2007 at 10:35 PM

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Posted By: slofy
Date Posted: February 10, 2007 at 5:42 AM

Thank you very much, but how should i trigger a relay to give me a ground pulse?

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(slofy) Charles Fols
Delaware

Posted By: hotwaterwizard
Date Posted: February 10, 2007 at 8:15 AM

with a Opto isolator where the led goes.

posted_image

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

Posted By: KPierson
Date Posted: February 10, 2007 at 3:00 PM

You could also use a voltage divider with a 5vdc reference.

If you set up the voltage divider with a 1:3 ratio it will divide your car voltage by 3. You can then use a seperate trimpot to to set your scaled low voltage limit.j

Run both of these values in to a comparator and transistorize the output and you're done.

No pretty pictures from me though! :)

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Kevin Pierson

Posted By: KPierson
Date Posted: February 10, 2007 at 3:02 PM

You may also want to do a bit of research on this before you install it. Any big shifts in load will cause a voltage drop, which could provide false triggers. You may want to consider adding a time delay to the circuit to prevent the truck from starting any time you turn something on.

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Kevin Pierson