Electronics > Beginners
Voltage sensing DC 12/volt
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m42665:
 Let me put it this way we have a 12/volt battery on a table that has a light on but in time the battery is dying  with voltage going lower and lower the light brakes at 9/volt and lower . It is this I want to stop , put something on the line that would sense the lower voltage and shuts off and only let 12v or high come through . I am a newbie and I have to get this done so please somebody help.

2ND POST --Thank you so much for your help ! I need you to remember that I am new at this  I didn't know what a comparator was and had to look it up and a lm385 too but it looks like they are the same thing , now this doesn't mean that am good now.  I still need help putting this together , some part numbers or names of what is needed to do the job.
Well it looks like what I need is the comparator I saw a video on it and want to try it . Looking at a picture I posted I believe I can do but if you can hold my hand and help with maybe a picture or how to wire it would mead a lot to me !
As far as my example of the battery going dead was so it could be understood what I wanted to do .
The battery never go's dead but the wire I have to use and must use changes voltage from 6 to 12volts so I want to stop anything less then 12volts coming out on a wire I'm going to tie to it that is going to a relay that could turn on at the wrong time . Please get back thanks in advance Mitch .

3RD POST---Thank you so must for your help !!!!! I put up a 2nd post where I say it's not so much the battery I care about , it's the wire I must use that has 6 and 12volt at different times , I want to tie on to that wire but I only want it when it has 12volt not 6volt . So I must stop 6volt from coming out of that wire that I must use . Please get back with something like you sent me and how to wire it up .
I Thank everybody so much for your ideas and remember that I am new at this so make it easy so I can understand .
JS:
A comparator, check a known voltage from a reference, like lm385 or other voltage regulator with a division of your rail, that comparison will tell you if the battery voltage is higher or lower.

As your battery will recover some voltage after the load is disconnected you need to deal with that. Hysteresis might help but likely not enough, as recovery voltage is quite high, depending on bat chemistry. I'd probably use a way to dissconnect everything from the battery till a charger is detected, and only then reconnect the comparator and light. Something like a mosfet powering the comparator and light, keept on by the comparator till it hits a threshold and then a separate circuit which powers the mosfet while detects a charger.
  That way once you hit the level ypu want the light off it turns off and as soon as a charger gets plugged it goes back on, till voltage drops again.

JS

m42665:
Thank you so much for your help ! I need you to remember that I am new at this  I didn't know what a comparator was and had to look it up and a lm385 too but it looks like they are the same thing , now this doesn't mean that am good now.  I still need help putting this together , some part numbers or names of what is needed to do the job.
Well it looks like what I need is the comparator I saw a video on it and want to try it . Looking at a picture I posted I believe I can do but if you can hold my hand and help with maybe a picture or how to wire it would mead a lot to me !
As far as my example of the battery going dead was so it could be understood what I wanted to do .
The battery never go's dead but the wire I have to use and must use changes voltage from 6 to 12volts so I want to stop anything less then 12volts coming out on a wire I'm going to tie to it that is going to a relay that could turn on at the wrong time . Please get back thanks in advance Mitch .     
Zero999:
You can buy them off ebay, if you don't feel like making your own.
https://www.ebay.co.uk/itm/Battery-Against-Excessive-Discharge-Controller-12V-Low-Under-voltage-Protection-/252928364204

Another possibility is the TL431. How much current does the light draw? If it's more than a couple of hundred mA, use this circuit to switch a relay or use a P-MOSFET for Tr1.
m42665:
Thank you so must for your help !!!!! I put up a 2nd post where I say it's not so much the battery I care about , it's the wire I must use that has 6 and 12volt at different times , I want to tie on to that wire but I only want it when it has 12volt not 6volt . So I must stop 6volt from coming out of that wire that I must use . Please get back with something like you sent me and how to wire it up .
I Thank everybody so much for your ideas and remember that I am new at this so make it easy so I can understand .
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