Rise in voltage detector help

[deadgroovy]

Hi, hope one of you guys can help me out.

I need a simple circuit to detect a rise in voltage from about 12.6 vdc to switch on a relay when the voltage reaches about 13.8 vdc.
Its for an automotive project. The rise is the alternator charging voltage.
I would like the relay to stay energised even if the voltage drops below 13.8 v and to be de energised when the ignition is turned off.
Hope I've explained clearly!

 

[MikeMl]

Read this current thread. You want the inverse. If you need some help reconfiguring my posts to your specific needs, write back.

 

[deadgroovy]

Cheers, MikeMl, thanks for the link, I think this might do it, if I replace the LED with a relay http://www.reuk.co.uk/OtherImages/lm741-voltage-indicator.jpg
Do you know what the resistor values should be to drive a relay with a coil current of 0.15A @ 12v?

 

[MikeMl]

Here's how I would do it with the TL/LM431. Red, Blue, Green, ltBlue, and Purple traces are with the trimpot 100Ω, 300Ω, 500Ω, 700Ω and 900Ω, from the top, respectively. Horizontal (X) axis is input voltage. Vertical (Y) axis is current through the relay.

 

[deadgroovy]

Thanks a lot, Mike, I'll give it a try.