Hmm.. Good question... I'd like to do that too. Speking of idiot lights, if you have autometer electrical gauges, they make a 3-channel light driver that goes in between the sender and gauge. You set the warning level to whatever you want, and it will activate an output when it hits that level. You can use it for any electrical gauge - pressure, temp, fuel level.. I don't think it can be used for volts though.
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