Seems everyone is looking for a simple way to drive a relay compared to a reference voltage. Here's another Stupid Engineer Trick, use a zener. Well a fancy zener, like a LM/TL431. This amplified 2.5V zener is adjustable up to 36V and will drive over 100ma. Add a pot to the adjustment pin and feed a voltage to it. When that pin voltage exceeds 2.5V the zener conducts driving the relay. Just need to make sure that the supply voltage minus 2.5V is enough to drive the relay. Big problem is there is no hysteresis and the monitored voltage has to be over 2.5 volts. In many aplications this may not be a problem. I find it an easy way to drive an LED to indicate when a voltage is over a certain level. Three pins and 15 cents!
