Simple Under-Voltage Cut-Out

Sensitive low-current relay coils often operate at much lower voltages than their typical ratings. This can be undesirable in some applications, where low supply voltages can result in erratic system behaviour. In some instances, this problem could be overcome simply by inserting an appropriate value zener diode in series with the relay coil. This reduces the voltage seen by the relay coil, so causing it to drop out earlier. If desired, a LED could be included when a spare set of "normally closed" contacts are available to indicate relay dropout.

Circuit diagram: