Cylinder valve controller for an uninterrupted supply of cryogenic liquid

A simple, inexpensive, solid-state controller is described which maintains an uninterrupted supply of liquid nitrogen by switching a solenoid valve from one supply cylinder to another when the first becomes exhausted. The device is used in conjunction with a commercial liquid-nitrogen level controller. The circuit uses a 7474 flip-flop as a bistable memory element, a thermistor temperature sensor, and a clock to wait an appropriate interval for the liquid-nitrogen transfer line to become cold. With minimal modification the circuit could be used with other cryogenic liquids or with compressed gases.