Gas monitor circuits

Circuits are described in which the sensing and the reference electrodes (12, 16) of a gas sensor housed within a gas monitor are connected together through the source and drain of a field effect transistor (FET) (20). The gate of the FET is connected to a power source to apply a first voltage to the gate when the monitor is operational so that the resistance between the source and the drain of the FET is high and the two electrodes are effectively isolated from each other; when the monitor is not operational, however, a second voltage, which is preferably zero volts, is applied to the gate that greatly reduces the resistance between the source and the drain of the FET so that a short circuit is established between the electrodes. The circuit has output lines (32, 34) leading to an output device, e.g. a display or an alarm. A resistor (22) of large resistance is connected between the two output lines (32, 34) and, when a gas is sensed a large voltage change occurs across the resistor which can be used directly, i.e. without an intervening amplifier, for driving the output device.