Abstract: A detector for smoke and/or selected gases includes a sensor means to which the smoke or gas is applied. The sensor is connected at one terminal to ground, the other terminal going through a current meter, through a control potentiometer to a selected positive DC voltage. A potential detector is connected at a selected point in the potentiometer so as to determine when a selected magnitude of current is flowing through the sensor. The output of the detector goes through an alarm control means, and an alarm, which is sounded whenever the current through the sensor is greater than a selected minimum value. The system also includes a scanning detector system where a plurality of separate sensors are used, each in a separate location.