a relatively inexpensive modular component that can be substituted for , or used in conjunction with , an existing smoke detector in a commercially available residential fire - detection alarm comprises : an amperometric sensor for carbon monoxide , in which the reference and counter electrodes are combined either internally into a single auxiliary electrode or externally by electrical shorting ; and a simple current - to - voltage converter circuit that converts the current signals from the sensing electrode into amplified voltage signals . the converter circuit is powered by the same battery and is connected to the same alarm - triggering circuit that are used in existing residential fire alarms . the sensor - and - converter module is designed to fit into a commercially available smoke - detector - type fire alarm unit . the electrical output of the module is designed to be compatible with the electrical input requirements of the commercial detection circuitry . an alarm fitted with this module provides early warning of hazardous conditions , such as a smoldering fire , a leaky furnace , or an otherwise heavily polluted atmosphere .