Carbon monoxide detector
A carbon monoxide detector is a device that detects the presence of the toxic gas carbon monoxide (CO), a colorless and odorless compound produced by incomplete combustion and lethal at high levels. If a high level of CO is detected, the device sounds an alarm, giving people in the area a chance to ventilate the area or safely leave the building.
CO detectors do not serve as smoke detectors and vice versa. However, dual smoke/CO detectors are also sold. Smoke detectors detect the smoke generated by flaming or smoldering fires, whereas CO detectors can alarm people about faulty fuel burning devices. Carbon monoxide is produced from incomplete combustion of fossil fuels. In the home CO can be formed, for example, by open flames, space heaters, blocked chimneys or running a car inside a garage.
When an alarm sounds, action must be taken immediately. This may include evacuating the premise if experiencing CO poisoning symptoms (confusion, headaches, nausea, dizziness, vomiting, etc), ventilation of the premises, and contacting professionals to inspect CO emitting sources. If you suspect you have suffered CO poisoning, go outside immediately and call the emergency number. At high levels, breathing CO gas is fatal within minutes.

# Installation
The devices, which retail for $20-$60USD and are widely available, can either be battery-operated or AC powered (with or without a battery backup). Batteries should be replaced regularly to guarantee proper operation.  (They generally last about a year; most detectors are designed to signal a low-battery condition.)
It is important to install the detector in accordance with the manufacturer's instructions. CO detectors can be placed near the ceiling or near the floor as CO is very close to the same density as air. Detectors should be placed outside the bedrooms.
Since CO is colorless and odorless (unlike smoke from a fire), detection in a home environment is impossible without such a warning device. In North America, some state, provincial and municipal governments require installation of CO detectors in new units - among them, the U.S. states of Illinois, Massachusetts, Minnesota, New Jersey, and Vermont; the Canadian province of Ontario; and New York City.
All CO detectors have "test" buttons and, like smoke detectors, should be tested regularly (weekly or monthly).
Common modern (2007) battery-powered models have a limited life (of about seven years), and are designed to signal a need to be replaced after that timespan.

# Sensors
Early designs were basically a white or beige element which would fade to a brownish or blackish color if carbon monoxide were present. As carbon monoxide related deaths increased during the 1990s, audible alarms became standard.
The alarm points on carbon monoxide detectors are not a simple alarm level as in smoke detectors but are a concentration-time function.  At lower concentrations (eg 100 parts per million) the detector will not sound an alarm for many tens of minutes.  At 400 parts per million (PPM), the alarm will sound within a few minutes.  This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to relatively common sources of carbon monoxide such as cigarette smoke.
There are three types of sensors available and they vary in cost, accuracy and speed of response.  However, since CO levels normally increase very slowly and there is a large ratio between safe and unsafe levels and levels that cause minor and severe symptoms, all three are entirely adequate for use.  All three types of sensor elements typically last from 3 to 5 years and should be replaced after that time.  At least one CO detector is available which includes a battery and sensor in a replaceable module.  Most CO detectors do not have replaceable sensors.

## Biomimetic
A biomimetic (chem-optical or gel cell) sensor works with a form of synthetic  hemoglobin which darkens in the presence of CO, and lightens without it.  This can either be seen directly or connected to a light sensor and alarm.

## Electrochemical
A type of fuel cell that instead of being designed to produce power, is designed to produce a current that is precisely related to the amount of the target gas (in this case carbon monoxide) in the atmosphere.  Measurement of the current therefore gives a measure of the concentration of carbon monoxide in the atmosphere.  Essentially the electrochemical cell consists of a container, 2 electrodes, connection wires and an electrolyte - typically sulfuric acid.  Carbon monoxide is oxidised at one electrode to carbon dioxide whilst oxygen is consumed at the other electrode.  For carbon monoxide detection, the electrochemical cell has advantages over the other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of 5 years or greater).  Until recently, the cost of these cells and concerns about their long term reliability had limited uptake of this technology in the marketplace although these concerns are now largely overcome.

## Semiconductor
Thin wires of the semiconductor tin dioxide on an insulating ceramic base provide a sensor monitored by an integrated circuit. CO reduces resistance and so allows a greater current which if high enough will lead to the monitor triggering an alarm.

# Digital
Although all home detectors use an audible alarm signal as the primary indicator, some versions also offer a digital readout of the CO concentration, in ppm.  Typically, they can display both the current reading and a peak reading from memory of the highest level measured over a period of time.  These advanced models cost somewhat more but are otherwise similar to the basic models.
The digital models offer the advantage of being able to observe levels that are below the alarm threshold, learn about levels that may have occurred during an absence, and assess the degree of hazard if the alarm sounds.  They may also aid emergency responders in evaluating the level of past or ongoing exposure or danger.

# Alarm Manufacturers
- BRK Electronics (BRK Brands, Inc.)
- Dicon Safety Products
- DuPont
- FireAngel (Sprue Aegis plc)
- First Alert (BRK Brands, Inc.)
- Kidde