how do you make a carbon monoxide detector?

Answer 1

You start with a component which changes electrical properties when exposed to the gas you're interesting in detecting (CO in your case). See here for details on how these components work:
http://www.figarosensor.com/products/general.pdf

Next you need an amplifier to convert the change in component characteristics into a voltage. This is followed by a comparator circuit which will change output state when the voltage signal reaches a certain level. You then use this output to trigger an alarm.

Answer 2

Are you sure you want to make a carbon monoxide detector? These are difficult. They are available in stores at very nice prices.
If you need to ask about this, you probably need to build up your electronic skills first.
How about a SMOKE detector?
Much easier.
Build a light detector. Available in kits. Or build your own with a CdS light sensor and a battery.
Then you need a threshold detector to measure when the light goes dim.
Set a light some distance away, so that when you make smoke between the light and the detector, the smoke blocks some light and the threshold detector sends a signal.
Juggle the parts for best performance. Experiment.
Read the books by Forrest Mims available at Radio Shack, and they have parts too.
If you are more experienced, get Nuts and Volts magazine and get parts from the places that advertise in there.

Answer 3

i dont know but i have heared about them in fores fire alarm

Answer 4

Buy a canary...