Respiration circuit

I want to create a circuit that can measure respiration rate. The device is use to measure breathing intervals and not heart pulses.

Can anyone direct me to a suitable site or advice me on an approach to creat this ciruit.

I've seen some samples using a belt wrap to the chest and linked to a PC to gather the info but it doesnt specify any circuit.

Really appreciate it if anyone knows how to create one.

Thanks

search out those baby breathing monitors, the ones that you put a pad in their cot with them...

the belt method could be done in a number of ways. I assume it can't be restrictive. the antistatic conductive foam has variable resistance when you compress it - use a divider into a comparator. or use a photo interruptor in a sleeve. chest expansion pulls a blocking "paddle" out and a spring/elastic helps it to return. Maybe even a simple contact that expansion separates and a spring/elastic returns (or the other way around). Or use capacitance between two plates that slide against each other - the greater the expansion, the lower surface area in common and the lower the capacitance. measure capacitance via an oscillator or directly measure V rise time across the cap. Or maybe use an accelerometer (seems overkill). Or a strain gauge. Or...

Then, it's just a case of reading the output of the sensor and rejecting impossible data.

edit: the baby breath monitor seems to not monitor respiration but activity. It's a pad that goes under the mattress. Even when quietly asleep, a baby has some movement. no activity for 20 seconds and they trigger an alarm. I assume they have some sort of cutoff if the crib is empty. Must be pretty sensitive. I think it could to be too sensitive to use on awake people.

I spent a few minutes on this a few months ago. Apparently an ECG/EKG is modulated by respiration. I do not know if you can capture enough information with the two pads on an atheletic type HRM chest band. I would expect the circuitry to be less complicated that the software that you might need to separate the information on respiration. You might google on ECG or EKG and respiration.