- Tue Dec 08, 2009 1:19 pm
#86747
I've spent some time working with the MQ-7 CO sensor ( http://www.sparkfun.com/commerce/produc ... ts_id=9403 ) in a hydrogen application. This sensor requires a varying heater voltage (the documentation http://www.sparkfun.com/datasheets/Sens ... c/MQ-7.pdf suggests alternating between 5v for 60 seconds and 1.4v for 90 seconds). Apparently the heater needs to drive off adsorbed moisture and gases but the active element is desensitized while it is hot. The specs suggest sampling at the end of each of the two alternating periods (and, perhaps, using two different load values for each period - if I interpret the poor Engrish correctly) and it suggests using several 2.5-minute-total sample periods, presumably averaging the samples. That makes the sensor sampling slow. Further, when the heater voltage switches, the sensor produces immediate spikes as the element temperature changes quickly.
I've found another method to use the sensor by driving the heater with a slow sine. I chose a 285-second period (based on reaching 90% of the steady-state heater temperature) of a 2-volt-to-5-volt PWM'd sine drive. The sensor seems to saturate more easily at lower heater temperatures, so a 3-volt low end - or a smaller load resistance - might reduce that behavior.
The MQ-7 is sold as a CO sensor, but is actually more sensitive to hydrogen than it is to carbon monoxide, according to the documentation. Here is the response using the suggested 5v/1.4v rectangular heater drive. I made two small hydrogen releases of unknown concentration: http://rightime.com/images/Misc/CO_Sens ... ogen_2.GIF
Here is a ~200ppm hydrogen release (25cc into a 30-gallon container) using sine drive: http://rightime.com/images/Misc/MQ-7_5v ... 0kLoad.GIF
The obvious advantage of the sine method is that the sensor output is continuously available using a predictive filter to watch the resulting sine signal, not just single samples at 2.5-minute intervals.
Anyone else working with these sensors?
Tom
I've found another method to use the sensor by driving the heater with a slow sine. I chose a 285-second period (based on reaching 90% of the steady-state heater temperature) of a 2-volt-to-5-volt PWM'd sine drive. The sensor seems to saturate more easily at lower heater temperatures, so a 3-volt low end - or a smaller load resistance - might reduce that behavior.
The MQ-7 is sold as a CO sensor, but is actually more sensitive to hydrogen than it is to carbon monoxide, according to the documentation. Here is the response using the suggested 5v/1.4v rectangular heater drive. I made two small hydrogen releases of unknown concentration: http://rightime.com/images/Misc/CO_Sens ... ogen_2.GIF
Here is a ~200ppm hydrogen release (25cc into a 30-gallon container) using sine drive: http://rightime.com/images/Misc/MQ-7_5v ... 0kLoad.GIF
The obvious advantage of the sine method is that the sensor output is continuously available using a predictive filter to watch the resulting sine signal, not just single samples at 2.5-minute intervals.
Anyone else working with these sensors?
Tom