I recently purchased an MQ-7 CO sensor and, as per my usual approach, did so without reading the datasheet.
After receiving the unit, I was all set to connect it to my Uno and start getting readings. However, after actually reading the datasheet (attached), I now see that it is not quite as trivial to use as I had imagined.
Specifically, the unit must be ‘heated’ at 5 V for 60 s to desorb the gases, then powered at 1.4 V for 90 s during the actual reading. Initially this also sounded trivial (easily solved using a PWM pin). Unfortunately, the datasheet also indicates that the MQ-7 heating resistance is 33 Ohms, which should be a current draw of 150 mA (5 V) or 42 (1.4 V) mA, both of which are beyond the Uno PWM pin capability of 40 mA.
I found this tutorial which discusses this issue, and suggests using an NPN transistor as a variable resistor, and connecting the power of the MQ-7 to the 5 V pin of the Uno (can handle > 200 mA).
In this tutorial, the author recommends to desolder some of the components. My question is: Has anyone here used this module before, and if so, did you have to desolder any components to have the board function properly? Also, any tips/ideas on how to use this unit would be greatly appreciated.
MQ-7.pdf (52.2 KB)