Hello everybody, I have bought CO gas sensor module (27931) and after read all information, I know that there are 2 phases, heating and sensing.
The first one is turn to low state HSW pin about 60 seconds and watching schema I can see that it means to put MQ7 sensor to 5 V. And the second one is using a PWM signal to reach 1.4 V RMS about 90 seconds.
To test this sensor I use a microchip (PIC18LF2550) with 2n3904 transistor (http://www.parallax.com/Portals/0/Downloads/docs/prod/sens/27904_27930-1-2-GasSensor-v2.2.pdf
), so, HIGH level in my HSW pin means to turn on the heating.
In this PDF we can see a BASIC Stamp 2 code (last page). Well, I want to program in other language the 2 main routines (heating and sensing) but I have these questions:
- When I turn on my HSW pin (heating phase) I can see (using a voltimeter) 4.83 V and 190 mA (total current consumption) on H pin (MQ7 sensor) instead 5.0 +-0.1 V that appears in sensor datasheet. So I suppose this is ok, or no?
- Watching the proposed code for sensing phase, I guess that the period of the signal is about 18 ms (15 ms HSW on, remember 2n3904 transistor, and 3 ms HSW off). The code does that 1710 times, about 90 seconds, but if I multiply 18 ms x 1710 its only aprox 30 seconds instead 90. What am I doing wrong?
- What characteristics are recommended for PWM signal to reach 1.4 V on H gas sensor pin? (period or frequency, duty cycle for HSW pin)
I hope your answer, thank you very much.
PS: If you have any question I'll try to explain it again using other words.