Using RCTIME for MQ-3 Alcohol Gas Sensor

davidrudd

I am new to electronics (but have much software experience), so please forgive the perhaps simplicity of my question. I have been searching for examples of circuits for reading the MQ-3 Alcohol sensor. The examples online I come across all seem to point to using what I think are "analog" circuits (something called a "voltage divider"?). The MQ-3 documentation refers to "Connecting five volts at either the A or B pins causes the sensor to emit an analog voltage on the other pins." After completing Chapter 5 of "What's a Microcontroller" which introduced resistance-capacitor time, it seems to me that there are some sensors that are resistive (no current produced) and some that "emit a voltage". The "voltage divider" method of reading the sensor does not seem that practical with the BASIC Stamp (not having any analog inputs), whereas something like http://www.noisemantra.com/BASICStampAnalogInput.htm which shows an example of connecting an IR sensor to the base of a transistor to create a resistance between its Collector and Emitter so that the RCTIME function can be used seems VERY practical. Do I have this right? Can I route the Vout of the MQ-3 gas sensor to the Base of a transistor and set up an RCTIME circuit to accurately measure the full range of gas sensing being delivered? If I took this approach, would I still use the 220k "load" resistor across the Vout and Vss? I don't quite understand how to translate the noisemantra circuit to the MQ-3 and how the Diode fits in exactly.

Tracy Allen

The schematic at noiseMantra might work, but take note of the second circuit with the added resistor, which would be much better. The first one would be hideously affected by the choice of transistor and temperature. Either circuit there it would have problems at low voltages as it gets anywhere near the 0.6V base-emitter voltage of the transistor. The resistors have to be chosen to match the expected input range.

If you have a multimeter, hook it up between test points TP1(+) and TP2(common) and report back what voltage the module puts out as you subject the sensor to alcohol fumes.

Here is another circuit you might take a look at. It has an op-amp to allow for better translation from small voltages to RCTIME on the Stamp. You would connect the input of that via a voltage divider or potentiometer to the MQ-3 test points, but again, the choice of divider depends on what input range you expect to see from the module, which you find by checking with a multimeter.

I'm not clear on what you mean by "220k "load" resistor across the Vout and Vss" and the "how the Diode fits in".