Reading Sensor Values

I am working on a class project that involves a CO sensor that receives a supply voltage and depending on the impurities (Carbon Monoxide) sensed in the air, it will output a voltage. For example: Supply voltage 1.5 V, No CO Vout=.5V, CO Sensed Vout=1V. We are taking this value and using a microcontroller to determine if there is any CO sensed, but we are having issues with the inputting aspect. We wouldn't be able to just hook it up to an input pin due to the fact that Vout always will output a small voltage, meaning it will always trigger the input pin. My group has discussed using an ADC, but we haven't been able to find anything that fits our needs. We have also discussed using a voltage comparator and providing it with a reference voltage to compare to, but we are having issues with that as well. Any suggestions?

If you are looking for just a go/nogo result, the voltage comparator will be the simplest way. Depending on the accuracy you need (doesn't sound like it needs much), a simple resistive voltage divider may be sufficient for the reference. The output level of the comparator just need to match the input requirements of the microprocessor.

If you can provide more specifics about the system (operating voltage, microprocessor type, sensor type, schematic, problems you're having so far, etc.) that will be helpful.

Probably the best way to solve this is to use a microcontroller that has a comparator built in. For instance the:

* PIC12F683

Then you can use the Comparator voltage reference to set a voltage half way between the input high voltage (1V) and the input low voltage (0.5V):

Vref = (Vhigh - Vlow)/2 + Vlow = (1-0.5)/2 + 0.5 = 0.75V

So, with the PIC12F683, and a 5V supply there would be a 0.78V level you could use.

You could also do this with a built in ADC--most microcontrollers include one (as does the PIC12F683).

The only drawback is setting up the comparator or the ADC is a bit tricky.

Another possibility is to use a transistor to buffer the CO output and level translate to the microcontorller. Attached is a diagram that shows how to do that.

I added a little more detail to that diagram in case certain assumptions were not clear (now it's unambiguous ;)

The transistor can be just about any general purpose small signal NPN bipolar. Another common possibility:

* PN2222A

Steve's circuit will do the job. I would add a 33k resistor from base to ground to move the low temperature trip point above 0.5 volts and provide some noise margin.

Even with this addition the noise margin is only about 100 mV. A comparator can be configured for 200 mV of noise margin.

Using just a series diode changes the 0.5/1.0V signal to 0/0.3 to 0/0.5 volts, which will not meet the LOGIC HIGH level of the microcontroller.

Or if one wants a precise 0.75 volt trip point without an IC this discrete comparator will do the job.

The plot shows the input slewing from 0 to 1.5 volts and the output tripping at 0.75 volts. The multiple green lines show the response from 0 to 100 degrees C in 20 degree steps.

But, the output does not output "legal" TTL levels. The next post has an addition to output legal TTL levels.

This circuit trips at 0.75 volts and outputs legal TTL levels.

Go to home depot and buy an off the self CO2 sensor and be done with it. Why re-invent the wheel?