Need some help

[jchase88]

I am building a breathalyzer but I cannot use any programmable chips. We are using a comparator circuit to analyze the voltage output of the alcohol sensor. The problem we are having is that the output voltage of one of the comparators is 2V while the other is at the 5V we need to turn on the l.e.d. We are looking for a way to clean up the signal so that only one led is on. We tried using an inverter but 2V makes it go high so we are looking for another option. Thanks in advance for the help!

 

[MikeMl]

Post your circuit. Do you have a url of the sensor data sheet?

 

[jchase88]

https://www.electro-tech-online.com/custompdfs/2010/03/MQ-3.pdf This is the url for the sensor.
We are taking the output from the sensor and feeding it to the inverting end of one comparator and the non inverting into the other comparator. The other input of each comparator is tied to a voltage divider to create the desired reference voltage to determine whether the output of the comparator is high or low. When the voltage is below Vref, there is 5V at one comparator and 2V at the other. We need a way to make the 2V zero because that voltage is allowing the wrong l.e.d. light up. I'm sorry I don't have a schematic... we have been troubleshooting this for a while. The voltage coming out of the comparator that should have a zero voltage is a little about 2V which means we can't use an inverter to clean the signal.

 

[jchase88]

The actual project is to create a breathalyzer in which a green l.e.d. will light up below .08 and a red l.e.d. to light up at .08 and above.

 

[MikeMl]

Are you using the RL shown on the sensor data sheet?

 

[jchase88]

We actually changed the load because the voltage levels weren't where we wanted coming out of the sensor. Vref is set to 4.68V. Is there a simple way to clean up the output? We just need to be able to have one at 5V output and the other at 0V so that only one led is illuminated at a time

 

[MikeMl]

We actually changed the load because the voltage levels weren't where we wanted coming out of the sensor. Vref is set to 4.68V. Is there a simple way to clean up the output? We just need to be able to have one at 5V output and the other at 0V so that only one led is illuminated at a time

What comparators are you using?
Are they powered off 4.68V?
What value of RL are you using?

 

[jchase88]

we are using the lm 741. They are running off a 5V source. I believe that we used 10k for the Rl

 

[jchase88]

the 10k actually gave us a better range for the output

 

[MikeMl]

we are using the lm 311. They are running off a 5V source. I believe that we used 10k for the Rl

10K is much, much too low. The sensor is ratio-metric, expecting a load resistance of ~200K!

Figure out how to POST YOUR CIRCUIT!

 

[jchase88]

yes but is there a simple way to just clean up the output... I just need to be able to get that 2V to 0

 

[MikeMl]

yes but is there a simple way to just clean up the output... I just need to be able to get that 2V to 0

I cant tell without looking at the circuit!