I am trying to design a basic analog fuel control circuit, I can almost see it but I can't quite put it down on paper... My electronic engineering skill is at about a 6.5 out of 10.....

Anyway, this is what I need to do:

I have an O2 sensor with an ideal voltage of 500mv, and a 3 wire control valve. I need a design that will open the valve at a reading of less than 500mv and close the valve at a reading of more than 500 mv. It would also be nice if it was static at 500mv. It seems so simple but I can't seem to grasp it. My initial idea was a Zener diode and a small amp to a 2 position relay, but I don't think it would work well..... Would appreciate some ideas

 

What are the three wires on the control valve for, please? Needed for a complete answer.
It is correct that this is all powered by a car battery?
If it was me, I would start with a comparator IC that would compare the 500mv signal to a 500mv fixed reference. I would add a little hysteresis to the circuit with feedback. That would prevent the valve from oscillating rapidly right at 500mv input. The output from the comparator would feed a transistor driver circuit that would connect to the control valve.
Search for "comparator circuit designs". Find something that fits in a car, and ask more questions after you tell us about you valve.
Welcome to AAC!

 

It is a standard 3 wire valve, middle wire is ground, one wire opens valve and other closes the valve, it will operate on a 12v dc system, it is for a propane blast furnace

 

It is a standard 3 wire valve

Can you post a link to its spec? It helps to know its full electrical properties, particularly coil resistance/current ratings.

 

After some research, I think a comparitor circuit is exactly what I need. I can use a comparitor to essentially make a low level and a high level alarm circuit, low level set at 475mv and the high level set at 525mv, sending the output to a couple of control relays. The low level will activate the relay to open the valve and the hi level will activate the relay to close the valve, also leaving a 50mv dead zone at the nominal voltage. Sounds like it could work. Agree?

Thanks sailer joe for the idea!