OK, here's what I have. I'm looking at using a Wide Band O2 sensor that has a 0-5 VDC output. 0V = 10 AFR, 5V=20 AFR Air Fuel Ratio. Voltage output goes from 0v=10 to 0.05V= 10.1, 0.1V= 10.2, 0.15V= 10.3, here's a link to the table to show the range.
What I want to do is use this signal to trigger an alarm when the AFR reaches a preseleted level, too lean. I want to be able to select the level with a digital display, but for now I'd be happy to use a potentiometer and a Fluke meter if I can to select the preset point. I guess what I want to do is be able to select when the 0-5V input rises above a preset point.
I'm a 50 year old "newbie" to working with electronics, so please bear with me, I'm still learning. I know what I want to do and have reasonable understanding of how componets work. I think what I need is some type of comparator with an way to adjust the input voltage preset point and use a mosfet to trigger the alarm. I'm open to ideas.
Using a voltage comparator would in fact be a good way to go about this. Give this a read to get a good understanding of how it would be done. Using a comparator with a pot to set a trigger threshold would be a good start. Read the link and see how the comparator can have an active output above or below a setpoint depending on how it is configured. Then if you have more questions just ask away.