Edited to increase R8 to 100k.

Q1 will likely not fully saturate with such a low base current (forced Beta of about a 100) giving an On voltage of perhaps a volt or so, but that's probably not a problem in this application.

 

Q1 will likely not fully saturate with such a low base current (forced Beta of about a 100) giving an On voltage of perhaps a volt or so, but that's probably not a problem in this application.

It probably won't with the lowest spec BC337, but with Ic being only 8 - 10mA its not really an issue. There's more than enough volts to play with. Worst case, we can reduce R8 to 82k or even lower if need be.

 

IT WORKS !!!!! .... just what I was wanting ..... awesome .... however, it'll have to stay as a breadboard until I get back in about a month .... then I'll finish it ..... and send photos ..... thanks again ..... and stay safe .....

 

Great, glad to have helped...

Stay safe & have a great sail!

 

Irving .... good evening .... back from the Bahamas .... 21 days on a 36 foot sailboat ...... an adventure ..... so, got home and went to work ..... put it together on a perf board .... had some wiring problems ....... got them sorted out but still couldn't get it to work ... it did well on the breadboard .... so I breadboarded a second one ..... and I did make some changes ..... on the advice of one of the LM311 data sheets that I researched they recommended that I connect pins 5 and 6 together .... also C3 and C1 are redundant so I moved C3 to between LM311 pin 3 and ground .... again based on a data sheet suggestion .... my last mod was to drop R8 to 22K .... I'm using a BC337 for Q1 and it didn't seem to consistantly trigger at 100K ..... so that's what I've got ..... and I'm having issues ..... I'm using a trimmer for RV1 .... 10K .... and I'm wondering if that might contribute to some of my issues .... if I set it to switch the red LED off and the green one on at 6 mv, when the thermocouple cools and the voltage drops below 6 mv, the red and green don't switch .... and sometimes both LEDs are on at the same time ..... or did I blow a chip ? ...... help please ..... thanks ...

 

There are special IC's already designed to handle this and have reference temperature junction compensation built in:

https://cdn-shop.adafruit.com/datasheets/AD8494_8495_8496_8497.pdf

If you don't want to build that a pcb for the part:

https://www.adafruit.com/product/1778

Works from 3-18V, so 12V fits your requirement.

The only other thing you would need is an open collector (open drail) comparator to turn the LED on. A small protoboard after this board could handle that.

 

Good morning all ..... it took some time but I figured out my problem ..... it was me ..... I didn't pay attention and I didn't do things correctly .... and, yes, I burned out some chips .... hopefully I'm smarter now .... sorry for not following the advice from those of you who have tried to help me .... dcbingaman .... I took your advice and purchased that board from adafruit .... it got me past my lack of skills .... and things are better after I figured out that the polarity of thermocouple is not what I assumed it to be .... the copper sheath is positive and the button on the end is negative ..... I'm back to the original diagrams that Irving offered except that the adafruit board replaced the AD623 with it's filter circuitry .... and I'm using a BC337 in lieu of the 2N3904 .... the resistors and capacitors are as per Irving's diagram .... and I did replace the 10K trimmer that I was using with a better quality 10K trimmer ..... however, I still have a problem .... the red LED is always on ..... adjusting the trimmer pot willl turn the green LED on and off but the red LED is always on ...... here some voltage readings .... with the green LED on: +12v to LM311 pin 7 = 11.62v, pin 7 to gnd = 197.8 mv ..... with the green LED off: +12v to pin 7 = 1.854 v, pin 7 to gnd = 10.04v .... when I remove the circuitry for the red LED (1k, LED, BC337 and 100k) I get the following readings .... with the green LED on: +12v to LM311 pin 7 = 11.21v, pin 7 to gnd = 198.7mv, with the green LED off: +12v to pin 7 = 90.3mv, pin 7 to gnd = 10.37v ..... how do I get that red LED to switch off when the green LED is on and vice versa ? ..... thanks ....

 

I am a little confused. You stated you are using Irvings circuit but also using the adafruit module. Can you paste your latest schematic?

 

dcbingaman and Delta Prime ...... good afternoon .... I'm using the adafruit module to drive the LM311 ..... in place of the AD623 chip and it's associated filtering circuitry that is in Irving's diagram .... that filtering circuitry is included in the adafruit module ..... I've attached my circuit ..... pardon my scrible ..... I just went and poked at my breadboard and, guess what ?, I might just have a bad breadboard ..... now, the only time that the red and green LEDs are on at the same time is during the transition .... if there's an easy fix for that, I'd be interested .... otherwise I'm just going to leave this circuit alone ...... and get a new breadboard ..... any suggestions there ? ......

 

Delta Prime , dcbingaman , Irving ,
Gentlemen ….. thank you all for your help with this project ….. it’s done and it works very well …. yes, my perfboard skills are not the best so I had to go with the adafruit module …. the rest of the circuit is as Irving suggested …… and I did have to get a new breadboard …. the pilot light sensor electronics are stuffed in the metal box that is on the bottom right ….. the red and green LEDs are located just above the PID ..... and I’m smoking some chicken …. again, thank you all ….. this project is done …. this thread can be closed ...