CHT question

mondshine · Post by **mondshine** » Wed Jan 10, 2007 11:34 am

Greetings-
I am in the process of re-assembling a '71 squareback. I've had many acvw's and understand the need to monitor CHT; however, I don't want to hack the interior. I have some Westach spark plug thermocouples which I keep for another car with a CHT gauge. Here's the question: If I know these thermocouples output ~12 millivolts at 400 degrees, how could I create a device to trigger a relay (for a warning light, like the bulb in the emergency flasher switch, for example) at an adjustable voltage between 11 and 12.5 MV.
Are there any electronics wizzards out there that could help me with this?

Thanks, Gary Mondshine

MedicTed · Post by **MedicTed** » Wed Jan 10, 2007 11:44 am

That's a very interesting idea. Let me chew on it.

All_talk · Post by **All_talk** » Wed Jan 10, 2007 12:27 pm

Should be easy enough to do, nice thing about thermocouples is that pretty much operate at logic level outputs. I think all you would need is a comparator circuit to switch an output transistor. Electronic is not my daily bag so it would take me a little while to come up with a circuit for ya but I’m sure some wizard will come along to help.

You also might be able to find a ready made thermocouple reader with an alarm output, not sure exactly where to look or how much it might cost. I think Radio Shack use to have something like this but all they sell any more is cell phones and really crappy R/C cars.

Gary

All_talk · Post by **All_talk** » Wed Jan 10, 2007 1:20 pm

Something like this would work (link), its an industral prosses control thermometer with an on/off & alarm output.

http://www.omega.com/toc_asp/frameset.h ... ile=DP7000

But I think you could build something simple and much cheaper, just need the right person to layout the circuit and parts list.

Gary

mondshine · Post by **mondshine** » Wed Jan 10, 2007 4:27 pm

Thanks guys-
That omega controller may be overkill for this purpose; but thanls for looking into it.
Ted- nice to hear from another 181 guy.
Best regards, Mondshine

Speedy Jim · Post by **Speedy Jim** » Wed Jan 10, 2007 5:13 pm

HaHa! I looked thru my pile of old sketches and the last time
I worked on a CHT (for my 181) was in 1974!

For your alarm requirement, I would use a very simple
comparator, such as the LM311. They are antiques
and priced at $.50

They operate off a single 5V supply, so add a 78M05 5V regulator for $.50.

Typical circuit with pinout:
http://www.du.edu/~etuttle/electron/elect18.htm

You have to be a little careful on the inputs when dealing with
tiny voltages to keep things precise, but "no hill for a climber".

Another possible choice is the NE555. Normally thought of as a timer,
it is really a very robust comparator in sheep's clothing.

Speedy Jim
http://www.nls.net/mp/volks/