Another part of the problem could be the small MOSFET.
The gas value typically takes a few hundred mA from the thermocouple, and that could generate a significant voltage drop across the MOSFET's on-resistance when its ON.
There's not a lot of voltage to spare when the thermocouple only outputs about a half volt.

 

The TS mentioned it was only .84ma in post #8 and it worked consistently with a gate voltage of 4 volts and above.

 

A mosfet such as a FDS8896 may solve your problen as it has an on resistance of about 0.007 ohms.
A PVI5013RPbF (Or similar may solve the isolation problem.). This opto isolator gives an output voltage. (Not just a conection via a transistor or mosfet.)
I have only seen the thermopile system used as a flame failure device to shut of the main supply when the pilot fails. In that system it does not close the valve, it only holds it closed after it is manualy close while lighting the pilot.)

Les.

 

JimStokes can you reply to post #39 please?

 

I think the current reading in post #8 is incorrect. I think the current meter used had a high internal resistance that was limiting the current reading.

Les.

 

You mentioned the unit will be battery operated. What is the battery voltage?

9V. By the way I got your suggestion to work, using 2 BS170s and a 1M resistor.
I like the optocoupler suggestion as well, but no local suppliers.

Thanks again.
Jim

 

I think the current reading in post #8 is incorrect. I think the current meter used had a high internal resistance that was limiting the current reading.

Les.

I've tried 2. The Mastercraft gave Mr the first reading

I think the current reading in post #8 is incorrect. I think the current meter used had a high internal resistance that was limiting the current reading.

Les.

I took two reading. The 0.84 was with the Mastercraft. The fireplace started when I connected the meter the first time, but not on subsequent tries. The second reading was with Totaline and fireplace didn't start up when connected. Weird, to me.


Jim

 

If you have 2 meters with one meter set to the current range that you usedthen with the other meter measure the resistance between the test leads on the first meter.
I think you will also find that the meter has a different resistance on different current ranges.
Les.

 

Thanks all. So, I used Sgioto's suggestion and used two BS170 Mosfets and set GPIO high for controller off, low for controller on. This seems to work reliably. Thanks to all for all your suggestions. I am going to find out more about optocouplers as this seems like a perfect situation for the two circuits to be isolated from each other. In the configuration I now have, the grounds are shared between my PICO circuit and TH on the gas valve; not a problem, but isolation just seems intuitively better.

I've attached a sketch to show the final configuration. Now to box it up....

Probably should have shown this before, below is how the the millivolt valve was previously wired, with a receiver controlled by a remote control. After probably 30 years, the remote/receiver combo failed. So, I decided to learn something and try to innovate a solution.

Thanks for all your help

Jim.

 

I still favor the CPC1002 for isolation.

 

I still favor the CPC1002 for isolation.

Battery powered systems are usually rather isolated.

 

I still favor the CPC1002 for isolation.

Isolation from what?

 

Isolate the entire Raspberry circuit from the furnace controls I would suspect.

 

JimStokes, what model buck converter is that in the schematic?
Wondering if a LDO regulator would work better.

 

JimStokes, what model buck converter is that in the schematic?
Wondering if a LDO regulator would work better.

See attached.

 

OK, so now it is working as desired. Is it powered by a 9 volt battery?? Or is that a mains power operated supply that I see in post #55? Probably not a mains operated supply. So how long will the battery source hold up??

 

OK, so now it is working as desired. Is it powered by a 9 volt battery?? Or is that a mains power operated supply that I see in post #55? Probably not a mains operated supply. So how long will the battery source hold up??

Not mains. Powered by 9V battery. Post 55 is the Buck-Boost converter, to get a stable 5V source.

Good question on how long the battery source will hold up. I don't use the fireplace often, mostly only for ambience, and it will be fully off most of the time. We'll see how long it lasts. One battery per winter season would be better than the remote/receiver option I had before which took 2AAAs and 4AAs about twice per season.

Thanks for your interest/help.

Jim.

 

A simple, small, mains powered transformer isolated external supply might be worth considering if the battery does not last very long. An isolated 5 volt supply should not be that difficult to find, at a reasonable price even.