I have had the fridge out to seal up the burner box and stop it going out all the time. It's the original electrolux 3 way fridge fitted in the Westy.
While it was out, I set myself a little mission this week to make a flame status indicator for gas burner. The original - a little green and white needle gauge - is defunct, and I had no luck with a Wanted ad. It is a total pain trying to work out whether the thing has lit/gone out etc. The part does seem to be available from the US or Australia - but for about £100 with postage... a bit excessive!
It was quite a fun task so I thought I would put a brief account/description here in case others would find it interesting/useful!
The first step was to investigate the signal going to the old broken gauge. This took two connections. One wire was earthed to the fridge body, the other came from the burner thermocouple via this wire:
A multimeter told me it was at about -12mV when the burner was on full, and about -5mV with the burner on low, relative to the body.
What to do with these tiny voltages?
After a bit of investigating I learned that an op-amp set up as an inverting amplifier would be a good choice to amplify this into a useable positive voltage for an LED. I then got a bit ambitious and decided to set up 2 op-amp circuits, with one set up to light an LED at the slightest voltage from the signal wire, and the other calibrated to light up a second LED only when the voltage reached about -10mV - so the indicator would distinguish between the low and high burn levels, as determined by the fridge thermostat.
A single 50p chip is availabile with 2 op amps on it, which i picked with some other (all v cheap components) at Maplins.
There is a brilliant Android app (probably also available on other platforms) called Every Circuit which allows you to easily draw and then simulate circuits to check ideas, and also to calibrate component values. Below a screenshot of the Every Circuit simulation of the final circuit i worked out (sorry the layout is a bit all over the place). The resistor values are set up such that one of the op-amps will "saturate" at the slightest input voltage. The second one is set up so that when the input voltage hits -10mV, there is just enough output to trigger the transistor to open and allow the second LED to light. This all took a bit of trial and error, but that's very easy with the app!
Next step was a "prototype" on an easy "push to connect" breadboard.. a bit more tweaking of resistor values was then needed, as on the first go the red led was on even with no flame and the green was on even at low flame. (It was actually at this point i worked out that i needed to add the transistor to get a clean switch-on point at the higher input voltage for the green led).
Then once working..planned a layout on a project board and soldering everything in place. All a bit fiddly as I am not very practiced with soldering.
This is the end result...
And tidied up a bit:
...and it seems to work really nicely! The red led lights straight up as soon as the fridge lights, and the green one comes on a few seconds later as it hits full heat... the green one goes out as intended when the thermostat drops the burner down to low. Hurrah - no more desperately trying to squint to see through the hopeless burner peep hole, or listening with tubing through the condensation drain tube to hear if the burner is lit!
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