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Old 04-13-2010, 03:12 PM   #1
jlandin
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Default Electronics for Pilot Light Indicator?

Sometimes it's difficult to see the pilot light especially on a bright sunny day. I thought it would be neat to have an LED or something that indicates when the pilot light is on/off based on the thermocouple signal. I realize that the voltage on the thermocouple is ridiculously low (26-32mV). I'm using these thermocouples with my valves which do have support for this ECO connector to tap into the voltage change. It's such a low voltage though. Maybe I'm wrong, but I don't think there are LEDs that low.

Anyone have any ideas?

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Old 04-13-2010, 03:21 PM   #2
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you would have to amplify the signal with a opamp or comparator. This would get you a useable voltage/current to drive an LED or to drive a transistor to turn on an LED. My only concern would be that you would mess with the signal going to the temp controller (assuming that's what the thermocouple is going to) if you add anything to it. Not sure how you could tap into the signal without altering it.

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Old 04-13-2010, 03:29 PM   #3
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Quote:
Originally Posted by Bjornbrewer View Post
you would have to amplify the signal with a opamp or comparator. This would get you a useable voltage/current to drive an LED or to drive a transistor to turn on an LED. My only concern would be that you would mess with the signal going to the temp controller (assuming that's what the thermocouple is going to) if you add anything to it. Not sure how you could tap into the signal without altering it.
Thanks. The thermocouple is connected to the Honeywell solenoid gas valve. It controls the internal safety switch which shuts the valve down when the thermocouple cools (pilot out).

Code:
thermocouple ------+-----> solenoid valve (safety switch)
                    \
                     \-----> amplifier -----> LED
Is it possible to amplify just the LED signal from the thermocouple, and not affect the leg going to the solenoid valve?

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Old 04-13-2010, 03:35 PM   #4
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Code:
thermocouple ------+-----> solenoid valve (safety switch)
                    \
                     \-----> amplifier -----> LED
that's pretty much what I mean. I just don't know if it will affect the signal (reduce it would be the big problem) enough to mess with the valve. As long as you grab the signal right at the terminals (the ECO connector) you should be ok. I make not guarantees
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Old 04-13-2010, 04:04 PM   #5
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I was just looking through some components on Jameco.com ... There is obviously a lot to digest here... Maximum Single Supply Voltage, Maximum Input Offset Voltage, etc...

Anyone want to give me some specifics on picking the right parts and making a circuit that will work for this?

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Old 04-13-2010, 04:38 PM   #6
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Why not try a photo resistor from radio shack wired in series with power to LED. Mount in 1" long tube to limit ambient light influence, point at flame, and power with 12-24 Vdc. It might take a little trial and error testing to select the correct one from the pack and position it so ambient light will not turn it on, but it is the simplest way to get a flame indication. At about about $3 for a pack of 5, the cost is not a big factor.

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Old 04-13-2010, 04:41 PM   #7
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check out this amplifier: http://focus.ti.com/lit/ds/symlink/ina128.pdf

Datasheet even has application circuits. should get you what you need. You'd need a +/-12V power supply (wall wart maybe) and then the driving circuit for the LED. A 10k resistor to a 2N4401 transistor should do the trick. You'd have to calculate the collector resistor value (probably around 500 ohms) to work with whatever LED you pick.

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Old 04-13-2010, 06:32 PM   #8
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Thanks! I'm going to do some reading and give this a try.

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Old 10-10-2011, 10:03 PM   #9
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curious how this idea went? I want to have a panel light on my system that simply tells me the pilot is burning (not out) .. What did you end up doing??

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