# Comparator circuit for temp indicator

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I have two circuits at

to show what I am thinking. I am trying to make a lead acid charging temperature monitor. The circuits are from existing url's that I found by searching Google. My question is repeated on the page as follows: I want to set up a temperature monitor for charging lead acid batteries. I think the sensor can be imbedded into the negative battery post. Distance from the sensor to a LED indicator would be around 15 feet. I don't think there is a need to have exact temperatures. Likely a range within 10 degrees will suffice.

I would like to have the output from the temperature sensor light up a red LED when the temperature exceeds 130° Fahrenheit in place of the voltmeter in the first circuit (1). Since the first circuit is supposed to scale the output and provide 10 mV/degree, could a proper comparator circuit be made to indicate when the voltage exceeds 1300 mV or 1.3V? (Circuit B)

I have no clue how to calculate what resistor values would be needed in the comparator circuit (2). I am assuming there will need to be a trimmer to vary when the LED would turn on, maybe at V - REFERENCE? Would I need to supply the comparator voltage from a different source and what would that be?

Any help is certainly appreciated; Don

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Look into the lm34 temp sensor ic's, they look like a transistor. Its output is a voltage that is linear to temp change.

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One input of the comparator can be connected to where the voltmeter is in the other circuit; and the other input can be connected to the wiper of a trimmer. The trimmer can be connected between the power supply rails (like RV), or you could connect the top end of the trimmer to the +5V regulated output from the LM7805. The trimmer could be anything from 10K to 100K; it's not critical. You might also connect a large resistor (>1M) between the comparator output and the (+) input to provide positive feedback for hysteresis.

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Thank You so very much for the reply. I think I can follow your instructions and give it a try. Thanks Again; Don

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Try this: (view in Courier)

+12V>--+---[78L05]--+-------+----------[750R]--+ | | | | | [0.33µF] | [0.1µF] | | | | | | | +------+-----+ | | | | [HLMP4700] | | | | | | | +------+-------+ | | | | | | | | | +---|--[1M]--+-[750R]--+ | [3740R] | | | | | | | | | | | +------|---+--|+\\ | | | | | >------+ | | [LM34]---|-/ U1A | | | | | [1000R] | +-------+ | | | | | | | | +--|+\\ | | | | | | >-+ GND>---+-----+------+-------+--|-/ U1B LM393

Use the 12V from the battery to run it, wire it up on a little piece of perfboard and keep everything pretty close together. Instead of running a 15 foot set of leads to the sensor and building everything around the LED, wire everything around the sensor and run the leads to the LED.

I'd use a twisted shielded pair for the LED. with the shield grounded.

```--
John Fields
Professional Circuit Designer```
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```--
Better yet:

+12V>--+---[78L05]--+-------+--------------------/ /--+```
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AIUI the OP wants it to trip at 130F. Wouldn't your divider be a bit low (I calculate ~1.055V) or does leakage thru the LED pull it up? Couldn't the OP use a 5K linear pot in place of the 3.74K resistor to fine tune the trip point?

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```--
No, you\'re right.  It should be 1300 ohms.  I did the math properly
but transcribed the 1300 onto the post as 1000.  Good catch, thanks!```
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No biggy. :-)

I was playing around and with 5% resistors, the trip point could vary between ~120F - ~139F. Sounds like some kind of adjustment would be a good idea. 1% resistors would narrow that down to something that's probably negligible (127F - 131F).

Not a good place to live. Is there any worse place (on Earth) for electronics than in an automotive environment?

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The 78L05 has an output voltage which can vary from between 4.75V to

5.25V, so with an output voltage of 4.75V and worst case resistances for the 3740 ohm and 1300 ohm 1% resistors for a low reference out we have: 4.75V | [3774R] | +----1.208V ~ 121F | [1287R] | GND

At the high end we have:

5.25V | [3737R] | +----1.365V ~ 137F | [1313R] | GND

Which with the OP's request for a 10% machine would be fine.

Unless he actually meant +/- 5%, in which case the pot would be needed to adjust out the variation in the 78L05's output.

```--
Downhole.```
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a

Oops, I didn't even think about the voltage variation.

Ah, I've heard it's nasty in there.

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Thanks for all the expertise. Actually, this will be used to monitor charging on a large 12 volt 1200 amp hour battery bank with 150 amp alternator and a 120V 70 amp charger when on shore power. Realizing that we don't get all those amps in reality but it's a start. I'll get busy and order parts. Thanks again for everyone's help and the time for circuit drawing; Don

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