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Ohms to trigger a switch

Alan P

Mar 12, 2015
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Hi Guys,

I'm trying to design a circuit to trigger a relay based on an ohms reading. I need to trigger the relay at 150 ohms. I believe I could use an LT1721 to do this, however I'm unsure of the resistor values or the circuit design. Any help would be appreciated. Thanks.
 

Arouse1973

Adam
Dec 18, 2013
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Hi Alan can you explain a bit more about your project? Where is this 150R coming from?
Adam
 

Alan P

Mar 12, 2015
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Mar 12, 2015
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Hi Adam,

The signal is coming from a sending unit which is being used to monitor fluid levels in liquid storage tanks. The sending units are similar to fuel sending units which basically act as potentiometers with a reading of 16 ohms empty and 158 ohms at full. I need to create a safety switch to prevent overfilling of the tanks. By creating a circuit which could cut out the pump at approx 150 ohms, I could use the existing sending units as the source input signal.
 

Alan P

Mar 12, 2015
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Mar 12, 2015
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@ADRT, If I"m understanding this right, R3 provides for an adjustable trigger signal? Also, is there a specific Comparator used in this circuit?
 

Arouse1973

Adam
Dec 18, 2013
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Yes that circuit was just what I had in mind :). Nice one
Adam
 

Alan P

Mar 12, 2015
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I ran a few more tests on sender. At Empty it's around 5vdc and Full is about .8vdc which is my trigger voltage. Any suggestions on the comparator?
 

ADRT

Nov 25, 2014
192
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Nov 25, 2014
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This is the original source of the circuit. If you read further down you will find some recommendations on comparitors as well as some equations for figuring out which resistors to use for your application. Hope this helps and good luck. : ) oh and yes R3 is for adjustment.
 

duke37

Jan 9, 2011
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A comparator normally needs a pull-up resistor on its output.
 

ADRT

Nov 25, 2014
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Is that always duke37? I noticed that on some of the datasheets for different comparitors sometimes they use a pull up and sometimes they don't depending on the application. Do you think there should be one here?
 

duke37

Jan 9, 2011
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You will need enough base current to turn Q1 hard on so you will need a pull-up resistor if the comparator does not pull up.

If you wish to look at several different levels of voltage, then you could investigate the LM3914 which has 10 comparators on board.
 

Alan P

Mar 12, 2015
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Mar 12, 2015
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What's protecting the sender from +12v in the transistor circuit?
 
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Alan P

Mar 12, 2015
15
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Mar 12, 2015
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My circuit reading skills leave a lot to be desired. The fuel sender + enters at the bottom right? I think I got it. I'll wire it up on a bread board and give it a shot. Thanks.
 
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