relay, coolant level sensor

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dr_harika

Member - Posts: 8

Joined: April 19, 2007

Location: Australia

Posted: September 20, 2009 at 7:03 AM / IP Logged

Hi guys,

My dilema is trying to make a coolant level sensor work in my car (not equipped with one from factory) using a relay (or perhaps something else?).

I have installed a Coolant Level Sensor, and it has two resistances (nothing inbetween).

180ohm when there is coolant present

1450ohm when the coolant is low.

My intention was to somehow use the difference in resistance to activate a relay coil, so that it turns on a 12v light i will install on the dash.... I have been trying to connect one side of the relay coil to 12v ACC, and the other side of the coil to the coolant sensor and then ground; but once the relay activates, it stays activated until power is removed.

Any help or ideas is appreciated.

Thanks.

dr_harika.

KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: September 20, 2009 at 9:45 AM / IP Logged

There is a post on here somewhere from "I am an idiot" that shows how to make a low fuel light based on a change in resistance (it's an op amp circuit).

You can use the coolant switch as one half of a voltage divider and then use the voltage divider output to feed an op amp. The output of the opamp can drive a transistor that drives a relay.

Kevin Pierson

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: September 20, 2009 at 10:23 AM / IP Logged

Probably cheaper to buy and simpler to install an aftermarket temp. gauge, I'd feel safer with one of those than a warning light.

dr_harika

Member - Posts: 8

Joined: April 19, 2007

Location: Australia

Posted: September 21, 2009 at 6:58 AM / IP Logged

thanks for the idea. will need to look into it.

I couldn't find "i am an idiot"'s post or any that were relevant. He has 6000+ posts, so was hard to filter.

Could my idea work with a PIC microchip with some sort of analogue input? and an output to drive a relay?

I have a PIC programmer and know the basics (but only have delt with digital inputs)

Any help is greatly appreciated.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: September 21, 2009 at 7:26 AM / IP Logged

Using a microcontroller would greatly simplify things.

It's the same basic outline though, add a resistor to create a voltage divider and then feed that voltage in to an analog input. I'm not a PIC guy, so I can't give you specific details on the best route to go after that. Ideally, you'll have only an 8 bit ADC to make all the calculations "easy". If you have more then an 8 bit ADC shift out the LSBs to leave you with only the 8 MSBs. After that, especially with your application, it's pretty easy since you will always only have one voltage or the other. In fact, if you calculated the voltage that is directly in the middle of the two possible voltages you would only need one comparison.

You will still need the transistor and diode on the output of the PIC to drive a relay, and you'll still need a base resistor.

Kevin Pierson

dr_harika

Member - Posts: 8

Joined: April 19, 2007

Location: Australia

Posted: September 26, 2009 at 9:21 PM / IP Logged

Thanks for the help gents,

I solved the problem by using an OP amp (LM741).

I connected the inverting input (pin 2) to the top of a 5.1v zener diode which is limited by a 510ohm resistor.. 5.1v is the reference voltage for the basis of my voltage divider.

The non-inverting input (pin 3) was connected to the voltage divider made from the coolant sensor (connecting to ground), and the other side was a 430 ohm resistor connecting to ACC 12v+.

As for the output, it put a 510ohm resistor, and a 3.3v LED, and placed it in the dash.

works a treat. it is triggered just below the "low" level on the coolant bottle.

thanks for the help.. with this concept, i can now do my auto lights (with a light sensor).. never really knew how to use it.

I also want to learn how to use PIC's for anything other than basic switch circuits.

light

hotwaterwizard

Silver - Posts: 1,350

Joined: December 11, 2003

Location: California, United States

Posted: October 10, 2009 at 2:37 PM / IP Logged

relay, coolant level sensor -- posted image.

John DeRosa (Hotwaterwizard)

Stockton California

When in doubt, try it out !

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ckeeler

Gold - Posts: 1,461

Joined: June 20, 2008

Location: New Mexico, United States

Posted: October 12, 2009 at 12:32 PM / IP Logged

i actually saved that schematic you are talking about KP. here it is

relay, coolant level sensor -- posted image.

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relay, coolant level sensor

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