light led at specific resistance?

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astrosurfer

Member - Posts: 49

Joined: July 24, 2006

Location: Louisiana, United States

Posted: January 18, 2009 at 12:22 AM / IP Logged

Hi fellas.

Been a few months since my last visit.

Now I have a burning Q.

I am wondering if there's a way to incorporate a warning led into my fuel sender circuit to come on at a specific low level.

I'm sure the resistance from the sender changes and that's why the gauge changes.

I'd like a warning when fuel low like newer vehicles do.

Obviously all help appreciated.

Any diagram will help and list of components.

I'm thinking some sort of transistor setup would work but also thinking they work on voltage increase. Not real sure.

Thanks

james bond 007

Copper - Posts: 62

Joined: January 18, 2009

Location: Ontario, Canada

Posted: January 18, 2009 at 9:49 PM / IP Logged

If your gauge works with voltage, you could use a zener diode

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astrosurfer

Member - Posts: 49

Joined: July 24, 2006

Location: Louisiana, United States

Posted: January 25, 2009 at 10:12 PM / IP Logged

I have a little more info I can add

my fuel sender sends 240 ohms empty and 33 ohms full to my gauge.

Looking for a way to turn on a warning at say 180 ohms. Is this possible guys?

Again thanks.

I'm looking into the zener diode but I don't understand it.

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: February 04, 2009 at 5:17 PM / IP Logged

Mr. Bond is I think right, at empty it's returning a lower voltage than full, say 3 against 6 so a zner driving the LED selected at say 3 volts would light an LED. Raise the voltage, the Zener opens the circuit, LED turns off. I'm sure I am an Idiot or Mr. Pearson could enlighten you more if you pm them.

astrosurfer

Member - Posts: 49

Joined: July 24, 2006

Location: Louisiana, United States

Posted: February 06, 2009 at 7:54 PM / IP Logged

thanks for more info dude.

I am really uneducated on zeners for sure.

I did not know they did this.

I'll pm idiot....or mr.pearson and see if they could give me some more insight into this.

Thank you very much indeed.

And to MR BOND. Thanks.

i am an idiot

Platinum - Posts: 13,670

This member consistently provides reliable information

Joined: September 21, 2006

Location: Louisiana, United States

Posted: February 06, 2009 at 9:24 PM / IP Logged

Instead of the ohm reading of the unit, can you give me a voltage reading at whichever end of the range is easier. Full or empty will work. Black meter lead to ground and the red lead to the output wire of the sending unit, or the input wire of the gauge. Meter set to DC volts.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: February 06, 2009 at 11:02 PM / IP Logged

Who is this Mr. Pearson character?

I'm with I am an idiot on this one, voltage is the route to go, not resistance.

Kevin Pierson

howie ll

Pot Metal - Posts: 16,466

Joined: January 09, 2007

Location: United Kingdom

Posted: February 07, 2009 at 1:20 AM / IP Logged

Kevin. sorry I'm the guilty party.

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: February 07, 2009 at 7:39 AM / IP Logged

haha, not a problem at all!

Kevin Pierson

i am an idiot

Platinum - Posts: 13,670

Joined: September 21, 2006

Location: Louisiana, United States

Posted: February 07, 2009 at 5:52 PM / IP Logged

The Op amp in the diagram is a LM741 (NTE 941) single channel op amp. If you can not find one I can get an LM358 (NTE 928M) for you. It is a 2 channel op amp but you will only have to move power from pin 7 to pin 8 on the 358. If you can tell me what voltage you want the light to light up at, I can modify the adjustment pot to give you more precise control. Southern Radio 355-0396 or Ralph's Industrial Electronics 344-3761 should have the NTE parts.

https://www.the12volt.com/installbay/forum_posts.asp?tid=105885&KW=i+am+an+idiot

The part about the light coming on when 3 gallons remaining, you have to adjust it for that to happen.

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