how to have led come on at set voltage?

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Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=103296
Printed Date: May 15, 2025 at 12:07 AM

Topic: how to have led come on at set voltage?

Posted By: aaronk
Subject: how to have led come on at set voltage?
Date Posted: March 21, 2008 at 4:25 PM

Im trying to make a scaling-up shift light for my ice racer. Sometimes called a "Sequential Shift Light".

I have a tachometer signal that starts at 0.1V (idle) and ramps up to 5.0V (redline).

What Im trying to do is construct an LED shift light that will progressively light up as revs increase.

Example:
less than 4V = 0 LEDs on
4.1V = 1 LED on
4.3V = 2 LED on
4.5V = 3 LED on
.... and so on, up to 5V = all LEDs on.

The problem is how the heck to I do that?

Check this picture for an idea of what Im talking about:

posted_image

Replies:

Posted By: Ween
Date Posted: March 21, 2008 at 7:28 PM

hi,

so are you looking to build a circuit yourself....google LM3914. It's a display driver IC..and then with that IC, then look at expanded scale meters (google that also). With some changes in resistor values you should be able to make a meter/display that starts at 4.1V and has outputs in .2V increments. Don't worry about the piece having ten outputs, you don't need to use all of them. you may be able to find a kit and modify values to your liking.

hope this helps

mark

Posted By: aaronk
Date Posted: March 21, 2008 at 9:15 PM

That helps tremendously Mark.

Im on the research now and will probably be back with some application questions.

Thanks again.

Posted By: aaronk
Date Posted: March 21, 2008 at 9:32 PM

Im going to put this link here as there is a good diagram I (or anyone else with a similar question) can use for sorting this out.

https://www.mp3car.com/vbulletin/hardware-development/88578-lm3914-led-driver-questions.html

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