Thanks for the input. The voltage range I am interested in mostly is between 0.98 and 0.78 volts in .04 steps.Your resistor divider network is between pins 4 and 6 on the LM3914. There can be as little as 200mV difference between the two pins; providing that neither pin is within 1.5V of V+ and not less than V-. Use bar mode when you're that low for better accuracy. If your difference between pins 4 and 6 is 500mV or greater, you can use dot mode.
200mV difference between pins 4 and 6 will give you 20mV (0.02V) per output.
Pin 6 should be a more positive voltage than pin 4.
I suggest that you use an adjustable voltage regulator IC like an LM317 to set the upper voltage limit of your scale on pin 6 (properly bypassed with a capacitor, of course) and then use a pot from pin 4 to ground, bypassing that pin with a small cap as well.
Note that the LM3914's internal resistive divider will measure approximately 10K Ohms, but may vary from around 8K to 12K Ohms. You'll have to measure that yourself.
Let's say that you want to measure a possible voltage range of 3.0v to 3.15v in 0.05V increments. You're using just the lower four outputs of the LM3914 to power your LED's. You've measured between pins 4 and 6, and discovered the internal resistor network measures 11.2K Ohms.
3.0V + 10 x 0.05V = 3.5V (you have 10 outputs, and you want them to be 0.05v apart, starting at 3.0V)
Set your Vreg to 3.5V, and connect it to pin 6.
You now need to figure out the lower half of the voltage divider network. The upper half measures 11.2K Ohms
You could figure it by current:
I = E/R
I = 0.5V / 11.2K
I = 0.0446429 mA
Now, calculate the lower resistor by:
R = E/I
R = 3.0 / 0.0446429 mA
R = 67.2K
Or, just wire in a 100K Ohm pot from pin 4 to ground (don't forget a small bypass capacitor), and adjust it until you get 3.0V across it.
You might consider using the LM3915 instead of the LM3914. The LM3915 is logarithmic, as are the newer lambda sensors. By limiting yourself to only 4 LEDs over such a small voltage range, you're cheating yourself out of a lot of potentially valuable information.
I plan to use a green at the 'good' range, yellow in the warning range and one red in the 'something bad is about to happen' range. So from above 1.0v the green LED will be lit. At 0.99 to 0.95 the first yellow LED will light. At 0.94 to 0.90 the second yellow will light. Below 0.90 the red LED will light. I may add a 5th red LED to indicate that the reading is in the 0.80 range. I think I can do this by tying two outputs to one LED. I haven't tried this but I hope it will work. In this way I can have one LED light only for a .05v 'step' and another LED to light for a full .1 (two .05 steps). Make sense?Ok, I see.
Instead of just having red warning LED's though - how about using various color LED's to indicate the fuel mix and relative fuel economy? Or is fuel economy not a consideration?
Exactly which is why I want to adjust the LED stepping of the LM3914.Turbo-charged engines have very poor fuel economy when the turbo is doing its boosting of the power. The meter for the mixture is because a lean mixture and a high amount of boost will blow the head gasket or melt a piston.
I think a turbo engine likes a rich mixture to avoid detonation. Then at high amount of boost the fuel pump or fuel injectors might not be able to produce enough fuel so the mixture turns out lean and KA-BOOM!
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by Gary Elinoff
by Jake Hertz
by Jake Hertz