Saab: : : : Oxygen Sensor Gauge Hack : : :
I have created a 20 LED bar to show the siginal from the oxygen sensor.
Starting with two 10 LED bar graph driver IC's (National Semiconductor LM3914N) and two 10 bar LED graphs, I did a little math and came up with the wiring diagram and numbers below:

About the driver chip:
Each 18 pin chip drives 10 LED's. Each chip contains an internal voltage reference. Pin 7 is held at about 1.25v above the voltage applied to pin 8 (ref. adjust). The voltage range of the graph is determined by the voltage at pins 4 and 6. The current pulled through the led's (brightness) is about ten times the current pulled through pin 7.


(The twenty diodes across the the top are the LED's.)
The pinout is as follows:
  1. LED #1
  2. Ground
  3. + power
  4. Low Reference
  5. Siginal
  6. High Reference
  7. Reference Output
  8. Reference Adjust
  9. Mode
  10. LED #2
  11. LED #3 ...
Resistors:
  1. 1 Kohm
  2. 5 Kohm
  3. 1.05 Kohm
  4. 0-10 Kohm potentiometer
  5. 300 ohm
Note the driver contains ten 1K resistors, between pins 4 and 6 (marked as '10K' on the diagram).
For our application we will be using only one of the voltage reference units. R1 (which is on the un-used reference output) determines the brightness of the first unit.

We know we want a total range of zero to one volt. We attach pin 4 of the first unit to ground, pin 6 to pin 4 of the second unit, and pin 6 of the second unit to 1 volt. Since the internal voltage reference is above that, we need to place a resistor on that voltage to drop it down, this is R2. The 1v here is what we want, the 1.25 is what we have, and the 20K resistor is the twenty 1k resistors internal to the two units. 1v = 1.25v (20Kohm / (20Kohm + R2)).

R1 and R3 control the maximum amount of power that can go through the LED's, you don't want to burn them or the driver chip. We can figure out a value of R1, that will provide equal impedance to the internal circuit, R2 and R3 with this math: R1 = (20K + R2) R3 / (20K + R2 + R3)

R4 is a 10K potentiometer, used as a dimmer for both led's at the same time.

R5 simply limits the 14 volts possibly coming into the LED's to a reasonable voltage.
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