The Oxygen sensor Display

1. Components
Diode 1N4001 X 3
Zener Diode 9v1 X 1
Capacitor 0.1uF X 2
LED X 3 (Red, Yellow, Green)
Resistor X 7 (R2=1KΩ, R3=1KΩ, R4=1KΩ, R5=380Ω, R6=10KΩ, R7=270Ω, R8=470Ω)
Op-Amp LM324 X 1 (14pins)

reference by
http://elcodis.com/datasheet.php?c=1315884&c_name=LM324AN&doc=164605

2. Calculations
R5?
Power supply voltage is at 12v, Vd of crossing diode D2=0.6v, Vd of zener diode D1=9.1v
Vd of R5 = 12-0.6-9.1= 2.3v
I=5.6mA=0.0056A
R=V/I =2.3/0.0056 =411Ω
I have chosen a 380Ω resistor which is similar to 411Ω resistor.
R8 & R7?
R6=10KΩ Voltage drop in crossing R6 is at 8.47v
The voltage are at 9.1v and 0.63v before the R6 and after the R6 each.
So, Vd=9.1-0.63 = 8.47v
Using Ohm's law I=V/R =8.47/10,000 =0.000847A
R8 consumes the voltage which voltage drop is at 0.4v
Vd= 0.63 - 0.23 =0.4v
R=V/I =0.4/0.000847 =472Ω
R7 consumes the voltage which voltage drop is at 0.23
Vd= 0.23 - 0 =0.23Ω
R=V/I =0.23/0.000847 =271.5Ω
Total resistance RT=R6+R8+R7 =10,743Ω
I=9.1/10,743 =0.000847A
R2, R3 & R4?
I= 9.5mA =0.0095A
The voltage drop in R2 is at 9.6v, Vd=12 - 0.6 - 1.8 =9.6v
R=V/I =9.6/0.0095 =1010.5Ω
The voltage drop in R3 is at 9v, Vd=12 - 0.6 - 0.6 - 1.8 =9v
R=V/I =9.6/0.0095 =947Ω
The voltage drop in R4 is at 9.6v, Vd=12 - 0.6 - 1.8 =9.6v
R=V/I =9.6/0.0095 =1010.5Ω
3. A Technical Explanation

There are three different resistors(R6, R8 & R7) in this circuit and these resistors are connected in series which produces different voltage drop and available voltages and these voltages operate the Op-Amp.
A green LED turns on. If the sensor input (12 IN+) is lower than 0.23v and the 13IN- is at 0.23v the current flows through the green LED and runs from 14OUT to 11V- of rail voltage. The other two LEDs will turn off due to lower input voltage.

A yellow LED turns on. If the sensor signal is between 0.23v and 0.63v the 9IN- is at 0.23v to 0.63v. The current flows through the yellow LED and runs from 8OUT to 11V- of rail voltage.The other two LEDs will turn off due to lower and higher input voltages.

A red LED turns on. If the sensor signal is higher than 0.63v the 5IN+ is at 0.63v. The current flows through the red LED and runs from 7OUT to 11V- of rail voltage. The other two LEDs will turn off due to higher input voltage.

4. Test Procedure
- Building a circuit in a board : I have chosen correct components with calculated resistors.
  There are four Op-Amps in a LM324.

 reference by
http://elcodis.com/datasheet.php?c=1315884&c_name=LM324AN&doc=164605

 When the signal voltage is lower than 0.23v, the green LED turns on and other two LEDs turn off. When the O2 sensor signals are between 0.23v and 0.63v, the yellow LED turns on and other two LEDs turn off. Finally, When the signal voltage is higher than 0.63v, the red LED turns on and other two LEDs turn off. Therefore, this circuit was working properly without any problems.

- Voltage drop
When I tested the voltage drop in each resistor, the values were similar between calculated and actual measurement.

- Making an actual board with soldering
This circuit was working properly without any problems.

5. Problems
I have made a disconnection of the circuit. So I could find various faults and repair that problems.
The points of the broken wiring

Point1 : The yellow LED stays turning off.
The pin2 will get 0v and the pin3 of signal voltages is always higher than pin2. So, the current flows through the pin4 of voltage rail and D3 diode then the yellow LED always turns off.
Point2 : The yellow LED keep turning on over 0.63v.
The current will not flow through D3 diode and after the yellow LED. When the red LED turns on at over 0.63v, above current flowing will turn the yellow LED off. From the disconnection, the yellow LED keep turning on at over 0.63v.  
Point3 : The yellow LED keep turning on always.
The pin10 will get 0v and a pin9(signal voltage) is always higher than the pin10. Therefore, the current flows through the yellow LED and pin11(earth) of voltage rail and the yellow LED always turns on. 
Point4 : The yellow LED keep turning on always.
The pin10 will get 0v and a pin9(signal voltage) is always higher than the pin10. Therefore, the current flows through the yellow LED and pin11(earth) of voltage rail and the yellow LED always turns on.
Point5 : The green LED keep turning on always.
The pin13 will always receive 9.1v and the O2sensor signals are lower than 9.1v. So, the current flows through the green LED and pin11(earth) of voltage rail and the green LED always turns on.
Point6 : The all LEDs and Op-Amps are turned off.
The supply voltage is cut for the operation of all Op-amps and LED. So these components will not work.

6. Reflection
 It was a good opportunity that I have understood the operation of Op-Amps from building an O2 sensor display unit. In addition, I could make various voltages from the voltage divider with using different resistors. These voltages are used input of Op-Amps and two differnt inputs determine the flowing output voltage and it controls the switching LEDs on and off.   
 When I took the display on an engine, it works correctly. When the engine rpm suddenly goes up, the red LED turns on due to rich mixture. When the engine is idling, the LED lights are changed green, yellow and red in order which means that the O2sensor monitors the state of exhaust gases then the ECU controls fuel injection for proper engine running.