Components:
- R2=1000 ohm carbon film resistor
- R3=1000 ohm carbon film resistor
- R4=1000 ohm carbon film resistor
- R5=390 ohm carbon film resistor
- R6=10000 ohm carbon film resistor
- R7=270 ohm carbon film resistor
- R8=470 ohm carbon film resistor
- 2 x Capacitor C1 and C2 ,50 volts 0.1uF
- Zener diode 9v1 9.1 volts
- 3 x Rectifier diode 1N4001
- Opam LM324
- Red LED
- Yellow LED
- Green LED
Calculations:
R2 = V / I
R2 = 12 - 0.7 - 1.8 / 9.5 mA
R2 = 1000 Ohms
R3 = 12 - 0.7 - 0.7 - 1.8 / 9.5 mA
R3 = 926 Ohms ( since there is no such resistor as 926 Ohms we use the next one up which is 1 K Ohms)
R4 = 12 - 0.7 - 1.8 / 9.5 mA
R4 = 1000 Ohms
Voltagedrop = ( R above / R total ) Vin
Vd = 9.1 - 0.63 = 8.47 volts,
Vin=9.1 volts, R above=10000 ohms
8.47 = (10000 / Rtotal ) 9.1,
8.47 / 9.1 = 10000 / Rtotal
0.93 = 10000 / Rtotal
Rtotal =10752 Ohms
Rtotal = R6 + R7 + R8
10752 = R8 + R7 + 10000
10752 - 10000 = R8 + R7
752 = R8 + R7
Vd = 0.63 -0.23 = 0.4V
Vin = 0.63 volts , R above = R8
0.4 = ( R8 / Rtotal ) 0.63
0.4 / 0.63 = R8 / Rtotal
0.6349 = R8 / Rtotal
R8 = 0.6349 x Rtotal
Rtotal x 0.6349 = R8
R7 = Rtotal - 0.6349 Rtotal
R7 = 0.3650 x 752 = 274.48 = 274 Ohms
R7 = 274 ohms
or
R7 = ( 0.23 x 10000 ) / 8.47
R7 = 271.5 Ohms
R8 = ( 0.4 x 10000) / 8.47
R8 = 472.2 Ohms
This circuit is an oxygen sensor tester and is a potentialy divided circuit .
It is divided into 3 parts and depending on the sensor output it will have a different coloured LED
so we can visually see the difference in the signal volatge thats coming out of a O2 sensor in the exhaused.
Region 1: 0V to 0.23V sensor input and the green light is on indicating that the O2 sensor is reading a lean mixutre in the exhaused
Region 2: 0.23 V to 0.63 V sensor input and the yellow light is on indicating that the O2 sensor is reading a normal mixture in the exhaused
Region 3: 0.63 V to 1.05 V sensor input voltage and the red light is on indicating that the O2 sensor is reading a rich mixture in the exhaused
If we have this board correctly hooked up to a good working O2 sensor it will blink forward and thru the different colour LEDs.
If we could see these LEDs connected to the O2 sensor while the car is driving if we accelerate we would get the red light coming on as the mixture gets richer and if we decelarate by engine braking or going down a hill it will change to the green LED as the mixture gets real lean
Testing:
I used Voltage drop to make sure every component is getting the voltage they are suppose to be getting and to make sure I have not got a short or open circuit .
I checked available voltage at R6 and instead of being around 9.1 Volts which is the Zener diode spec and the zener diode is regulating the voltage to 9.1 because we are using 9.1 V zener ... it was around 0.23 mVwhich was not right but after fixing the issue it was to the correct available voltage which was 9.1 V
Available voltage :
Terminal 2,5 : 0.63
Voltage drop of 0.7 after each diode is due to the knee voltage of the rectifier diodes
Volyage drop of 1.8 volts after LED which is the knee voltage of this Light emitting diode
Problems:
First my circuit was only getting the red LED on all the time even by changing the signal voltage but by voltage drop and visual inspection I noticed that I had Resistor 6 or R6 instead of being connected to the positive rail after R5 it was soldered by mistake into the earth rail and after rectifying this problem I got my board working correctly .
Reflection:
Next time I wont rush as much and would do a good visual check to make sure it matches the diagram before I even start testing the circuit .
