The Oxygen Sensor Tester

This is my own work and represents my learning in this assessment. Any information sourced from elsewhere has been appropriately acknowledged and referenced. I have and will continue to maintain the privacy of any person I have referred to in this assessment and have sought their permission to have their information shared here. I acknowledge a copy of my work may be used for moderation purposes. I have a back up copy of the work I have presented in this assessment if it is required.

Vladimir Borzenko

References:   http://www.electronics-tutorials.ws

                       http://www.allaboutcircuits.com

                       http://www.circuit-magic.com

                       Automotive Electrics & Electronics Gregory’s First Edition 1999

The circuit diagram is on the photo:

The are two ways to calculate resistors for  this operational amplifier. R6, R7 and R8 are divider’s resistors. So, they can be calculated via the voltage drop formula.  V out/ V in = R bottom/ (Rbottom+Rtop). For our diagram (Va-Vb)/Va=R6/(R6+R7+R8). From this we have :

R7+R8=(Va*R6)/(Va-Vb)-R6=(9.1*10,000)/(9.1-0.63)-10,000=743 Ohms

Rtotal=R6+R7+R8=10,000+743=10,743 Ohms
R7=Rt*Vc/Va=10,743*0.23/9.1=271 Ohms

R8=((Vbc*Rtotal)/Va)-R7=((0.63-0.23)*10,743)/9.1V)-271Ohms=471 Ohms

I3=Va/Rtotal=9.1/10,743=847 uA

Second variant of R7 and R8 calculation:

I3=Vdab/R6=(9.1-.63)/10,000=847uA

R7=Vd/I=.23/847uA=271 Ohms

R8=(0.63-0.23)/847uA=471 Ohms

I1=I3+I2, where I2=IzRm=5.6 mA

I1=847uA+5.6mA=6.447mA

R5=(VRaw-Vdd2-Va)/I1=(12-.6-9.1)/6.447mA=356.76 Ohms

R2=(VRaw-Vdd2-Vdled)/Iled=(12-.6-1.8)/9.5mA=1010.5 Ohms

R3=(VRaw-Vdd2-Vdd4-Vdled)/Iled=(12-0.6-0.6-1.8)/9.5mA=1010.5 Ohms

R4=R2=(VRaw-Vdd2-Vled)/Iled

R4=R2=(12-.6-1.8)/9.5mA=1010.5 Ohms

Thus, results for resistors in accordance to E24(5% tolerance) row:

R2=R4=1 kOhm

R3=910 Ohms

R5=360 Ohms

R7=270 Ohms

R8=470 Ohms

layout of components was drawn with Lochmaster programme:

Voltage measurements and diagramm reading on the O2 senser tester circuit board:

Voltage input of 12.05 V is rectified by D2 and smoothed by C1. As a result, voltage at the LED’s anodes is 11.34V. From the very beginning I must mention that light difference in voltages between wire diagram and measured figures occurred due to electronic components and measuring equipment tolerance. From the figures above it’s clear that Vs is slightly higher than on the paper. For all the reason above, the zener diode D1 stabilises voltage at the point A at 9.09V. Current flows through divider circuit.  There is 0.67 V after R6 at the point which I named B and on the pins #2 and #6. There is 0.27 V after R8 at the point C. That gives the same potential on the pins #10 and #13. It’s clear from the first glance that op-amps are connected in pairs in a such specific way when incoming signal goes onto the non-inverting input of the upper op-amp and onto the inverting input of the low op-amp. Incoming signal from the PL2 contact comes onto the pins #12, #9, #6, and #3 simultaneously. If this signal level is within 0 - 0.27 V intervals there is 0.941 V on the pin#14 which is V out of the lower op-amp. Thanks to that the LED6, Green one, is forward biased. It glows. Once the incoming signal reaches 0.27 V and goes higher it overcomes voltage on the non-inverting input of op-amp. This voltage difference is amplified and as a result there is 10V on the pin#14, outcome of the low op-amp. Since this moment the LED#6 is no more forward biased. It’s off. At the same time this signal overcomes voltage on the pin#10 and comes onto the pin#9 which is inverting input for the op-amp connected to the Yellow LED5. At this moment op-amp amplifies voltage difference. But it inverts the incoming signal. For that reason the D5 is forward biased and glows. This figure remains negative but the highest op-amp “closes” LED5 forcibly after 0.67 V level of incoming signal. The #1 pin is connected through diode D3 straight away to the LED5 cathode. It gives high potential at the LED5 cathode. From this time Yellow LED is no more forward biased and it’s off. Once the input signal reaches 0.68 V and rises higher it allows to have the inverted outcome on the pin#7 what provides forward bias for Red LED1. It is on since this moment. The Yellow LED is off because of reason mentioned above. Resistors R2, R3 and R4 are limiting resistors.

The table below  supplements voltage reading. It helps to check voltage for falts finding also.

Pin#	Green LED on	Yellow LED on	Red LED on
1	0.006	0.003	10
2	0.7	0.7	0.7
3	0.3	0.3	0.3
4	11.56	11.57	11.54
5	0.7	0.7	0.7
6	0.3	0.3	0.3
7	10.3	10.27	0.9
8	10.3	0.9	0.9
9	0.3	0.3	0.3
10	0.3	0.3	0.3
11	0.0001	0.0001	0.0001
12	0.3	0.3	0.3
13	0.3	0.3	0.3
14	0.95	10.27	10.27

These photos illustrate that diodes change one another from the Green one to the Red one with increasing input signal. This proves that all the calculation and the diagram reading are correct. This tester can be used to check the O2 sensor output and conclude if fuel mixture is reach or lean.