Amplifying a wrx crank trigger sensor with an op amp

[Azezial]

I am having an issue with my stock crank angle sensor giving me a 8v pk to pk voltage where the spec is 14v pk to pk. The vehicle will not rev past 6800.

I built two circuits with op amps trying to amplify this signal and got two different outputs which I am unsure of and was hoping someone could shed some light.


this is the circuit:

**broken link removed**

I used two 9v batterys to create the positive and negative vcc's

I built one using a tlo82 BiFET op amp and when I t'd into the signal wire (just to check the output) I was able to amplify the signal by about 1 volt. However when i cut the wire and actually put the amp into the loop it shows 14v pk to pk at the input and output. (i dont understand why this is changing the input?)

With this circuit in place it wont rev to anything past idle so im assuming is out of phase and throwing the timing off??? Because the signal is the same before and after the op amp I really have nothing to compare it to for the phase.

So i tried again with a standard 741 op amp and the output was changed to a 2v pk to pk square wave. When cut into the loop this was able to rev to 5k but then expirenced break up in the ignition. Im guessing the slew rate wasent fast enough or maybe im going out of phase again...

Any advice or insight would be much appreciated.

Thanks, Mike

 

[Mikebits]

Your circuit is more like a buffer. How did you come up with those resistor values?
Check out this link.
Op-amp Varieties
BTW, what is a wrx crank trigger sensor?

 

[mneary]

If your crank sensor is putting out below spec, I would suspect a problem that won't be solved with amplification. More likely a failed sensor or maybe out of adjustment.

 

[Azezial]

its a subaru wrx and the crank position sensor is a hall effect sensor that tells the ecu where the engine is in its cycle...

The resistor values are used as a ratio of input to output if i am not mistaken???

I used 33 and 10 ohms? maybe i should use 1k and 330?



as far as the sensor being out of spec, its not... its just because of the high flow pump that the sensor is a little farther from the gear so its off, so im trying to fix a (theoretical) mechanical problem with electronics...

thanks, Mike

 

[audioguru]

Your extremely low resistor values are shorting the output of the opamp to ground. An ordinary opamp has trouble driving 1000 ohms and lower. Your resistors have a total of only 43 ohms. An ordinary opamp limits the output current to 25mA which is a peak voltage across 43 ohms of only 1V which you see.

If the resistors are 10k and 33k then the opamp will work but its gain is only 1.3 which is not enough.

If the battery is 13.2V then the max output of an ordinary opamp is only 12.0V, not 14.0V. Maybe the car's computer is looking for a swing from 0V to 14V.
With two brand new 9V batteries the ouput of an opamp with enough gain is from -7.8V to +7.8V which maybe is not high enough. The battery voltage and the opamp's output voltage drops as the batteries run down.

I don't know if the sensor has continuity to ground for the DC reference voltage of the opamp. If it doesn't then the opamp will not work. If it does then the output of the opanp will try to go negative as well as go positive. The computer in your car might not like the negative voltage.

 

[Mikebits]

There are op amps the swing rail to rail.
https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1045,P1323,D2762

 

[Azezial]

Crankshaft
position
sensor

Signal (+) B2 W 0 –7 — +7 Sensor output waveform
Signal (–) B11 G 0 0 —
Shield B21 SB 0 0 —

this is what the stock signal looks like:

**broken link removed**

My pspice simulated output using the two 9v's showed enough gain to amplify the signal... should the output be exactly in phase with the input? It only counts when zero is crossed.

here is my simulation:
**broken link removed**


I do want it to swing from -7 to +7, i guess I will try it with some higher value resistors...