reading analog engine signals to a pic

[jay543_uk]

Hi all, im new to pics and electronics but i do no quite abit about automotive engine sensors and signals.My question is how can i take readings from the engines sensors with out disturbing the signal going back to the ecu.All the signals on my engine are a max of 5volts.

would i have to use a op-amp so i wasnt puting any sort of load on the signal back to the ecu.

hope this makes sense.

i no how to do the a/d conversions once its at the pic, i just need to no how to read the voltage/signal with out desturbing it going back to my ecu.

thanks for any help

 

[Nigel Goodwin]

Yes, use opamp buffers, and low pass filters as well - it's essential you don't pass any frequencies that are too high.

See https://en.wikipedia.org/wiki/Nyquist_frequency

 

[jay543_uk]

hi thanks for your replie, im guess the filter would be before the opamp. would you have any diagram examples that i could look at.

im very new to electronics so im learning as i go along.

many thanks

 

[Nigel Goodwin]

hi thanks for your replie, im guess the filter would be before the opamp. would you have any diagram examples that i could look at.

No, place the filter after the buffer, the filter will have a low output impedance suitable for feeding the PIC.

There are plenty of examples out there, see here for one of them:

**broken link removed**

 

[jay543_uk]

iv attached a pic of a low pass filter, is this the type of filter i need to use. iv not used filters or opamps before so im just doing resource on the net at the mo.

thanks to any one who can help me

jason

 

[Ambient]

You can check Texas Instruments. They have quite a few pdf's with many single supply and dual supply op amp filters.

 

[Pommie]

If you can put a normal multimeter on the signal your interested in and it doesn't effect the ECU then you can connect it straight to your pic. Also, unless the signal is noisy, there is also no need to filter it.

What parameters are you wanting to read? If it's temperature and pressure type readings then I would imagine you would not need filtering. If it's crank position etc then you don't want filtering. Something like manifold pressure would need to be filtered.

Mike.

 

[Nigel Goodwin]

If you can put a normal multimeter on the signal your interested in and it doesn't effect the ECU then you can connect it straight to your pic. Also, unless the signal is noisy, there is also no need to filter it.

Unless you can 100% guarantee that there will never be any higher frequency components in the signals, it's essential to low-pass filter the input.

Check for 'nyquist frequency' to see why.

 

[Pommie]

Unless you can 100% guarantee that there will never be any higher frequency components in the signals, it's essential to low-pass filter the input.

Check for 'nyquist frequency' to see why.

I believe I stated, "unless the signal is noisy".

"Nyquist frequency" only comes into it is you are trying to reproduce a waveform. As the OP doesn't want to reproduce any frequencies then he should be fine.

Even if the signal is noisy, a simple RC filter will probably suffice. With an RC filter the input impedance requirements (of the pic) are easily met as the capacitor acts as a low impedance source.

Mike.

 

[ericgibbs]

hi,
For the small cost of the Res/Cap filters, I fit them as standard on any automotive/mobile installation.

Also I fit a resistor of around 47K/100K between the 'sensor' and the non-inverting input of the OPA buffer.
If the OPA input defaults to a short circuit due to a spike is dosn't kill the signal to the ECU.

 

[jay543_uk]

hi all,
so is this the setup i need to use.

https://en.wikipedia.org/wiki/Image:Active_lowpass_filter.svg

 

[jay543_uk]

If you can put a normal multimeter on the signal your interested in and it doesn't effect the ECU then you can connect it straight to your pic. Also, unless the signal is noisy, there is also no need to filter it.

What parameters are you wanting to read? If it's temperature and pressure type readings then I would imagine you would not need filtering. If it's crank position etc then you don't want filtering. Something like manifold pressure would need to be filtered.

Mike.

Why does a manifold pressure sensor need to be filtered.??????

sorry if its a stupid question.

 

[Diver300]

Why does a manifold pressure sensor need to be filtered.??????

sorry if its a stupid question.

The only stupid question is the one you don't ask......

The manifold pressures pulse lots and lots as there is a sharp pressure change each time a valve opens or closes, and as the pistons go up and down.

Often the first stage of filtering will be a restrictor feeding the pressure sensor.

 

[jay543_uk]

The only stupid question is the one you don't ask......

The manifold pressures pulse lots and lots as there is a sharp pressure change each time a valve opens or closes, and as the pistons go up and down.

Often the first stage of filtering will be a restrictor feeding the pressure sensor.

i didnt think of that, the ford equipment i use at work must give you an average reading and not a live reading.i wanna be able to read boost pressue in the engines manifold and was planning on using the engine map sensor,is it going to be ok to use a op-amp and rc filter for this and do a A/d conversion on the pic and use a look up table.