Hey guys,

I want to read out my W-contact on my alternator for my digital Tachometer.
I can have a AC sinewave with a voltage between 0,1V and 30V and want to reproduce it to DC wave with a voltage between 0 and 3.3V.
Does anyone have any suggestions how to build this on my custom PCB?

Kind regards,

Rens

 

What is the digital tach you are using, or is that the custom PCB you mentioned?

 

What is the digital tach you are using, or is that the custom PCB you mentioned?

Yes I want to read it out on my Rapsberry PI, but I building a custom PCB that connects to it to read out varies sensors, including the W-contact for my tachometer on the rapsberry pi.

Greets,

Rens

 

30V seems very high for the W terminal. How did you measure it? Can you post the waveform?

 

30V seems very high for the W terminal. How did you measure it? Can you post the waveform?

Hi Werecow, I measured already 8V at around 1000rpm with my mulitmeter on the diesel engine so I set a safe upper limit for this setup to about 30V. The engine can rev pretty high so I need to have a safe margin for it.

 

Hi Werecow, I measured already 8V at around 1000rpm with my mulitmeter on the diesel engine so I set a safe upper limit for this setup to about 30V. The engine can rev pretty high so I need to have a safe margin for it.

Hi, Rensieboy,

Before you start designing the interface to your Raspberry Pi, you might want to verify the W terminal signal, using an oscilloscope. Hint: It may not be the sine wave or voltage level you think it is.

 

Hi, Rensieboy,

Before you start designing the interface to your Raspberry Pi, you might want to verify the W terminal signal, using an oscilloscope. Hint: It may not be the sine wave or voltage level you think it is.

Hello Werecow,
Thanks for the tip!
I don't have a oscilloscope on my hands right now but verified it again with my multimeter on the dynamo were gonna use for the setup and it was: 6,34V AC and 800hZ on the multimeter at 2000rpm alternator rotation.

Do you have any tips, examples or circuits that are able to convert this AC signal to a DC signal?

King regards,
Rens

 

Good day, Ren,

My recommendation to use an oscilloscope is because what you're reading as AC voltage on your voltmeter may not be a sine wave with amplitude proportional to RPM, so you must confirm that before designing your tach circuit. The two multimeter readings you've provided (8V @ ~1000 RPM and 6,34V @ 2000 RPM) are not consistent with what you believe the output to be.

-B

 

I just did this yesterday to interface a digital tach to an alternator. Works great.

 

Hello Werecow,
Thanks for the tip!
I don't have a oscilloscope on my hands right now but verified it again with my multimeter on the dynamo were gonna use for the setup and it was: 6,34V AC and 800hZ on the multimeter at 2000rpm alternator rotation.

Do you have any tips, examples or circuits that are able to convert this AC signal to a DC signal?

King regards,
Rens

Have you verified that the frequency actually varies directly with the engine RPM?? I am not familiar with the "W" connection on your alternator. ALSO!! I suggest a low current in-line fuse close to the alternator connection, because most alternator connections can deliver a damaging amount of current if they are accidentally grounded.

 

Hi Werecow, I measured already 8V at around 1000rpm with my mulitmeter on the diesel engine so I set a safe upper limit for this setup to about 30V. The engine can rev pretty high so I need to have a safe margin for it.

Do you happen to be working on this project for a mk1/mk2 Diesel VW? I am currently trying to research how i would go about doing the same thing in my vehicle, since many didn't ship with a tachometer.